EPROM for 4K ZX80 ROM?

Discussions about Sinclair ZX80 and ZX81 Hardware
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1024MAK
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Re: EPROM for 4K ZX80 ROM?

Post by 1024MAK »

RetroByte wrote: Mon Feb 15, 2021 2:00 pm
1024MAK wrote: Mon Feb 15, 2021 1:54 pm Do your ‘failed’ EPROMs verify in the programmer?
Yes, they read OK, all FF's
FF (0xFF or 255 decimal) is returned for blank unprogrammed memory locations.

When programmed, the programmer flips bits in memory locations from logic 1 to logic 0. So if a memory location fails to program correctly, one or more bits will be 1 instead of the required value.

Mark
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1024MAK
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Re: EPROM for 4K ZX80 ROM?

Post by 1024MAK »

Some programmers do have an external power input. Often intended for a 12V DC supply. This runs all the electronics in the programmer. And indeed before USB became popular, this was fairly common for programmers controlled by parallel printer ports.

USB programmers use a voltage boost circuit to get the 12.5V, or 21V programming voltage. But often the cheap units don’t use a good design and/or good quality components. So some struggle to maintain 21V during the whole programming cycle. And as the original application for the cheap programmers was for programming memory chips for vehicle engine control units, they were never intended to programme the long obsolete 25V EPROM chips.

It is in theory possible to built an adapter, then you could use moggy’s suggestion of three 9V batteries wired in series.

Mark
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1024MAK
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Re: EPROM for 4K ZX80 ROM?

Post by 1024MAK »

Here is a simple circuit that will detect when you programmer is trying to program an EPROM, the circuit then switches a 27V battery supply to the programming pin.
Vpp adaptor for EPROM programmers
Vpp adaptor for EPROM programmers
B1 is three 9V PP3 alkaline batteries wired in series
R1 is a 2.2k resistor
R2 is a 2.2k resistor
R3 is a 8.2k resistor
All resistors are 250mW / ¼ watt types.
C1 is a 47nF ceramic capacitor
D1 is a 8.2V or 9.1V 500mW zener diode.
D2 is a 1N5819 diode
TR1 is a 2N3904 NPN transistor
TR2 is a 2N3906 PNP transistor

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Moggy
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Re: EPROM for 4K ZX80 ROM?

Post by Moggy »

Most excellent! :ugeek:
RetroByte
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Re: EPROM for 4K ZX80 ROM?

Post by RetroByte »

1024MAK wrote: Mon Feb 15, 2021 11:35 pm Here is a simple circuit that will detect when you programmer is trying to program an EPROM, the circuit then switches a 27V battery supply to the programming pin.


Mark
Nice, thanks, but I assume because I'm only going to use the 27v for these EPROMs, that a simple switch would be OK?
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1024MAK
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Re: EPROM for 4K ZX80 ROM?

Post by 1024MAK »

A simple switch may work, but it may be tricky as the software will likely assume that the Vpp voltage switching is fully under it's control. Hence how are you going to manually follow what the software is trying to do with the correct timing?

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Re: EPROM for 4K ZX80 ROM?

Post by Moggy »

The programmer bursts in fractions of a second if memory serves and there is no way you could keep up with that manually I assume.
RetroByte
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Re: EPROM for 4K ZX80 ROM?

Post by RetroByte »

I probably should have done this sooner, but I put a meter across the Vpp pin and com pin and found that there is a very low 17ohm resistance, so I'm thinking these were either always faulty, or I've damaged them.

I've ordered some more TMT2532A's.
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