Commodore Userport


 
The Commodore userport is not a true RS232 device, but RS232 communication is implemented in software in the Kernal of the computer. The userport provides all of the lines necessary for RS232 communications, but it is all TTL-level.

Here is a breakdown of the userport pins used to emulate RS232 communications.


 
Commodore Func. Direction Modem Func. Pin Name Description Uno Mega 2560
GND   GND A GND Protective Ground GND GND
RxD <-- TxD B + C FLAG2 + PB0 Receive Data (Must be applied to both pins) 0 19
RTS --> CTS D PB1 Ready to Send 2 22
DTR --> DCD E PB2 Data Terminal Ready 7 23
RI <-- RI F PB3 Ring Indicator 4 24
DCD <-- DTR H PB4 Data Carrier Detect 5 25
CTS <-- RTS K PB6 Clear to Send 6 26
DSR Unused DSR L PB7 Data Set Ready 3 27
TxD --> RxD M PA2 Transmit Data 1 18
GRND   GRND N GRND Signal Ground GND GND
 

credit: http://www.gamesx.com/hwb/co_C64Rs232UserPort.html