Dual-AC External Transformer & Internal Triple-Rail (`+5V / +12V / −5V`) Alert!

Inside the TI-99/4A, both the giant 64-pin TMS9900 16-bit CPU AND all eight 4116 Video RAM chips require THREE separate DC power rails (+5.0V, +12.0V, and −5.0V), while the proprietary TMC0430 GROM chips also require −5.0V (and −0.7V bias)! The external TI "brick" (AC9900 / PHP1100) outputs two separate low-voltage AC windings (18V AC and 8.5V AC) over a 4-pin rectangular plug into a slide-in internal Power Supply Regulator Board under the front-right of the console. If the internal −5V (7905 / 79M05) or +12V (7812) regulator fails — or if cracked solder joints develop on the 4-pin power jack — the TMS9900 CPU and all eight 4116 DRAMs can latch up and burn out!

1 Rear 4-Pin AC Power Socket Pinout

Viewed looking at the rear 4-pin male power receptacle on the TI-99/4A console (numbered 1 to 4 Left-to-Right):

Pin	External AC Brick Signal	Unloaded / Loaded AC Voltage
Pin 1 (Left)	18 V AC Winding (to +12V / −5V stage)	~18.0–20.5 V AC (measured to Pin 3)
Pin 2	8.5 V AC Winding (to main +5V stage)	~8.5–10.0 V AC (measured to Pin 3 @ 1.9 A)
Pin 3	AC Common Return	Common return for Pins 1 & 2
Pin 4 (Right)	NC / Safety Earth	Unconnected on 2-prong US bricks

1983 US Consumer Product Safety Commission Recall Note: Early brown/black transformer bricks without an internal thermal fuse were recalled in 1983 (TI issued a clip-on "Step-Down" inline fuse block). Check that your brick stays cool!

2 Internal PSU Board 4-Wire Harness

Before plugging the internal PSU board's 4-wire Molex plug into the main motherboard, verify all three DC rails referenced to the metal RF shield:

Wire / Pin	Target DC Voltage	Powers
Pin 1 (`+5V`)	+4.90 to +5.15 V DC	TMS9900, TMS9918A, TMS9901, 4116 V_CC, ROMs & TTL
Pin 2 (`GND`)	0.00 V Digital Ground	Common DC ground
Pin 3 (`−5V`)	−4.75 to −5.25 V DC	TMS9900 Pin 2 (`V_BB`), 4116 Pin 1 (`V_BB`) & GROMs!
Pin 4 (`+12V`)	+11.5 to +12.5 V DC	TMS9900 Pin 27 (`V_DD`), 4116 Pin 8 (`V_DD`) & Video!

3 Cartridge Slot Felt Dust-Trap Warning!

Look inside the front 36-pin Solid State Software Cartridge door: early Silver/Black TI-99/4As have a thick grey felt wiper pad glued around the cartridge throat to wipe dust off cartridge fingers.
After 40 years, this felt strip sheds conductive lint and snags on cartridge edge connectors, bending the delicate gold contact fingers inside the slot!
Fix: Carefully peel out disintegrating felt strips and clean the 36-pin cartridge connector with a stiff card wetted with DeoxIT D5.

System Architecture & Model Selector

Explore the TI-99/4A's unique "256-Byte 16-Bit Scratchpad + 16KB 8-Bit VDP RAM" architecture and compare the Silver/Black vs. Beige v2.2 QI consoles.

Original Silver & Black Brushed-Aluminum TI-99/4A 1981–1983 • Full Cartridge Compatibility

Replacing the chicklet-keyed 1979 TI-99/4, the Silver/Black TI-99/4A introduced a full-travel 48-key typewriter keyboard, added Graphics II bitmapped mode (TMS9918A instead of TMS9918), and became a 2.8-million-unit bestseller during the 1982–83 home computer price war. Its heavy die-cast internal clamshell RF shield doubles as a giant heatsink for the 64-pin TMS9900 CPU, the 40-pin TMS9918A VDP, and the eight 4116 Video DRAMs.

Key ICs on the Standard Motherboard

CPU: Texas Instruments TMS9900NL (64-pin DIP) @ 3.00 MHz (derived from 48 MHz/12 MHz crystal + TIM9904 4-phase clock generator)
Scratchpad CPU RAM ($8300–$83FF — 16-Bit Wide!): 2× MCM6810 / TMS4045 (128×8) SRAMs = 256 Bytes of zero-wait-state 16-bit RAM where the TMS9900 stores its register workspaces (R0–R15)!
System Console ROMs ($0000–$1FFF — 16-Bit Wide!): 2× 4 KB Mask ROMs (2332 pinout, 8KB total) holding the GPL (Graphics Programming Language) interpreter and floating-point math routines
System GROMs (GROM 0, 1, 2 — TMC0430): Three 16-pin 6 KB Graphics Read-Only Memories (18KB total) with built-in auto-incrementing address counters holding TI BASIC and the master title screen
VDP: TI TMS9918ANL (NTSC Composite) or TMS9929ANL (European PAL YPbPr)
Video RAM (16KB): 8× TMS4116-20NL (16K×1, triple-rail) connected privately to the VDP!
I/O Controller (CRU Bus): TI TMS9901NL (40-pin DIP) — Programmable Systems Interface scanning the keyboard, joysticks, cassette I/O, and VDP/external interrupts
Sound Generator: TI TMS9919NL / SN94624N (SN76489) — 3 square-wave tone channels + 1 noise channel

Silver/Black vs Early Beige (Non-QI) Identification

Not all Beige TI-99/4A consoles have the v2.2 lockout ROM! Here is how to tell in 2 seconds on the master title screen:

Title Screen Copyright Line	Motherboard Type	Runs Third-Party ROM Carts (Atarisoft / FinalGROM)?
`© 1981 TEXAS INSTRUMENTS`	Standard Silver/Black OR Early Beige (Standard Board)	YES — 100% Compatible with all GROM and ROM-only cartridges!
`© 1983 TEXAS INSTRUMENTS` (with `v2.2` beneath)	Late Beige "QI" (Quality Improved) Cost-Reduced Board	NO! Blocks ROM-only cartridges (`Atarisoft`, `Imagic`, `Parker Bros`, and `FinalGROM 99` ROM images) unless GROM 0 is swapped!

