Pinout diagram
Summary specifications
Full pin reference
| Pin | Name | Group | Direction | Levels | Description |
|---|---|---|---|---|---|
| 1 | C1+ | Analog | Bidirectional | Charge pump cap | Charge pump capacitor 1 positive. Connect to C1- (pin 3) through a capacitor. No signal — six of the sixteen pins are charge pump. |
| 2 | VS+ | Power | Output | +10 V (pump output) | Charge pump positive output, roughly +10 V. OUTPUT, for a storage capacitor only. TI's datasheet says so explicitly — do not try to power anything else from it; the pump has no useful current capability and loading it collapses the RS-232 levels. |
| 3 | C1- | Analog | Bidirectional | Charge pump cap | Charge pump capacitor 1 negative. Connect to C1+ (pin 1) through a capacitor. |
| 4 | C2+ | Analog | Bidirectional | Charge pump cap | Charge pump capacitor 2 positive. Connect to C2- (pin 5) through a capacitor. |
| 5 | C2- | Analog | Bidirectional | Charge pump cap | Charge pump capacitor 2 negative. Connect to C2+ (pin 4) through a capacitor. |
| 6 | VS- | Power | Output | -10 V (pump output) | Charge pump negative output, roughly -10 V. OUTPUT, for a storage capacitor only. Same warning as VS+: do not load it. |
| 7 | T2OUT | I/O | Output | RS-232 level | Transmitter 2 output. RS-232 side — connect to the DB9 connector. The two line pins are split either side of the logic pins. |
| 8 | R2IN | I/O | Input | RS-232 level (up to +/-30 V) | Receiver 2 input. RS-232 side. Receiver inputs have ~5 kohm to ground and tolerate +/-30 V, so an unconnected RX input sits at a defined level rather than floating. |
| 9 | R2OUT | I/O | Output | TTL level | Receiver 2 output. Logic side — connect to the UART. The four logic pins (T1IN, R1OUT, T2IN, R2OUT) are together in the middle on pins 9-12. |
| 10 | T2IN | I/O | Input | TTL level | Transmitter 2 input. Logic side — connect to the UART. |
| 11 | T1IN | I/O | Input | TTL level | Transmitter 1 input. Logic side. The drivers invert: a TTL low in gives a positive RS-232 voltage out, and vice versa, since RS-232 marks are negative. |
| 12 | R1OUT | I/O | Output | TTL level | Receiver 1 output. Logic side. The receivers invert back, so a TTL-to-TTL loop through two MAX232s comes out the right way round. |
| 13 | R1IN | I/O | Input | RS-232 level (up to +/-30 V) | Receiver 1 input. RS-232 side. Tolerates +/-30 V; ~5 kohm internal pull to ground. |
| 14 | T1OUT | I/O | Output | RS-232 level | Transmitter 1 output. RS-232 side — connect to the DB9 connector. |
| 15 | GND | Ground | Power | 0 V | Ground. |
| 16 | VCC | Power | Power | +5 V | Positive supply. Single +5 V rail. Decouple with 100 nF close to the pin. |
Common uses and repair diagnostics
The MAX232 is a dual RS-232 driver/receiver that generates roughly +10 V and -10 V internally from a single +5 V rail, so it can drive true RS-232 levels without external supplies. Six of the sixteen pins carry no signal at all: pins 1 to 6 are the charge pump. C1+/C1- and C2+/C2- are capacitor connections, and VS+ (2) and VS- (6) are the pump's own outputs — for a storage capacitor only, not to power anything else. That is the entire reason this part exists and it is what the five external capacitors are for.
The MAX232 is a dual RS-232 driver/receiver that generates roughly +10 V and -10 V internally from a single +5 V rail, so it can drive true RS-232 levels without external supplies. Used for adding a serial console or SD/WiFi adapter to almost any retro machine, and as the modern replacement for the period-correct 1488/1489 driver/receiver pair. Cross-link the 6850 ACIA page, which needs exactly this to reach real RS-232 levels.
Six of the sixteen pins carry no signal at all. Pins 1 to 6 are the charge pump: C1+/C1- and C2+/C2- are capacitor connections, and VS+ (2) and VS- (6) are the pump's own outputs. VS+ and VS- are OUTPUTS, for a storage capacitor only. TI's datasheet says so explicitly — do not try to power anything else from them; the pump has no useful current capability and loading it collapses the RS-232 levels. That is the entire reason this part exists and it is what the five external capacitors are for.
Capacitor value depends on the suffix. The plain MAX232 wants 1 uF capacitors; the MAX232A wants 0.1 uF. Fitting 0.1 uF to a plain MAX232 gives marginal output swing that works on a short cable and fails on a long one — a genuinely common and very confusing fault. Fitting 1 uF to a MAX232A is harmless.
The two channels are interleaved, not grouped. Logic side (connect to the UART): T1IN (11), R1OUT (12), T2IN (10), R2OUT (9) — pins 9 to 12. RS-232 side (connect to the connector): T1OUT (14), R1IN (13), T2OUT (7), R2IN (8). So all four logic pins are together in the middle of one side, and the line pins are split either side of them. The drivers invert: a TTL low in gives a positive RS-232 voltage out, and vice versa, since RS-232 marks are negative. The receivers invert back, so a TTL-to-TTL loop through two MAX232s comes out the right way round.
Two drivers and two receivers, so one chip handles TX, RX and one handshake pair (typically RTS/CTS). A full handshake needs a second chip or a MAX238. Receiver inputs have ~5 kohm to ground and tolerate +/-30 V, so an unconnected RX input sits at a defined level rather than floating.
- With power applied and nothing else connected, measure VS+ (pin 2) and VS- (pin 6) — roughly +10 V and -10 V confirms the charge pump and therefore the chip.
- If VS+ or VS- are low or absent, suspect the capacitors (wrong value, wrong polarity, or dried out) before the chip.
- Electrolytics fitted backwards on the pump are the single most common fault.
- Loop T1IN to R1IN externally and confirm data passes — a TTL-to-TTL loopback test.
- Fitting 0.1 uF capacitors on a plain MAX232 (not a MAX232A) gives marginal swing that fails on long cables. Check the suffix and the cap values.