Re: VIC-20 "all games in one" cartridge update

From: john/lori (
Date: 2001-04-01 19:29:50

> The ON Semiconductors (former Motorola) 74LVX245 data sheet, page 1
> <URL:> mentions
> something about "a parasitic diode" being formed between the bus and Vcc
> terminals.  I don't know if this is a problem; aren't there some internal
> NMOS pullups on the data bus?  According to the schematic diagram
> 251027-01 sheet 1, left side, the data bus on the cartridge port is wired
> directly to the 6502 and to a 74LS245 in the VIC-20.

Are you familiar with latchup?

Parasitic SCRs that switch on and short the device under certain
(like when the protection diodes are passing enough current).

I don't know how much of a problem this might be in modern devices,
ones designed to operate in mixed voltage environments.

I note that that link you posted say "latchup performance exceeds 300ma"
not sure what that means, either that you must inject >300ma to cause
or the SCR holding current exceeds 300ma (I'm guessing)

I also note that it says "do not force a signal on an I/O pin when it is
active output, damage may occur" I suspect trying to drive a 5V line
would be included there.

I know I've read several app notes on the subject, I'll see if I can
find them,
if you're interested.  (not sure if they were on line or in an
electronic form)

> I managed to reduce the number of vias to 25 (not bad, since 19 signals
> need to be routed from one side to another).  The board is now about 53 mm
> tall.  Three boards could be fitted on a standard 160 by 100 mm circuit
> board.  By making photo masks of exactly that size, it should be possible
> to align the bottom and top sides without special equipment.  And there's
> no need to drill any holes, since all chips are SMD.  Just solder wires
> around the edge of the card.  No vias are located under any chip.
>         Marko

Is the circuit viewable somewhere?

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