Beige "QI" (Quality Improved) `v2.2` Console Late 1983 • Custom Gate Array • ROM Lockout

In late 1983, TI released the cost-reduced "QI" motherboard in the textured beige case (with no metal trim around the cartridge slot). It replaced dozens of 74LS glue chips with a single custom gate array, soldered the three System GROMs onto a narrow riser strip, and updated GROM 0 to version v2.2. Infamously, TI altered the v2.2 GROM 0 menu scanner so it ignores the $6000–$7FFF CPU ROM cartridge header and only lists cartridges that contain a proprietary TI GROM chip — locking out unlicensed ROM-only games from Atarisoft (Pac-Man, Defender, Donkey Kong), Imagic, and Funware!

How to Unlock a `v2.2` Beige QI Console for All Cartridges

Swap GROM 0 (CD2155 / TMC0430) with a 1981 Silver/Black GROM 0: The lockout lives entirely inside the 16-pin GROM 0 chip! Replacing the v2.2 GROM 0 with an original 1981 GROM 0 (or a modern ATmega GROM 0 replacement board) immediately restores 100% compatibility with Atarisoft and FinalGROM 99 cartridges!
Note on QI Cartridge Port Pin 33 (/ROMG Enable): On some very late QI revisions, also verify that Cartridge Pin 33 (/ROMG $6000 select) is connected on the PCB!

Other Hardware Differences on the QI Board

No -5V on the Cartridge Slot (Pin 1): Because TI switched to single-voltage GROMs on late runs, verify power pins before using vintage prototype boards.
Lighter RF Shielding: Uses foil-lined cardboard instead of the two-piece die-cast aluminium brick from the 1981 Silver/Black console (runs slightly cooler as a result!).

Why the 16-Bit `TMS9900` Has Only 256 Bytes of Direct CPU RAM!

The TI-99/4A has one of the most unusual architectures in computing history. The 64-pin TMS9900 is a true 16-bit minicomputer processor (from the TI 990 series) with a 16-bit data bus and 16-bit address bus — and unlike the 6502 or Z80, it has no general-purpose registers (R0–R15) on the CPU die! Instead, a Workspace Pointer (WP) register points to 32 bytes in RAM where R0–R15 live, allowing instant context switches. However, because 16-bit RAM was expensive in 1979, TI only gave the TMS9900 256 bytes of 16-bit Scratchpad RAM ($8300–$83FF)!

Memory / Bus Tier	Bus Width & Wait States	How the TI-99/4A Uses It
Scratchpad RAM (`$8300–$83FF` • 256 Bytes) & Console ROM (`$0000–$1FFF` • 8KB)	Full 16-Bit Bus • ZERO Wait States!	Blazing fast! Assembly language games store their `WP` registers (`R0–R15`) and inner loops inside this 256-byte Scratchpad RAM for full 16-bit speed!
16KB Video RAM (`VRAM`) (8× `4116` wired privately to `TMS9918A`)	8-Bit Port via VDP (`$8C00` / `$8C02`)!	On an unexpanded TI-99/4A, all 16 KB of main RAM is actually Video RAM connected behind the VDP! To read or write a single byte of TI BASIC program memory, the CPU must send two address bytes and one data byte one at a time through the VDP's I/O window!
32KB RAM Expansion (`PEB` or Side-Car) (`$2000–$3FFF` & `$A000–$FFFF`)	16-Bit Multiplexed to 8-Bit (4 Wait States) (or 16-Bit 0-Wait Internal Mod!)	Adding a 32 KB RAM expansion (Required for `Extended BASIC` assembly, `Editor/Assembler`, and modern homebrew games!) gives the TMS9900 32 KB of directly addressable CPU RAM!

Why TI BASIC Was Slow (Double Interpretation!): In TI BASIC, the 16-bit TMS9900 executes the GPL interpreter (in 16-bit ROM), which fetches GPL bytecodes out of slow 8-bit GROMs (GROM 1/2), which in turn interprets your BASIC tokens stored inside VDP Video RAM! Plugging in the Extended BASIC cartridge (or FinalGROM 99) + 32K RAM bypasses VDP RAM for programs and runs dramatically faster!

Peripheral Expansion Box (PEB), Speech Synthesizer & Modern Upgrades

Servicing the heavy 8-slot PEB "Firehose" interface, the TMS5220 Speech Synthesizer side-car, and modern FinalGROM 99 + TIPI setups.

