Signal Description of the C64 User Port --------------------------------------- Pin(s) Signal Dir Description 1 GND - Ground (0V). 2 +5V DC - Supply voltage. Max. 100mA. 3 /RESET in/out RESET. When pulling this line to LOW, the CPU resets and waits for a LOW-HIGH transistion to load the PC with the value stored in $fffc/$fffd. After power-up, timer A in the NE556 pulls this line to LOW for a short time and half a second later back to HIGH. 4 CNT1 progr. Count1. Connected to CNT of CIA #1. 5 SP1 progr. Serial port 1. Serial I/O of shift register (SP) of CIA #1. 6 CNT2 progr. Count2. Connected to CNT of CIA #2. 7 SP2 progr. Serial port 2. Serial I/O of shift register (SP) of CIA #2. 8 /PC2 out Port control 2. Connected to /PC of CIA #2. Becomes LOW for one cycle after read or write access of port register B. Open collector output, therefore needs pull-up resistor. 9 SER ATN out Serial attention. Attention line of serial bus. Can be used as output (inverted PA3 of CIA #2) and via open-collector ouput as input, too (have to verify that). 10-11 9 VAC - Supply voltage. Max. 100mA. 12 GND - Ground (0V). A GND - Ground (0V). B /FLAG2 in A negative edge at this pin sets the FLAG bit in the interrupt control register of CIA #2. C-L PB0-PB7 progr. Port register B of CIA #2. Each line is individually programmable as in- or output. M PA2 progr. Port registers A, bit 2 of CIA #2. N GND - Ground (0V). * The 'ø' is used as Greek 'phi' here. progr. = programmable Author: Marc-Jano Knopp on 97/12/11 Source: http://mjk.c64.org