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

Signals

Pins D25

Pins D9

RX/TX

2, 3

2, 3

RTS/CTS

4, 5

7, 8

DSR/DTR/CD

6, 8, 20

1, 4, 6

Serial Port Loopback Diagram

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

Signals

Pins D25

Stobe/Select In

1, 13

Data0/Error In

2, 15

Auto FD/Paper End In

12, 14

Init/Acknowledge In

10, 16

Select Out/Busy In

11, 17

Parallel Port Loopback Diagram