1 Solid State Speech Synthesizer (`PHP1500`)

Plugs directly into the 44-pin right-hand side expansion port and passes the bus through to the PEB or 32K side-car. Built around TI's TMS5220 / TMC0285 Linear Predictive Coding (LPC) Speech Synthesizer + two 16 KB Speech Phrase ROMs (TMC0350, ~350 built-in words + infinite pitch/phoneme synthesis in *Parsec*, *Alpiner*, and *TI Extended BASIC*!):

Garbled Speech ("Static / Alien Screech") or Console Crashes When Speech Is Attached:
- 95% of Speech Synthesizer faults are caused by oxidised tin fingers on the sliding 44-pin pass-through edge connector!
- Slide the black plastic pass-through block out and scrub both the male fingers on the console's right side and the female spring contacts inside the Speech module with DeoxIT D5 and a folded business card.
Important Power Note: The Speech Synthesizer draws its +5V and −5V rails directly from the TI-99/4A console's internal power supply over the 44-pin side port! Always turn the console OFF before attaching or removing any side-car module.

2 Peripheral Expansion Box (`PEB` & "Firehose" Cable)

The tank-like 8-slot aluminium/steel PEB chassis (PHP1200) connects to the console's side port via the thick shielded Flex Cable Interface Card (PHP1210):

Flex Cable "Gooseneck" Foot Contact Care: The heavy metal foot that plugs into the right side of the console (or into the Speech Synthesizer) puts mechanical leverage on the 44-pin edge fingers. Support the foot level with the desk and clean its contacts with DeoxIT whenever the PEB LEDs fail to blink on boot!
PEB Card Voltage Regulation (Local 7805 / 7812 / 7905 on Every Card!): Just like an S-100 computer, the PEB backplane distributes unregulated +8V DC, +16V DC, and −16V DC, and every individual PEB card (32K RAM, Disk Controller PHP1240, RS-232 PHP1220) has its own local TO-220 7805 / 7812 regulators and axial filter caps! If one card stops responding, check its onboard 7805 regulator and tantalum decoupling caps first.

3 Essential Modern TI-99/4A Upgrades

FinalGROM 99 (by Ralph Benzinger): Plugs into the front cartridge port and loads any .BIN ROM or .G GROM cartridge image (up to 1 MB, plus 512 KB RAM mode!) straight from an SD card!
JEDIMATT42 32K Sidecar + TIPI (TI-99 to Raspberry Pi Bridge): A tiny modern PCB that plugs into the 44-pin right-hand side port (behind the Speech Synthesizer), providing 32 KB RAM expansion (or SAMS 1MB banked RAM!) + Raspberry Pi Wi-Fi network drive (DSK1–DSK9), mouse, and web browser without needing a 35-lb PEB box!
F18A / Tang Nano 9K VDP Upgrade: Drop-in FPGA replacement for the 40-pin TMS9918A/29A VDP chip that outputs crystal-clear VGA / HDMI and eliminates the 4-sprites-per-scanline flicker limit!

Diagnostic Checklist, 4116 VRAM Locator & Keyboard Repair

Decoding the "Screaming Continuous Beep", TI's backwards RD0–RD7 VRAM bit numbering, and restoring Mitsumi vs Stackpole keyboards.

▦

8× `4116` Video RAM Chip Locator & TI's Backwards Bit Numbering (`RD0 = MSB`!)

Because all 16 KB of console RAM is wired exclusively to the TMS9918A / TMS9929A VDP, a bad 4116 DRAM chip leaves the 16-bit TMS9900 CPU running normally — so the console still plays its normal short power-on BEEP and shows the cyan/colour bars on the title screen, but characters have corrupted letters or vertical pixel stripes!
CRITICAL TEXAS INSTRUMENTS BIT NUMBERING TRAP: Texas Instruments numbered every data and address bus in the 9900/9918 family in reverse big-endian order: Bit 0 (RD0 / D0) is the MOST Significant Bit (128 / $80) and Bit 7 (RD7 / D7) is the LEAST Significant Bit (1 / $01)!

TI-99/4A 8× `4116` VRAM Bit-to-IC Map (Bottom-Left of PCB)

VDP Pin & TI Bit	Standard Binary Weight	VRAM IC Designator (Top to Bottom / Left to Right)	Character Corruption Symptom on Title Screen
`RD0` (VDP Pin 24)	MSB (`$80` / 128)	U509 (or #1 in row)	Inverse/offset characters; leftmost pixel of every 8px tile stuck!
`RD1` (VDP Pin 23)	Bit 6 (`$40` / 64)	U510 (#2)	Letters turn into symbols/numbers (`T` ↔ `4`); 2nd pixel column stuck
`RD2` (VDP Pin 22)	Bit 5 (`$20` / 32)	U511 (#3)	Uppercase/lowercase or spaces turn into `@` (`$20` ↔ `$00` / `$40`)
`RD3` (VDP Pin 21)	Bit 4 (`$10` / 16)	U512 (#4)	`T` (`$54`) turns into `D` (`$44`); 4th pixel column stuck
`RD4` (VDP Pin 20)	Bit 3 (`$08` / 8)	U513 (#5)	`A` (`$41`) turns into `I` (`$49`); 5th pixel column stuck
`RD5` (VDP Pin 19)	Bit 2 (`$04` / 4)	U514 (#6)	`A` (`$41`) turns into `E` (`$45`); 6th pixel column stuck
`RD6` (VDP Pin 18)	Bit 1 (`$02` / 2)	U515 (#7)	`A` (`$41`) turns into `C` (`$43`); 7th pixel column stuck
`RD7` (VDP Pin 17)	LSB (`$01` / 1)	U516 (#8)	Adjacent ASCII letters swap (`A` ↔ `@`, `B` ↔ `C`, `1` ↔ `0`); rightmost pixel stuck!

