C64 RS232 User Port

From HwB

Available on the Commodore C64 & C128. Software emulated. The signals does not have true RS232 levels. It's TTL level, and RXD/TXD is inverted. It's just the normal User Port, used as a RS232 port.



24 PIN MALE EDGE (DZM 12 DREH) at the computer.

Pin Name RS232 Description
A GND GND Protective Ground
B+C FLAG2+PB0 RxD Receive Data (Must be applied to both pins!)
D PB1 RTS Ready To Send
E PB2 DTR Data Terminal Ready
F PB3 RI Ring Indicator
H PB4 DCD Data Carrier Detect
K PB6 CTS Clear To Send
L PB7 DSR Data Set Ready
M PA2 TxD Transmit Data
N GND GND Signal Ground



  • Usenet posting in comp.sys.cbm - Help on modem -> c64 by Lasher Glenn
  • Commodore 64 Programmer's Reference Guide