Loopback Diagrams

Serial and Parallel Port external loopback wiring diagrams.

Serial Port

The Serial Port external loopback testing requires a loopback plug. The connections for D25 and D9 serial ports are shown here.

Pins D25
Pins D9
RX/TX 2, 3 2, 3 Serial Port
RTS/CTS 4, 5 7, 8
DSR/DTR/CD 6, 8, 20 1, 4, 6

Parallel Port

The Parallel Port external loopback testing requires a D25 parallel port loopback plug wired as show here.

Pins D25
Stobe/Select In 1, 13 Serial Port
Data0/Error In 2, 15
Auto FD/Paper End In 12, 14
Init/Acknowledge In/CD 10, 16
Select Out/Busy In 11, 17
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