Jupiter ACE (again)

General Chit Chat about Sinclair Computers and their Clones
McKlaud
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Re: Jupiter ACE (again)

Post by McKlaud »

Hi Andy,

It is a very good idea :) I've got a silly question. All buffered lines have pull-ups. Do you want to keep them HIGH when they are nor used? In my understanding they should be LOW if they are not used/active.
Claudius
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1024MAK
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Re: Jupiter ACE (again)

Post by 1024MAK »

For the data bus, the normal practice is to pull it up to VCC rather than leave it floating (which will be case should the Z80 try to read any unimplemented memory or IO address).

As I wrote on StarDot...
I wrote:There’s no need to have pull-up resistors on the buffered address lines, as the 74xx244 buffers will always be driving the buffered address lines.

I presume that there is no intention to disconnect the Z80 from the busses via /BUSREQ.

There is one thing that is missing, and it’s rather important if you are going to allow the Z80 to read data from devices (memory or IO) from the external bus. There needs to be some circuitry to prevent the 74xx245 bidirectional buffer from driving the data bus on the Jupiter Ace board side when the Z80 is accessing any of it’s on board memory or built-in IO ports. Otherwise the on-board memory/IO chip will compete with the 74xx245 and cause a bus clash :twisted:

If you have room/space, it may be worthwhile buffering the following control lines: /WR, /RD, /MREQ, /IOREQ as its always good to have nice clean control signals with reasonably fast edges.
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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anightin
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Re: Jupiter ACE (again)

Post by anightin »

1024MAK wrote: Tue Sep 29, 2020 1:22 pm For the data bus, the normal practice is to pull it up to VCC rather than leave it floating (which will be case should the Z80 try to read any unimplemented memory or IO address).

As I wrote on StarDot...
I wrote:There’s no need to have pull-up resistors on the buffered address lines, as the 74xx244 buffers will always be driving the buffered address lines.

I presume that there is no intention to disconnect the Z80 from the busses via /BUSREQ.

There is one thing that is missing, and it’s rather important if you are going to allow the Z80 to read data from devices (memory or IO) from the external bus. There needs to be some circuitry to prevent the 74xx245 bidirectional buffer from driving the data bus on the Jupiter Ace board side when the Z80 is accessing any of it’s on board memory or built-in IO ports. Otherwise the on-board memory/IO chip will compete with the 74xx245 and cause a bus clash :twisted:

If you have room/space, it may be worthwhile buffering the following control lines: /WR, /RD, /MREQ, /IOREQ as its always good to have nice clean control signals with reasonably fast edges.
Mark
Thanks Mark, I have done some partial updates and re-attached the file to my original post.
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1024MAK
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Re: Jupiter ACE (again)

Post by 1024MAK »

For those following along at home (or at work!), see also this topic ;)

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer 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 later in the year.
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