Measurement Computing Serial488A - Manual (Page 27)

2.17
RS-232 Signal Levels Selected - Serial488A/OEM
Shorting Plug
J106
J105
RS-232
RS-422
2.8.2 Serial Signal Descriptions
The Serial488A is equipped with a standard DB-25S connector on its
rear panel and requires a standard DB-25P mating connector. The
Serial488A's connector is configured as DCE type equipment for RS-232
communications, which means the Serial488A always transmits data on
Pin 3 and receives data on Pin 2. The following list describes the RS-232
and RS-422 signals provided on the Serial488A. Note: since the
Serial488/512K does not support RS-422 communication, the pins labeled
+TxD and +RxD are not used. Any reference to RS-422 communication
does not apply to the Serial488/512K.