Four Wire Connection - IBM SuperSerial SST128P Hardware Installation Manual

Three Wire
For terminals and printers using XON/XOFF flow control.
Connection
Figure 5-3. Pinouts for a Terminal or Printer Cable
Four Wire
For terminals and printers using pin 20 hardware flow control.
Connection
Figure 5-4. Cable for a Terminal or Printer Using DTR Flow Control
Serial I/O Port to Device Cabling 5-3