This weekends projects
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Re: This weekends projects
Mystery Solved
It was switching noise.
I hadn't added the bypass caps to the 74xx chips
Lesson learned.
Never underestimate the power of the bypass cap across Vcc & GND to clean up switching noise.
Ignore them at your own peril and frustration.
BTW:
The hint was that moving C13 to smooth the signal going into IC6 p1&19 worked.
For that to make a difference it had to be a very short blip. Like, say, noise on the line.
Positive:
I learn so much more when I make mistakes.
I now know how pretty much every piece of this works.
Cheers,
Matthew
It was switching noise.
I hadn't added the bypass caps to the 74xx chips
Lesson learned.
Never underestimate the power of the bypass cap across Vcc & GND to clean up switching noise.
Ignore them at your own peril and frustration.
BTW:
The hint was that moving C13 to smooth the signal going into IC6 p1&19 worked.
For that to make a difference it had to be a very short blip. Like, say, noise on the line.
Positive:
I learn so much more when I make mistakes.
I now know how pretty much every piece of this works.
Cheers,
Matthew
Re: This weekends projects
Another example in Micro&Robot #7 (Amstrad Centronics port):
https://www.abandonware-magazines.org/a ... &album=oui
https://www.abandonware-magazines.org/a ... &album=oui
Xavier ...on the Facebook groupe : "Zx81 France"(fr)
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Re: This weekends projects
Look at the truth tables rather than the diagrams in the datasheets.
For the 74xx240, both /G inputs need to be logic low to enable the outputs. When logic high, the outputs are in the high impedance / tri-state mode.
This design, along with rather a lot of 1980s expansion designs does not appear to take into account the fact that a standard ZX81 has active memory at ALL addresses (due to incomplete address decoding). So when the Z80 tries reading from the address that the 74LS240 is mapped to (0x4021, 16417 dec), there will be a databus conflict with the RAM memory chip. Which is not good.
In a standard ZX81, the ROM chip appears in the Z80 memory area FOUR times. And the 1k byte RAM chip appears in the Z80 memory area 32 times. If a 16k byte RAM pack is in use, or an internal 16k byte upgrade is used, then the 16k of RAM appears in the Z80 memory area TWO times. See this topic
Mark
For the 74xx240, both /G inputs need to be logic low to enable the outputs. When logic high, the outputs are in the high impedance / tri-state mode.
This design, along with rather a lot of 1980s expansion designs does not appear to take into account the fact that a standard ZX81 has active memory at ALL addresses (due to incomplete address decoding). So when the Z80 tries reading from the address that the 74LS240 is mapped to (0x4021, 16417 dec), there will be a databus conflict with the RAM memory chip. Which is not good.
In a standard ZX81, the ROM chip appears in the Z80 memory area FOUR times. And the 1k byte RAM chip appears in the Z80 memory area 32 times. If a 16k byte RAM pack is in use, or an internal 16k byte upgrade is used, then the 16k of RAM appears in the Z80 memory area TWO times. See this topic
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
- 1024MAK
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- Joined: Mon Sep 26, 2011 10:56 am
- Location: Looking forward to summer in Somerset, UK...
Re: This weekends projects
So taking the points I talked about (sorry!) above into account, here is a revised design.
This version is mapped into the echo of the ROM between 0x2000 and 0x3FFF, the actual address being 0x2021 or 8225 decimal.
A transistor is included to disable the ROM chip when reading the status from the 74LS240 buffer chip. The status ("standby") signal now going to data bit 0. Note that the 74LS240 inverts the input signals. As this is now in the ROM area below the RAM area, the RAM check performed at power up by the ROM code won't affect the speech chip. I also took the opportunity to include the Z80 /MREQ signal so that it's not affected by I/O port operations. Mark
This version is mapped into the echo of the ROM between 0x2000 and 0x3FFF, the actual address being 0x2021 or 8225 decimal.
A transistor is included to disable the ROM chip when reading the status from the 74LS240 buffer chip. The status ("standby") signal now going to data bit 0. Note that the 74LS240 inverts the input signals. As this is now in the ROM area below the RAM area, the RAM check performed at power up by the ROM code won't affect the speech chip. I also took the opportunity to include the Z80 /MREQ signal so that it's not affected by I/O port operations. Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
-
- Posts: 122
- Joined: Wed Apr 08, 2020 4:48 am
- Location: San Francisco
Re: This weekends projects
You Mark =
- 1024MAK
- Posts: 5189
- Joined: Mon Sep 26, 2011 10:56 am
- Location: Looking forward to summer in Somerset, UK...
Re: This weekends projects
Well, before you cast me in bronze and stick me on a plinth, do try it first, just in case I’ve dropped the ball somewhere...
Mark
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
Re: This weekends projects
Hi,
I can't find the BUS expender in previous issus…
Is there any buffers/BUS amplifier/power supply in this ghost kit ? to explain this miscelencious data/address synchro...
I can't find the BUS expender in previous issus…
Is there any buffers/BUS amplifier/power supply in this ghost kit ? to explain this miscelencious data/address synchro...
Xavier ...on the Facebook groupe : "Zx81 France"(fr)
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- Posts: 122
- Joined: Wed Apr 08, 2020 4:48 am
- Location: San Francisco
Re: This weekends projects
I received my boards
Now I just need to solve an annoying display problem before I can try them out
And a little MtM adapter because it seemed usedful
Now I just need to solve an annoying display problem before I can try them out
And a little MtM adapter because it seemed usedful
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- Posts: 122
- Joined: Wed Apr 08, 2020 4:48 am
- Location: San Francisco
Re: This weekends projects
Process pics:
Cheers,
Matthew
Missed a GND trace but it was an easy fix
Complete
Looking forward to testing it once I get my RAM issue figured outCheers,
Matthew
- 1024MAK
- Posts: 5189
- Joined: Mon Sep 26, 2011 10:56 am
- Location: Looking forward to summer in Somerset, UK...
Re: This weekends projects
It’s not proper without at least one bodge wire...
Looking good
Mark
Looking good
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.