Computer bus ports paralleled?

[neazoi]

Hello, I have a zx80/zx81 computer from the 80's and I have found a 3-port expansion bus for it. It is similar to the ISA bus but there are not so many pins like the ISA bus. The bus accepts different expansion cards on it.

As far as I can notice all the bus ports (connectors) are connected in parallel. This means that each pin of one port is connected to the same pin on the other ports. I.e. all the ports are tapped to a common bus.

here is the schematic of the computer without the expansion bus **broken link removed**

I was wondering, can I tap more ports (connectors) to the same bus in order to be able to connect more expansion cards to it?
What is the limitation to it, i.e how many of them can I add?

 

[FvM]

Depends on the involved bus drivers. Logic family fan-out and capacitive load are causing the basic restrictions.

P.S.: I see, that the original ZX81 is exposing the Z80 bus without additional bus buffers. In this case, connecting more than 3 modules without buffering the signals may be a problem.

 

[neazoi]

Thank you very much

Maybe I could try it by making an adaptor to parallel one or two more slots to the existing bus.
But then I need to have all slots filled up with cards to try it right? Do they have to work simultaneously or just left connected there and try to access each one at a time?

 

[FvM]

To have multiple cards connected simultaneousy without conflicts, they must use different addresses (either in IO or memory space). Obviously no card must respond to an address that's assigned to a resource of the mainboard.

 

[neazoi]

Ok in that case I think there is no reason to alter the original 4-port (3 cards and one for external RAM) bus produced by a company specially for this computer.

This bus was connected directly to the mainboard using connectors. I would like to make an extension cable to have the bus about 30cm apart from the mainboard instead.

Will this extra cabling oppose problems?

 

[FvM]

Will this extra cabling oppose problems?

Possibly. I would at at least expect an increased susceptibility to electrical interferences, e.g. switching a room light, operating a mobile phone in the vincinity.

 

[neazoi]

Possibly. I would at at least expect an increased susceptibility to electrical interferences, e.g. switching a room light, operating a mobile phone in the vincinity.

Even if both boards exist inside the same metal enclosure?
This is the case with the ribbon cables in modern computers.

 

[FvM]

Even if both boards exist inside the same metal enclosure?

That sounds better. I didn't expect a metal enclosure, because ZX81 is just a plastic box. The drive strength of the unbuffered Z80 bus and respective peripheral devices is however small compared to PC IO standards. So apart from electrical interferences, there may be still a problem with capacitive load. But I don't feel able to predict the results, perhaps other edaboard members can report experiences with ZX80 hardware.

 

[neazoi]

That sounds better. I didn't expect a metal enclosure, because ZX81 is just a plastic box. The drive strength of the unbuffered Z80 bus and respective peripheral devices is however small compared to PC IO standards. So apart from electrical interferences, there may be still a problem with capacitive load. But I don't feel able to predict the results, perhaps other edaboard members can report experiences with ZX80 hardware.

I am able to make an extension cable to test it without altering the circuit.
The point is that i do not know how to judge when the system will operate ok. If it operates ok at once is this enough? or some not so obvious problems may arise later on? Sometimes a circuit may have unpredictable behaviour die to capacitances and inductances introduced by the cabling. But I think the speed of the z80 is quite low to counter on these 20cm cabling losses?
Just a thought...

 

[FvM]

The point is that i do not know how to judge when the system will operate ok.

You can run a continuous memory test on the external RAM.