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

From: Frank Kontros (
Date: 1999-08-03 12:17:18

Hello Marko,

Thursday, 29 July 99, you wrote:

Marko> The prlink cable uses the user-port contacts A through M, that is, eight
Marko> data lines and two handshake lines (FLAG/PA2 on the C64, CB1/CB2 on the
Marko> VIC-20 and CA1/CB2 on the PET).  On the PET, CA2 (which would allow
Marko> hardware handshaking) goes to pin 11, but it is also called GRAPHIC, which
Marko> is probably meant for reading the light pen.  Even worse, pin 11 is 9
Marko> volts AC on the VIC-20 and C64.  How can I make the same cable work on all
Marko> three types of computers?  Using just a diode won't work.

Bad thing with pinouts. Maybe two jumper would help something, but
extreme care should be taken when... Better build two or three cables,
than burn two or three computers. At least you can connect all commies
at once to one PC and serve all at once.

Marko> Your cable uses PC and FLAG for handshake, right?

Right. Hardware handshake works nice and fast.

Marko> PC2 is on pin 8, which is cassette
Marko> switch on the VIC-20 and cassette #1 read on the PET.  So, I think the
Marko> interface you sent me and I patched would work on the PET, except that it
Marko> won't take advantage of hardware handshaking, unless I modify the software
Marko> so that it will write also to port B when sending data from the PET.  
Marko> Hmm, good idea!

Good idea, and would work lot better than software generated PC2.

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

Right the handshaking circuit (74HCT74) same, but bidirectional data
transfer used (needs PS/2, EPP, ECP LPT port). So, no extra chips.
You can simply wire chip into USERPORT house without PCB.

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


Marko> On the C64 and VIC-20, pin 2 is +5V.  On the PET, pin 2 is video out.  
Marko> Not a very good power source. :-( Could I maybe use two diodes with the
Marko> "arrowheads" connected to the chip's +5V input and the other ends
Marko> connected to pins 2 and 5 (DIAG, goes to a 6520's pin PA7)?  Pin 5 is a
Marko> joystick input on the VIC-20 and SP1 on the C64.  Is just one PIA output
Marko> (with the voltage drop caused by the diode) enough for driving the
Marko> 74HCT74?

Should be enough. Maybe schottky diodes with low drop voltage would work better.

I don't have any PET configuration, so someone should check above
things. I just got a dead VIC-20, maybe later I could try my interface
with it. Thomas Hechelhammer going to try EasyPort with C65. Maybe
someone then sometime could try with Plus/4.

Would be very nice if near complete line of C='s would communicate
with the same cable (with one PC :). The software's on C='s are
different, but same (or one?) on the PC side.

Best regards,

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