These pinouts are for the NEC 5510/7710, 5515/7715, 5520/7720, and 5525/7725.
Apple III Serial Port Cable Printer Connector
Frame Ground ............(FG)...... 1 <-----> 1.....(FG)
Transmit Data (Tx) ..... 2 <-----> 3.....(Rx)
Receive Data (Rx) ..... 3
Request to send (RTS) .... 4 <-----> 5.....(CTS)
Clear To Send (CTS) .... 5
Data Set Ready (DSR) .... 6 <-----> 19....(SCTS)
Signal Ground ...........(SG)...... 7 <-----> 7.....(SG)
Data Carrier Detect (DCD) .... 8
Secondary Clear To Send (SCTS) ... 19
Data Terminal Ready (DTR) .... 20 <-----> 6 & 8.(DSR) & (DCD)
NOTE: Printer dip switches need to be set as shown;
SW1 1 - OPEN
2 - CLOSED
3 - OPEN
4 - OPEN
5 - CLOSED
6 - OPEN
7 - CLOSED
8 - OPEN