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pinout (распиновка)

C64 RS232 User Port connector

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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.

UNKNOWN connector  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

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