"Black Screen + Continuous Screaming Beep" Decision Tree

Why the TI-99/4A Screams on a Failed Boot: Just like the Oric and BBC Micro, the TMS9919 / SN94624 sound chip powers up with a random continuous tone until the TMS9900 CPU executes the Console ROM reset routine ($0000) and mutes the sound chip!
Step 1 — Check All Three PSU Rails (+5V, +12V, −5V): If −5V or +12V is missing on the 4-wire internal PSU harness, the TMS9900 CPU cannot run at all, leaving the sound chip screaming on a black screen!
Step 2 — Check the Two 6810 / 4045 Scratchpad SRAMs (U612 & U613): If either of the two 24-pin/18-pin Scratchpad SRAMs ($8300–$83FF) is dead, the TMS9900 loses its R0–R15 workspace registers and crashes before silencing the sound chip!
Step 3 — Check TIM9904 Clock Generator & Console ROMs (U610/U611): Verify the 4-phase 3 MHz clocks (Φ1–Φ4) entering the TMS9900 (Pins 8, 9, 25, 28) and check for a shorted 4116 VRAM dragging down the +12V or −5V rail!

⌨

TI-99/4A Keyboard Variants & Mitsumi Membrane Fix

TI used four different keyboard manufacturers (all plugging into the 15-pin motherboard header P5).

1. Mitsumi Flexible Membrane Keyboard (Common in 1983 Silver & Beige Units):
- The 2-layer Mylar membrane inside Mitsumi keyboards is notorious for failing as the foam separator pads turn to dust and the silver traces oxidise at the 15-pin ribbon neck!
- Fix: Open the metal backing plate, clean the silver contacts gently with IPA (or conductive silver ink pen), or drop in a modern mechanical Cherry MX replacement TI-99/4A keyboard PCB!
2. Stackpole / Hi-Tek ("Space Invader" / Waffle Frame): Individual dual-leaf contacts inside square plastic wells. Do NOT pry keycaps sideways or the brittle plastic stems will snap! Flush dirty leaf contacts from above with DeoxIT D5.
3. ALPS Mechanical Keyboards (1981–82 Silver/Black): Discrete soldered mechanical switches — by far the most durable and sought-after TI-99/4A keyboard.

⚙

`TMS9901` PSI (Dead Keys/Joysticks) & Stuck GROM `READY`

Console boots to the colour-bar title screen, but PRESS ANY KEY TO BEGIN never responds, or locks up with a continuous tone when a cartridge is inserted.

Dead Keyboard AND Dead Joysticks (TMS9901NL at U600): The 40-pin TMS9901 PSI scans the keyboard/joystick matrix directly without buffering diodes against static discharge! ESD from touching the joystick port or keyboard frequently blows TMS9901 input pins (P0–P7 / INT3–INT10).
Locks Up Solid When Reading GROMs: Every GROM chip (GROM 0, 1, 2 and Cartridge GROMs) holds the TMS9900 CPU paused (READY line low) until the slow PMOS GROM finishes its internal address cycle (~447 kHz GROM clock from VDP Pin 37 GROMCLK). If a dirty cartridge or failed GROM fails to release READY, the CPU halts permanently!

5-Pin NTSC vs. 6-Pin PAL Component Video, Dual Joystick & Tape Pinouts

Connecting NTSC Composite or European PAL TMS9929A YPbPr Component Video to modern upscalers, plus the single-DE9 dual-joystick diode circuit.

5-Pin DIN (NTSC US) vs. 6-Pin DIN (PAL Europe `YPbPr`!) Video Port

See also our dedicated TI-99/4A Dual-Joystick DE-9 & Cassette and 4-Pin Dual-AC Power pinout pages. North American TI-99/4A consoles use the TMS9918A VDP with a 5-pin 180° DIN socket (Composite Video + Audio — 100% compatible with a standard 5-pin Commodore 64 / VIC-20 / Atari 800XL A/V cable!). European PAL consoles use the TMS9929A VDP with a 6-pin 240° DIN socket that outputs native Component YPbPr (Y, R-Y, B-Y) Video!

Pin	US NTSC 5-Pin DIN (`180°`) (`TMS9918A` Composite)	European PAL 6-Pin DIN (`240°`) (`TMS9929A` Component `YPbPr`!)
Pin 1	+12 V DC Out (Powers external RF modulator — leave NC in RCA cables!)	+12 V DC Out (Powers external PAL modulator)
Pin 2	GND (0 V Shield)	`Y` (Luminance + Sync Out — Green `Y` RCA!)
Pin 3	Audio Out (Mono) → White/Red RCA	`R-Y` (`Pr` Red Colour Difference Out — Red RCA!)
Pin 4	Composite Video Out → Yellow RCA	`B-Y` (`Pb` Blue Colour Difference Out — Blue RCA!)
Pin 5	NC	Audio Out (Mono) → White/Red RCA
Pin 6 (Center)	—	GND (0 V Shield)

European PAL TI-99/4A Direct Component (YPbPr) to OSSC / RetroTINK 5X/4K Trick: On a 6-pin PAL TI-99/4A, you can wire Pin 2 (Y), Pin 4 (Pb / B-Y), and Pin 3 (Pr / R-Y) (with Center Pin 6 = GND and Pin 5 = Audio) directly to Component RCA plugs into a RetroTINK 5X/4K or OSSC for razor-sharp, lag-free component video without an external PAL modulator!

Non-Standard Single `DE-9` Dual-Joystick Port & Atari DB9 Adapter

WARNING: Do NOT plug a standard Atari/C64 joystick directly into the TI-99/4A's side 9-pin D-Sub socket! TI wired BOTH Joystick 1 AND Joystick 2 into a single 9-pin male socket as part of the keyboard scanning matrix (Select Joy 1 = Pin 2, Select Joy 2 = Pin 7 instead of Pin 8 GND)!

