All Macintosh computers with the external 25-pin SCSI port have the pinouts listed below. Note that the Macintosh II and Macintosh SE do supply terminator power on pin 25, unlike the SCSI port on the Macintosh Plus.
| Pin | Signal name | Signal description |
| 1 | REQ/ | Request |
| 2 | MSG/ | Message |
| 3 | I/O/ | Input/Output |
| 4 | RST/ | SCSI bus reset |
| 5 | ACK/ | Acknowledge |
| 6 | BSY/ | Busy |
| 7 | GND | Signal ground |
| 8 | DB0/ | Data bit 0 |
| 9 | GND | Signal ground |
| 10 | DB3/ | Data bit 3 |
| 11 | DB5/ | Data bit 5 |
| 12 | DB6/ | Data bit 6 |
| 13 | DB7/ | Data bit 7 |
| 14 | GND | Signal ground |
| 15 | C/D/ | Common/Data |
| 16 | GND | Signal ground |
| 17 | ATN/ | Attention |
| 18 | GND | Signal ground |
| 19 | SEL/ | Select |
| 20 | DBP/ | Data parity |
| 21 | DB1/ | Data bit 1 |
| 22 | DB2/ | Data bit 2 |
| 23 | DB4/ | Data bit 4 |
| 24 | GND | Signal ground |
| 25 | TPWR | Terminator power |
IMPORTANT: Because the SCSI port is a DB-25 connector, it looks like a RS-232 port. It is NOT a RS-232 port. Do NOT plug an RS-232 device into the SCSI port. Doing so will most likely damage the SCSI chip on the logic board.