Pin 2 to pin 3 -- TXD to RXD
Pin 4 to pin 5 -- RTS to CTS
Pin 8 to pin 11 -- DCD to SSD[1]
Pin 6 to pin 20 -- DSR to DTR
Loopback Test
When a serial interface is installed, the menu print-out will have at the end the group "Serial I/F". The fourth item in that group will be "Diagnostic Test No". Set that item to 'Yes' and exit menu mode. The printer will commence running the test, whether the loopback plug is in place or not.
If the loopback plug is off, the test will likely report "RAM = GOOD I/F = BAD". On plugging in the loopback, the test report should change to "RAM = GOOD I/F = GOOD" followed by a print of the ASCII character set.
Pressing SEL will stop and start the test, but as long as the test remains enabled with a serial interface installed, all the printer will do is run the test.[2]
To disable the test, switch off the printer. With forms loaded, switch on while holding SHIFT and MENU. That forces the machine to come up in Menu Mode. Get to the Serial I/F group and set the 'Diagnostic Test' item back to 'No'.
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Notes:
[1] 'SSD' stands for 'Supervisory Send Data'. SSD appears to be something proprietary to Oki; it's not part of the standard RS-232 interface specification.
[2] This characteristic of the test can result in a puzzling situation for someone unfamiliar with the test and its operation. If the serial interface is removed with the 'Diagnostic Test' left enabled, the printer will work normally as a parallel printer. Should someone later install a serial interface in it, the printer will come ready wanting to do nothing other than run the test. Always disable the test when done with it.
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