TI `DE-9` Pin	TI-99/4A Matrix Signal	Atari / C64 DB9 Adapter Target (With `1N914`/`1N4148` Diodes!)
Pin 1	NC	—
Pin 2	/SELECT JOYSTICK 1 (Active-low strobe)	Joy 1 Pin 8 (Common Return — via diode cathode!)
Pin 3	UP (Shared Joy 1 & Joy 2)	Joy 1 & Joy 2 Pin 1 (Anode to Pin 3, Cathode toward Pin 2/7)
Pin 4	FIRE (Shared Joy 1 & Joy 2)	Joy 1 & Joy 2 Pin 6 (Fire Button)
Pin 5	LEFT (Shared Joy 1 & Joy 2)	Joy 1 & Joy 2 Pin 3 (Left)
Pin 6	NC	—
Pin 7	/SELECT JOYSTICK 2 (Active-low strobe)	Joy 2 Pin 8 (Common Return — via diode cathode!)
Pin 8	DOWN (Shared Joy 1 & Joy 2)	Joy 1 & Joy 2 Pin 2 (Down)
Pin 9	RIGHT (Shared Joy 1 & Joy 2)	Joy 1 & Joy 2 Pin 4 (Right)

Why 5 Blocking Diodes Per Joystick (1N4148) Are Essential: When building a dual Atari-to-TI joystick splitter, put a 1N4148 diode in series with each of the 5 switch lines (cathode pointing toward TI Pin 2 for Joy 1, and toward TI Pin 7 for Joy 2) — this prevents pressing diagonal directions from ghosting onto the TI's `` key line!

Rear 9-Pin `DE-9` Female Cassette Port

Pin 1 & 2 = Motor 1 Relay (+/-), Pin 3 = Audio Out to Tape MIC (Red), Pin 4 = GND, Pin 5 = Audio Out Shield, Pin 6 & 7 = Motor 2 Relay (+/-), Pin 8 = Audio In from Tape EAR (White), Pin 9 = Audio In Return (Black).

Chip & GROM / Console ROM Reference

Interactive IC Pinout Finder cards plus TMS9900, TMS9901, TMS5220, and TMC0430 GROM notes.

DIP-64

TI TMS9900 16-Bit CPU

3.0 MHz 16-bit microprocessor with memory-to-memory workspace registers, CRU serial bus, and triple-rail supply.

View Pinout →

DIP-40

TI TMS9918A / 9929A VDP

Video Display Processor — 256×192 graphics, 16 colours, 32 sprites, GROM clock output, and 16 KB private 4116 VRAM bus.

View Pinout →

DIP-28

TI TMS5220 / TMC0285 Speech

LPC-10 vocal tract synthesizer inside the PHP1500 Solid State Speech Synthesizer side-car module.

View Pinout →

DIP-16

TMS9919 / SN94624 (`SN76489`)

3-channel square-wave + 1 noise channel Programmable Sound Generator mapped at $8400.

View Pinout →

DIP-16

TMS4116 DRAM (×8)

Eight triple-rail (−5 V, +12 V, +5 V) 16K×1 DRAMs providing the 16 KB Video/System RAM.

View Pinout →

Texas Instruments 9900-Family Silicon Reference

Chip	Package	Key Pins & Bench Notes
TMS9900NL (16-Bit CPU)	DIP-64 (Large Ceramic/Plastic)	Requires 3 rails (`Pin 1 = +5V V_CC`, `Pin 2 = −5V V_BB`, `Pin 27 = +12V V_DD`, `Pin 26 = GND`) + 4-phase 12V clock (`Φ1–Φ4` from `TIM9904`)! Note: `A0` is MSB (`Pin 49`) and `A14` is LSB (`Pin 35`); `D0` is MSB (`Pin 41`) and `D15` is LSB (`Pin 56`)!
TMS9901NL (PSI I/O)	DIP-40 (`U600`)	CRU (Communications Register Unit) bit-addressable I/O & interrupt controller. Static-sensitive on keyboard/joystick lines!
TIM9904 / 74LS362 (Clock Gen)	DIP-20 (`U601`)	Divides the 48 MHz/12 MHz crystal into four non-overlapping 12 V 3.0 MHz clock phases (`Φ1–Φ4`) for the TMS9900.
TMC0430 (`GROM 0, 1, 2`)	DIP-16 (PMOS)	Proprietary 6 KB serial-counter Graphics ROMs mapped at `$9800`/`$9802`/`$9C00`/`$9C02`. Can be replaced or overridden using an `ATmega` UBER-GROM board!
TMS5220 / TMC0285 (Speech)	DIP-28 (Inside `PHP1500`)	LPC-10 speech synthesizer with 128-bit FIFO buffer (`READY` handshaking with TMS9900).

Console ROMs (`U610`/`U611`) & Single-Rail `4164` VRAM Mod

Even & Odd 4KB Console ROMs (U610 & U611, $0000–$1FFF):
- The two 24-pin 4 KB mask ROMs (U610 = High Byte D0–D7, U611 = Low Byte D8–D15) use the standard 2332 mask ROM pinout — meaning two TMS2532 (4 KB, 24-pin) UV EPROMs drop straight in with zero adapters!
16-Bit 32KB Internal Fast-RAM Mod (0-Wait-State! Piggybacked on Console ROMs):
- One of the coolest hardware mods for the TI-99/4A is soldering two 62256 (32K×8) static RAMs directly on top of the 16-bit Console ROM sockets (U610/U611) with 3 logic gates: it puts all 32 KB of expansion RAM ($2000–$3FFF & $A000–$FFFF) directly on the 16-bit zero-wait-state CPU bus, speeding up Extended BASIC and assembly games by 50% to 110%!
Single-Rail 4164 VRAM Upgrade: If replacing bad 4116 VRAMs, you can fit modified +5V-only 4164 DRAMs (bend out Pin 1, bridge Pin 8 to Pin 9 +5V) to cut heat inside the RF shield in half!

