*** M E M O R U M ***

Hi Mark,

it all makes sense (I think), the drivel I wrote on the web page (temporarily removed) was based on my thinking that the diodes were on the ROM card as they did not have component numbers. As you'll know, they are in fact on the MTX board, close to 5B

regards
Dave

Off topic a bit, but...

Dave, while on that web page of yours I noticed some errors with the PAL logic equations. The second line for the /CEA equation should have A13 true and not inverted. Also both the equations for /CE64 should have A13 true and not inverted. I have not checked any others, but you may want to.

Mark

Hi Mark,

thanks for that - you are correct of course - I did check it before posting it, but obviously missed that one.

To much cutting & pasting & not enough proof reading

regards
Dave

@ Martin, if a revised design is produced whereby the /ROM7 signal is generated on board, you may also want to consider adding !RE/CPM to the logic. Use the last free OR gate in U1 with one input being !RE/CPM and the other input being A15. Then route the output to U2B (74LS00) in place of the A15 shown on my sketch.

This will prevent bus contention from the EPROM/FLASH in "ROM7" mode, should the RELCPMH bit (D7) in the page port (Output Port 0) be set incorrectly to CP/M all RAM mode.

Mark

Shhhhh Don't tell Dave, who I've just sent the V4 schematic to, that I I'm looking at doing a V5 board with all these suggestions included.

Though for ROM protection would ORing the ROM chip-select with *RE/CPM not be more logical ? that way if the ram only mode is set, them the ROM can never be selected as the chip select is forced high

What's all this about a V5!

I just updated the page for V4

V5 is built and lightly tested, it seems to be working fine. It's now down to 6 chips.

It has all of Mark's suggestions implemented, plus *grom instead of the 74xx138 doing rom 7 detection.

The MTX500 I tested it on has one of Andy's Rememorizor PAL's fitted, but that shouldn't make any difference.

Software wise, running the V0.15 Rom unaltered.

Martin A wrote:V5 is built and lightly tested, it seems to be working fine. It's now down to 6 chips.

It has all of Mark's suggestions implemented, plus *grom instead of the 74xx138 doing rom 7 detection.

The MTX500 I tested it on has one of Andy's Rememorizor PAL's fitted, but that shouldn't make any difference.

Software wise, running the V0.15 Rom unaltered.

The only downside to a forum is the lack of a like button

Mark

There's an nice bonus that comes with using your mods to the circuit. I don't need to store the extra 3 shadow copies of the ROM 7 code that were required to "fix" the original prototype. That's 24k more game space!

I've currently got 38 games squeezed in, it's now just about reached the limits of one screen (and the alphabet!). If there are enough games left that could work, they'l probably have to go in a new ROM image.

I'm a bit late, but I'd buy one of these