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Serial Bus

Employed in: C64, C128, C264 series, SX64, VIC20

Nearly all external drives connect to the serial bus. There are also printers and harddisks available for it.

Connection scheme

Picture (ASCII version below)
ASCII version:

      /       \
     / 5     1 \
     |    6    |
     | 4     2 |
     \    3    /

Signal Description

(Click here for ASCII version)

Pin(s) Signal Dir Description
1/SRQINin/out Service request. This line is an input only at the C64's serial jack which connects to the /FLAG input of CIA #1. It was intended to be used by peripherals to request being served by the C64. The C128 uses this line for the fast serial transfer.
2GND- Ground (0V).
3/ATNin/out Attention. Set to low by the C64 to indicate the beginning of a serial data transfer.
4/CLKin/out Clock. Used for software handshaking.
5/DATAin/out Data! Check the warp drive!
6/RESETin/out Reset. Input at serial bus jack of peripherals, output at C64's serial bus jacks. When pulled to LOW, resets every peripheral hooked up on the serial bus. In old C64 board versions, a serial reset also resets the C64.

The following schematic shows the complete serial bus circuitry for the C64:

[picture of serial bus circuitry]

Note that the reset diode is found only in newer versions of the old board. The protection circuit consisting of (up to) 8 diodes is missing in the old boards.

Note: All pictures show the plug side.

Updated: December 12th, 1997
Created: February 4th, 1997
Status : Verified

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!