Resources & Further Reading

Blog posts, interactive bench tools, and essential TI-99/4A communities & archives.

Smiggy's Retro Computing Posts

Bench Tools & Machine Hubs

IC Pinout Finder — TMS9900, TMS9918A, TMS5220, TMS9919, 4116
Connector Pinout Directory — 5-Pin NTSC & 6-Pin PAL YPbPr Video, Dual-Joystick DE-9 & Cassette, 4-Pin Dual-AC Power
Recap & Repair Reference — capacitor replacement guide
Ohm's Law Calculator
C64 Machine Hub — Commodore 64 reference
VIC-20 Machine Hub — Commodore VIC-20 reference
Commodore PET Hub — PET 2001, 4000 & 8000 reference
Plus/4 & C16 (TED) Hub — C16, C116 & Plus/4 reference
C128 Machine Hub — C128, C128D & DCR reference
SX-64 Machine Hub — Executive Portable C64 reference
Amiga Machine Hub — A500/A600/A1200 reference
Big-Box Amiga Hub — A1000, A2000, A3000 & A4000 reference
Atari ST Hub — 520ST, 1040STFM & STE reference
Atari 8-Bit Hub — 400/800, 800XL & 130XE reference
Apple IIe Hub — Unenhanced, Enhanced & Platinum reference
BBC Micro Hub — Model B, B+ & Master 128 reference
Acorn Atom & Electron Hub — Atom & Electron reference
ZX Spectrum Hub — 16K/48K & +2/+3 reference
Sinclair QL Hub — 68008, Microdrive & ULA reference
Dragon 32/64 Hub — Dragon 32 & 64 reference
TRS-80 & CoCo Hub — Model I/III/4 & CoCo 1/2/3 reference
Oric-1 & Atmos Hub — Oric 6502A & ULA reference
Amstrad CPC 464 Hub — CPC 464 & Datacorder reference
Amstrad CPC 6128 Hub — 128K & 3″ disc reference
MSX Machine Hub — MSX1, MSX2, MSX2+ & Turbo R reference
Osborne 1 & Kaypro II Hub — CP/M luggable reference

External TI-99/4A Resources

Thierry Nouspikel's TI-99/4A Tech Pages — the definitive cycle-by-cycle hardware, CRU, GROM, and PEB engineering reference
AtariAge TI-99/4A Forum — the main active global TI-99/4A hardware repair & development community
Ninerpedia — TI-99/4A hardware, FinalGROM 99, TIPI, and Geneve 9640 wiki
Mainbyte's TI-99/4A Site — step-by-step console teardown, 32K internal fast-RAM mod, and schematics

Capacitor & Power Stage Reference

TI-99/4A — Capacitor & Recap Reference

Board revisions, electrolytic capacitor lists, voltage/farad ratings, and Through-Hole servicing notes.

TI-99/4A — Printable Board Recap & Capacitor BOM ReferenceFormat this system's capacitor replacement list, diagnostic flowchart, and IC pinouts into a high-contrast bench PDF.

Texas Instruments TI-99/4A — TI-99/4A Mainboard & Internal LPS-2 Power Supply Board

16-Bit TMS9900 + TMS9918A VDP + triple-rail LPS-2 regulator • Mount: Through-hole • Difficulty: Beginner friendly

Full Printable Recap Sheet →

Internal Power Supply Regulator PCB (LPS-2) (6 Capacitors)

Ref	Capacitance	Voltage	Package / Type
`C1`	4700 uF	16 V	Axial / Radial
`C2`	1000 uF	35 V	Axial / Radial
`C3`	220 uF	25 V	Radial
`C4`	100 uF	16 V	Radial
`C5`	47 uF	25 V	Radial
`C6`	10 uF	25 V	Radial

TI-99/4A Main Logic Board (4 Capacitors)

Ref	Capacitance	Voltage	Package / Type
`C101`	100 uF	16 V	Axial
`C102`	22 uF	25 V	Axial
`C103`	10 uF	25 V	Axial
`C205`	10 uF	25 V	Radial

Printable Diagnostic Flowchart & Voltage CheckpointsFormat this step-by-step fault isolation tree into a high-contrast bench reference sheet.

TI-99/4A — Interactive Diagnostic Flowchart & Voltage Checkpoints

Enable JavaScript to run the client-side interactive fault-isolation flowchart engine.

Complete Diagnostic Flowchart: Capacitive Load, ESR & Power Rail Ripple Diagnostic Tree (Universal Retro Mainboards (C64, Amiga, ZX Spectrum, Game Gear, Consoles))

STEP 1: Measure 100Hz/120Hz Peak-to-Peak AC Ripple on the +5V / +12V Bulk Filter Cap [node-1]

Set your oscilloscope to AC Coupling, 200 mV/div, and 2.5 ms/div (20 MHz bandwidth limit ON). Probe directly across the main bulk filter capacitor (e.g. C19 2200µF / C88 1000µF on C64, or C303/C304 on Amiga) under full system load. Measure the peak-to-peak sawtooth ripple amplitude (mVpp).

