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mikew8760 · 7th Mar 2020, 10:36 am

Hi,
Thanks again for taking the time and trouble to reply. I have now discovered that the main board is OK, the problem seems to stem from 'dodgy' UV EPROMS. I have quite a collection of old devices which can be erased and re-programmed, but simply do not work. The logic analyser only sees the prom memory space as all FF's, and scoping the data lines shows them as being marginal, with low level outputs and slow(ish) rise and fall times. I had expected the problem to be with the prom CS and/or OE, but substituting a 6116, and lifting the WR pin and connecting it to the WR line works fine. I'm puzzled by this, but it seems to indicate some degradation of the proms, which is odd since they erase, program, and verify OK. I've ordered a 28C16 eeprom to replace the 2716; how much easier if flash memory devices had been available 40 years ago!!
I shall have a go at reverse engineering my somewhat bodge job of the kbd/display unit; I used an LED display from an old calculator with lots of transistors (16!) to drive it. The other problem is that I seem to have robbed one of the only two ic's and not sure what it might have been. The remaining one is a 74LS155, so perhaps the other is the same.
BTW, I have two Logic analysers, an old Paratronics beast I got as a 'dumpster' find, but it's so big and heavy I never use it now, had cost someone £20,000 in 1980, and a £10 one about the size of a matchbox, which just about does the job. Isn't technology wonderful?

7th Mar 2020, 10:36 am	#8
mikew8760 Triode Join Date: Mar 2020 Location: Bristol, UK. Posts: 18	Re: Maplin Z80 SBC Hi, Thanks again for taking the time and trouble to reply. I have now discovered that the main board is OK, the problem seems to stem from 'dodgy' UV EPROMS. I have quite a collection of old devices which can be erased and re-programmed, but simply do not work. The logic analyser only sees the prom memory space as all FF's, and scoping the data lines shows them as being marginal, with low level outputs and slow(ish) rise and fall times. I had expected the problem to be with the prom CS and/or OE, but substituting a 6116, and lifting the WR pin and connecting it to the WR line works fine. I'm puzzled by this, but it seems to indicate some degradation of the proms, which is odd since they erase, program, and verify OK. I've ordered a 28C16 eeprom to replace the 2716; how much easier if flash memory devices had been available 40 years ago!! I shall have a go at reverse engineering my somewhat bodge job of the kbd/display unit; I used an LED display from an old calculator with lots of transistors (16!) to drive it. The other problem is that I seem to have robbed one of the only two ic's and not sure what it might have been. The remaining one is a 74LS155, so perhaps the other is the same. BTW, I have two Logic analysers, an old Paratronics beast I got as a 'dumpster' find, but it's so big and heavy I never use it now, had cost someone £20,000 in 1980, and a £10 one about the size of a matchbox, which just about does the job. Isn't technology wonderful?