Pinout diagram
Summary specifications
Full pin reference
| Pin | Name | Group | Direction | Levels | Description |
|---|---|---|---|---|---|
| 1 | A0 | Address Bus | Input | TTL, 0 V / 5 V | Address input, bit 0 (LSB). Selects which of Y0-Y7 is active. |
| 2 | A1 | Address Bus | Input | TTL, 0 V / 5 V | Address input, bit 1. |
| 3 | A2 | Address Bus | Input | TTL, 0 V / 5 V | Address input, bit 2 (MSB). |
| 4 | /E1 | Control | Input | TTL, 0 V / 5 V, active low | Enable input, active low. The decoder is enabled only when /E1, /E2 are both low and E3 is high; otherwise all outputs are high. TI marks this pin G2A on some datasheets; NXP and others mark it E1. |
| 5 | /E2 | Control | Input | TTL, 0 V / 5 V, active low | Enable input, active low. Same gating as /E1. TI marks this pin G2B on some datasheets; NXP and others mark it E2. |
| 6 | E3 | Control | Input | TTL, 0 V / 5 V, active high | Enable input, active high. The decoder is enabled only when E3 is high and /E1, /E2 are both low. TI marks this pin G1 on some datasheets; NXP and others mark it E3. |
| 7 | /Y7 | Control | Output | TTL totem-pole, active low | Active low. Decoded output 7. Low only when enabled and A2 A1 A0 = 111; high otherwise. |
| 8 | GND | Ground | Power | 0 V | Ground return. |
| 9 | /Y6 | Control | Output | TTL totem-pole, active low | Active low. Decoded output 6. Low only when enabled and A2 A1 A0 = 110. |
| 10 | /Y5 | Control | Output | TTL totem-pole, active low | Active low. Decoded output 5. Low only when enabled and A2 A1 A0 = 101. |
| 11 | /Y4 | Control | Output | TTL totem-pole, active low | Active low. Decoded output 4. Low only when enabled and A2 A1 A0 = 100. |
| 12 | /Y3 | Control | Output | TTL totem-pole, active low | Active low. Decoded output 3. Low only when enabled and A2 A1 A0 = 011. |
| 13 | /Y2 | Control | Output | TTL totem-pole, active low | Active low. Decoded output 2. Low only when enabled and A2 A1 A0 = 010. |
| 14 | /Y1 | Control | Output | TTL totem-pole, active low | Active low. Decoded output 1. Low only when enabled and A2 A1 A0 = 001. |
| 15 | /Y0 | Control | Output | TTL totem-pole, active low | Active low. Decoded output 0. Low only when enabled and A2 A1 A0 = 000. |
| 16 | VCC | Power | Power | +5 V | Positive supply. Decouple with 100 nF close to the pin. |
Common uses and repair diagnostics
The 74LS138 is the 3-to-8 line decoder that turns up on almost every memory board of the era. Three address lines select one of eight active-low outputs; three enable inputs (two active low, one active high) gate the whole device, so several 74LS138s can be paralleled to decode a larger address space without extra glue. It is the part you reach for when a board needs to chip-select one of eight ROM, RAM or I/O blocks from a three-bit address field, and the active-low outputs mate directly with the active-low chip-selects of most memory and peripheral chips.
The 74LS138 is the memory decoder of choice on 8-bit and 16-bit boards. Three address lines select one of eight chip-selects, and the three enable inputs let several 74LS138s be ganged to decode a larger address field without any extra gates — a 5-to-32 decoder takes four 74LS138s and one inverter. The active-low outputs mate directly with the active-low chip-selects of most ROM, RAM and peripheral chips, which is why the part is so common on memory maps. It also works as a demultiplexer: feed the data into one of the active-low enables and use the address lines to route it to one of the eight outputs. The 74HC138 and 74HCT138 are the CMOS equivalents and share the same pinout.
- With power off, probe pin 16 (VCC) to pin 8 (GND) for a low-ohms supply rail; a dead short or open points to a decoupling capacitor or the chip itself.
- Power up and check pin 16 sits at 4.75 to 5.25 V. A sagging rail can make the decoder assert the wrong output, which on a memory board looks like a bad chip.
- Confirm the enable inputs are in the active state: E1 and E2 low, E3 high. If all eight outputs sit high, the enables are usually the reason, not the address lines.
- Drive A0-A2 through each of the eight combinations and check that exactly one output goes low each time. Two outputs low means a shorted output; none means a dead decoder.
- If the board intermittently selects the wrong device, scope the enable inputs — a slow edge on E3 can let the decoder respond to a stale address for a few nanoseconds.