Expected Reading: < 50 mVpp clean regulated rail (post-regulator) or < 350 mVpp on raw unregulated bulk DC before the 7805/7812.

STEP 2: Check Linear Regulator (7805 / 7812) Input-to-Output Dropout Margin [node-2]

Switch oscilloscope or multimeter to DC Volts and measure Pin 1 (Input) of the 7805 (+5V) or 7812 (+12V) regulator at the bottom of the ripple trough. Verify that Input Voltage remains at least 2.0V above the regulated output (e.g. ≥7.2V into a 7805, ≥14.2V into a 7812).

Expected Reading: 7805 Input ≥ 7.2V DC; 7812 Input ≥ 14.2V DC. Below this dropout threshold, ripple punches straight through to the logic rail.

STEP 3: Check Local IC Decoupling & Audio/Video AC Coupling Capacitors [node-3]

Probe audio output coupling caps (e.g. C91 100µF on C64 SID Pin 27, or C324/C334 22µF SMD on Amiga Paula) — compare upstream vs. downstream signal amplitude. Inspect SMD electrolytic pads (Amiga 600/1200, Game Gear, DMG-01) under magnification for fishy odor, dull grey solder corrosion, or lifted copper traces.

Expected Reading: Full audio/video swing across the coupling capacitor with zero DC leakage on the output side and clean, shiny solder pads.

[ISOLATED FAULT: RES-BULK-CAP] Fault Isolated: High-ESR Bulk Filter Capacitor (>100 mVpp Rail Ripple) VERDICT

Excessive 100/120Hz sawtooth ripple on the DC rail confirms that the primary reservoir/filter electrolytics have dried out and lost effective capacitance.

[ISOLATED FAULT: RES-REG-DROPOUT] Fault Isolated: Linear Regulator Dropout / Bridge Rectifier Diode Open VERDICT

The pre-regulator DC rail is sagging below the minimum 2V dropout headroom required by the 7805/7812 regulator.

[ISOLATED FAULT: RES-COUPLING-CAP] Fault Isolated: Open/Leaky Audio or Video Coupling Capacitor & Trace Corrosion VERDICT

Signal attenuation, muffled audio, or DC offset hum across the coupling stage indicates dried-out or leaking coupling capacitors.

[ISOLATED FAULT: RES-RAILS-HEALTHY] Verification Passed: Power Rails, Ripple & Coupling Capacitors Within Spec VERDICT

All measured DC supply rails, AC ripple levels (<50 mVpp), and coupling stages pass bench specification. If performing a preventive recap, use the verified board BOM below.

Interactive Pinouts, Visualizers & Disassemblers

TI-99/4A — System Bench Tools & IC Pinouts

Click any chip below to open its interactive DIP/PLCC pinout diagram, pin-by-pin signal table, and oscilloscope test waveforms.

TI-99/4A — Printable IC Pinout & Chipset SummaryPrint this system's custom IC pinout summary or open any chip card below for its full DIP/PLCC pinout sheet.

DIP-20 74-Series Bus Interface

74LS245

SN74LS245N / DM74LS245

Octal bidirectional 3-state bus transceiver — one direction pin and one active-low enable, the standard bidirectional bus driver.

Open Interactive Pinout →

DIP-64 TI 9900

TI TMS9900

TMS9900NL / TMS9900JL

The first single-chip 16-bit microprocessor — three supply rails (-5V, +5V, +12V), a four-phase 12V clock, and reversed big-endian bit numbering.

Open Interactive Pinout →

DIP-16 TI SN764xx

TI SN76489

SN76489 / SN76489A / SN76496 / SN94624 / TMS9919

TI's write-only sound chip — D0 is the MOST significant bit, opposite to every other chip in this library. Read the data bus warning.

Open Interactive Pinout →

DIP-40 TI VDP

TI TMS9918A

TMS9918A / TMS9928A / TMS9929A / TMS9118

The VDP behind the MSX, ColecoVision and TI-99/4A — every bus is numbered backwards: CD0, AD0 and RD0 are the MOST significant bits.

Open Interactive Pinout →

DIP-28 TI Speech

TI TMS5220 / TMC0285 Speech

TMS5220NL / TMS5220CNL / TMC0285 / TMS5200

Texas Instruments' Linear Predictive Coding (LPC-10) vocal tract synthesizer — used in the TI-99/4A Speech Synthesizer, Acorn BBC Micro, Apple II Echo II and arcade classics.

Open Interactive Pinout →

DIP-16 Dynamic RAM

4116 16K x 1 DRAM

4116 / MK4116 / TMS4116 / MCM4116 / uPD416

16K x 1 DRAM with FOUR supply pins and THREE rails. A 4164 fitted here receives +12 V on its 5 V pin and will not survive.

Open Interactive Pinout →

Visualizer

Signal & Trace Visualizer (Scope)

Compare healthy vs. faulty CPU clocks, reset ramps, PLA strobes, and 5V PSU ripple waveforms.

Disassembler & ROM Lab

Retro File Utilities (Rust/Wasm)

6502/6510 disassembler with IC register links, C64 .D64 parser, and EPROM CRC32 verifier/patcher.

Logic & Pinouts

Connector & Port Pinout Reference

59 interactive RGB SCART, DIN A/V, PSU polarity, joystick, floppy, and expansion bus pinouts.

Logic & Pinouts

Logic & RAM Substitution Guide

Cross-reference 4116→4164 DRAM mods, 2364→27C256 EPROM adapters, and 74LS/74HCT swaps.

Workshop Rescue & Community Rehoming Free UK Shipping Never Resold

Dead Texas Instruments TI-99/4A on the Bench — or Clearing Out Spare Kit?

