RE: EPP or ECP based data transfer

From: Marko Mäkelä (
Date: 2000-01-07 09:51:29

On Fri, 7 Jan 2000, COPLIN, Nicholas. wrote:

> There was already a commercial cart for downloading games two via either
> tape or even via the SID output of a C64.  I too have thought about building
> a simple RAM-cart for the 2600, but its limited native addressing space
> means that you'll also need latches etc to emulator the bank switching on
> carts of greater than 4kB.

Yes, I have read about the Supercharger.  But this cartridge I'm planning
would be much faster.  At first, it would only support 4k cartridges, the
full space directly addressable through the 2600 cartridge port.  (Are
there any commonly used addressing schemes for bigger memories?)  The
transfer speed would be limited by the PC.  At 1 MB/s, a cartridge image
could be transferred in a 1/256 of a second.

The hardware is very simple: one 12-bit counter (stepped by the data
strobe from the PC) and one flip-flop for generating acknowledgement
signals.  The address and data busses need to be buffered and switched.  
When the cartridge is used by the 2600, the busses of the RAM chip are
connected to the cartridge port.  When a game is being transferred, the
data bus will be connected to the parallel port and the address bus to the
counter.  Some lines will be used for resetting the counter and for
selecting the operation mode (transfer/play).  A similar cartridge could
be constructed also e.g. for the VIC-20 or the C64, but then it would be
tempting to design also a circuit for reading the memory contents.  (That
shouldn't be too difficult on the C64: stop the computer with the -DMA
signal and connect the (16-bit) counter to the cartridge port.  In this
case, it would be advisable to have a means for initialising the address
counter to the desired value first, so that one doesn't always need to
transfer all 64 kilobytes.)

Last night I had another look at the documents provided by Warp Nine
Engineering (<URL:>,
<URL:>).  It appeared to me that the 74LS74
handshake circuit designed by Frank Kontros can be directly used with EPP.
The "-write" (-strobe) line can be safely ignored; only the actual EPP
data strobe line (-autofeed) is absolutely required.

I still haven't found an easy way to power the 74LS74 in the PC<->PET
transfer cable.  Does anyone have the power specs for 74LS74, 74HCT74 and
CD4013?  Could any of them be powered through some (possibly several)  
lines on the PC's parallel port?  Does the IC survive without damage if
its Vcc is inadequately low (say, 1 volt) and some of its inputs are
higher than Vcc (5 volts)?


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