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User Port

Employed in: C64, C128, SX64

One of the most commonly used ports at all. Many extensions like parallel floppy speeders, eprommers, relay cards, etc. connect to the user port. Please note that the SX-64's user port seems to be slightly INCOMPATIBLE which means that certain user port devices can cause DAMAGE! See description of pin 10-11.

Connection scheme

[Picture (ASCII version below)]
ASCII version:

      1   2   3   4   5   6   7   8   9   10  11  12
  |     |                                   |         |
      A   B   C   D   E   F   H   J   K   L   M   N

Signal Description

(Click here for ASCII version)

Pin(s) Signal Dir Description
1GND -Ground (0V).
2+5V DC- Supply voltage. Max. 100mA.
3/RESETin/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.
4CNT1progr. Count1. Connected to CNT of CIA #1.
5SP1progr. Serial port 1. Serial I/O of shift register (SP) of CIA #1.
6CNT2progr. Count2. Connected to CNT of CIA #2.
7SP2progr. Serial port 2. Serial I/O of shift register (SP) of CIA #2.
8/PC2out 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.
9SER ATNout 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).
10-119 VAC- Supply voltage. Max. 100mA. WARNING! It is said that one of the pins is GROUNDED in the SX-64! This means for example that you cannot use your Commodore Vicmodem, because it will damage your SX-64! [2]
12GND- Ground (0V).
AGND - Ground (0V).
B/FLAG2 in A negative edge at this pin sets the FLAG bit in the interrupt control register of CIA #2.
C-LPB0-PB7 progr. Port register B of CIA #2. Each line is individually programmable as in- or output.
MPA2 progr. Port register A, bit 2 of CIA #2.
NGND - Ground (0V).

* The 'ø' is used as Greek 'phi' here.
progr. = programmable

Note: All pictures show the plug side.

[1] Hardware-Basteleien zum C64/C128, Der Userport, pp.270-271, ISBN 3-89090-389-4
[2] Eric Pass in his article in comp.sys.cbm.

Updated: January 16th, 1997
Created: February 10th, 1997

Site copyright © 1997 by Marc-Jano Knopp
This document is part of MJK's Commodore 64 & LCD Page
Brought back to life by Peter Schepers, Dec 10, 2005 because I really liked this site!