Worked through the fault checklist and found a dead custom IC, severe trace rot, or a repair you don't have the gear to tackle? Don't scrap your Texas Instruments TI-99/4A — donate faulty machines, donor boards, joysticks, cables, or peripherals to Smiggy's Workshop. We cover UK shipping and never resell a single item.

Donate Faulty or Unwanted Tech →

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Where Refurbished Donations Go — Free to the Community

Donated joysticks, controllers, cables, peripherals, and restored computers/consoles are tested on Smiggy's bench and rehomed completely free of charge (including UK postage) to enthusiasts, makers, and STEM projects.

Browse Rehoming Bench → Donate → Refurbish → Rehome Free

Texas Instruments TI-99/4A Machine Hub

Dual-AC External Transformer & Internal Triple-Rail (+5V / +12V / −5V) Alert!

1 Rear 4-Pin AC Power Socket Pinout

2 Internal PSU Board 4-Wire Harness

3 Cartridge Slot Felt Dust-Trap Warning!

System Architecture & Model Selector

Original Silver & Black Brushed-Aluminum TI-99/4A 1981–1983 • Full Cartridge Compatibility

Key ICs on the Standard Motherboard

Silver/Black vs Early Beige (Non-QI) Identification

Beige "QI" (Quality Improved) v2.2 Console Late 1983 • Custom Gate Array • ROM Lockout

How to Unlock a v2.2 Beige QI Console for All Cartridges

Other Hardware Differences on the QI Board

Why the 16-Bit TMS9900 Has Only 256 Bytes of Direct CPU RAM!

Peripheral Expansion Box (PEB), Speech Synthesizer & Modern Upgrades

1 Solid State Speech Synthesizer (PHP1500)

2 Peripheral Expansion Box (PEB & "Firehose" Cable)

3 Essential Modern TI-99/4A Upgrades

Diagnostic Checklist, 4116 VRAM Locator & Keyboard Repair

8× 4116 Video RAM Chip Locator & TI's Backwards Bit Numbering (RD0 = MSB!)

TI-99/4A 8× 4116 VRAM Bit-to-IC Map (Bottom-Left of PCB)

"Black Screen + Continuous Screaming Beep" Decision Tree

TI-99/4A Keyboard Variants & Mitsumi Membrane Fix

TMS9901 PSI (Dead Keys/Joysticks) & Stuck GROM READY

5-Pin NTSC vs. 6-Pin PAL Component Video, Dual Joystick & Tape Pinouts

5-Pin DIN (NTSC US) vs. 6-Pin DIN (PAL Europe YPbPr!) Video Port

Non-Standard Single DE-9 Dual-Joystick Port & Atari DB9 Adapter

Rear 9-Pin DE-9 Female Cassette Port

Chip & GROM / Console ROM Reference

TI TMS9900 16-Bit CPU

TI TMS9918A / 9929A VDP

TI TMS5220 / TMC0285 Speech

TMS9919 / SN94624 (SN76489)

TMS4116 DRAM (×8)

Texas Instruments 9900-Family Silicon Reference

Console ROMs (U610/U611) & Single-Rail 4164 VRAM Mod

Resources & Further Reading

Smiggy's Retro Computing Posts

Bench Tools & Machine Hubs

External TI-99/4A Resources

Texas Instruments TI-99/4A — Unified Workbench Reference

TI-99/4A — Capacitor & Recap Reference

Texas Instruments TI-99/4A — TI-99/4A Mainboard & Internal LPS-2 Power Supply Board

Internal Power Supply Regulator PCB (LPS-2) (6 Capacitors)

TI-99/4A Main Logic Board (4 Capacitors)

Complete Diagnostic Flowchart: Capacitive Load, ESR & Power Rail Ripple Diagnostic Tree (Universal Retro Mainboards (C64, Amiga, ZX Spectrum, Game Gear, Consoles))

TI-99/4A — System Bench Tools & IC Pinouts

74LS245

TI TMS9900

TI SN76489

TI TMS9918A

TI TMS5220 / TMC0285 Speech

4116 16K x 1 DRAM

Signal & Trace Visualizer (Scope)

Retro File Utilities (Rust/Wasm)

Connector & Port Pinout Reference

Logic & RAM Substitution Guide

Theme Palette

Audio & Display

System Info

Dual-AC External Transformer & Internal Triple-Rail (`+5V / +12V / −5V`) Alert!

Beige "QI" (Quality Improved) `v2.2` Console Late 1983 • Custom Gate Array • ROM Lockout

How to Unlock a `v2.2` Beige QI Console for All Cartridges

Why the 16-Bit `TMS9900` Has Only 256 Bytes of Direct CPU RAM!

1 Solid State Speech Synthesizer (`PHP1500`)

2 Peripheral Expansion Box (`PEB` & "Firehose" Cable)

8× `4116` Video RAM Chip Locator & TI's Backwards Bit Numbering (`RD0 = MSB`!)

TI-99/4A 8× `4116` VRAM Bit-to-IC Map (Bottom-Left of PCB)

`TMS9901` PSI (Dead Keys/Joysticks) & Stuck GROM `READY`

5-Pin DIN (NTSC US) vs. 6-Pin DIN (PAL Europe `YPbPr`!) Video Port

Non-Standard Single `DE-9` Dual-Joystick Port & Atari DB9 Adapter

Rear 9-Pin `DE-9` Female Cassette Port

TMS9919 / SN94624 (`SN76489`)

Console ROMs (`U610`/`U611`) & Single-Rail `4164` VRAM Mod