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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 22nd Nov 2022, 7:54 am   #21
Mark1960
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Default Re: 6502 NOP generator

I think the zx80 and zx81 had resistors between the ram and the z80 so they could force NOPs into the z80 while the video circuit reads the ram. These would probably prevent any serious bus contention.
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Old 25th Nov 2022, 10:52 am   #22
Slothie
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Default Re: 6502 NOP generator

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Originally Posted by Mark1960 View Post
One thing that always worries me about the 2114 type ram on z80 or 8060 type bus is that they have a R/W control instead of separate OE and WE. Its not a good idea to drive R/W using WE from z80 or WDS from 8060 because both of these processors drive the data bus before the write enable disables the output of the 2114.
According to this PDF by the highly regarded Steve Ciarcia the trick is to gate the chip enables OE and WE with the MREQ signal (pages 117, 126)
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Old 25th Nov 2022, 4:58 pm   #23
Mark1960
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Default Re: 6502 NOP generator

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Originally Posted by Slothie View Post
According to this PDF by the highly regarded Steve Ciarcia the trick is to gate the chip enables OE and WE with the MREQ signal (pages 117, 126)
The Ciarcia design is a good starting point for a simple z80 system, but it still doesnít avoid chip enable to the 2114 before the write enable is active. The timing diagram he used shows memory request is earlier than write enable. The only mitigation I can see is that memory chips were smaller and slower so there would be delays in chip enable from the longer address control decode path and the ram would take longer to put data on the bus, though I donít think the 2114 output buffers would be so slow to turn on that they would not try and drive against the data bus buffers.

He could have disabled the data bus buffers during write but he only uses the read enable to control direction.

I wonder how many people built his design and found interupt mode 2 would not work.
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