Harlequin 128 Rev 2D strange issue with external ROM cart

[DaIceMan]

Hi all,

I recently built the Superfo Halrequin 128 kit rev 2D from Bytedelight. Everything apparently went smoothly as it booted right up into the diagrom which passed all tests; once switched to the 128 ROM the classic 128 menu screen came up as expected. As I was testing it first without the AY (not the original but the Emu version which is now supplied) and a limited current PSU for safety I thought everything was fine so I popped the AY - however I would just get a garbled coloured screen and nothing else. I deduced that the AY was problematic so I popped in an original working AY which I had but the same thing happened. I suspected some issue along the data bus or address lines but before I went on to start probing with my scope I swapped out the Z80 first thing (I have a few luckily) and it booted up properly with both AY chips! I then ran through various diag tests and even ran some games from the divMMC cart and tested it with the ZX-HD and everything worked. HOWEVER, when I tried a classic external ROM cartridge (I have one with 4 selectable ROMs using a switch) I can't get any of the 4 images to boot up (they work fine with my 48K). I know the Halrequin supplies only +5 and +9 volts (no negative rails) so any special cartridge with on board DRAM wouldn't work, but this is cartridge has only a 27C512 Winbond eeprom which works with a single +5V (in normal operation) voltage rail. Maybe I missed something and there are compatibility issues with some cartridges? Thanks for any heads up!

[DaIceMan]

Update: I found that the Harlequin 128 board was not the issue, it was this specific Diag Cart: it uses 2 transistors (actually 4, 2 on each side of the pcb) instead of the 74LS32 Quad OR and that causes issues one the Harlequin, possibly because different logic level transition from 0 to 1, while it works fine on my other Spectrums (48K, +2, +3). I assembled a simple pcb cart using a 74LS32 and had no issues using the exact same 27C512 flash used on the other board (called Diagnostic Rom Kart Rev G).

[1024MAK]

The Harlequin 128 board should be compatible with most expansions / interfaces (unless they need one of the unused voltage supplies).

However, as you appear to have found, if individual transistors are used instead of 74 series logic chips, then problems may occur.

In electronics, there are various timing requirements as well as logic levels. It may be that the circuit using transistors is borderline in some respect. So will work on some machines / boards, but not on others.

Sorry about not replying to your first post, but it did have me scratching my head.

Mark

[DaIceMan]

No worries, I was also scratching my head but I was sure something was up with this specific diag board as everything worked with the DivMMC and ZX-HD! So lesson learned, the Harlequin with all discrete logic ICs requires "fast" rise and fall times on the bus and compatible TTL logic to avoid issues.