A parallel interface for PET, VIC-20 and C64/C128

From: Marko Mäkelä (msmakela_at_cc.hut.fi)
Date: 1999-07-29 13:42:50

An old message from Frank Kontros got buried in my mailbox a long time
ago, when I asked about my 8032-SK on the list.

> Would you like to try port my interface to PET? I looked at the schematic
> and hopefully enough lines free for use.

The prlink cable uses the user-port contacts A through M, that is, eight
data lines and two handshake lines (FLAG/PA2 on the C64, CB1/CB2 on the
VIC-20 and CA1/CB2 on the PET).  On the PET, CA2 (which would allow
hardware handshaking) goes to pin 11, but it is also called GRAPHIC, which
is probably meant for reading the light pen.  Even worse, pin 11 is 9
volts AC on the VIC-20 and C64.  How can I make the same cable work on all
three types of computers?  Using just a diode won't work.  Your cable uses
PC and FLAG for handshake, right?  PC2 is on pin 8, which is cassette
switch on the VIC-20 and cassette #1 read on the PET.  So, I think the
interface you sent me and I patched would work on the PET, except that it
won't take advantage of hardware handshaking, unless I modify the software
so that it will write also to port B when sending data from the PET.  
Hmm, good idea!

What are the differences between your new EasyPort interface and the one
you sent to me?  Is the handshaking circuit the same?

> Right. The +5V is really lack. But probably some open-drain 6522 pins
> enough for driving one little 74HCT74 chip.

On the C64 and VIC-20, pin 2 is +5V.  On the PET, pin 2 is video out.  
Not a very good power source. :-( Could I maybe use two diodes with the
"arrowheads" connected to the chip's +5V input and the other ends
connected to pins 2 and 5 (DIAG, goes to a 6520's pin PA7)?  Pin 5 is a
joystick input on the VIC-20 and SP1 on the C64.  Is just one PIA output
(with the voltage drop caused by the diode) enough for driving the


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