Expansion plane project

[Littlejohn]

I would like to build an Expansion Plane for the back of the ZX81. ( As stated in another thread.)
On this "EP" there will be 5 sockets, 1 feed through, a joystick adapter, headers for an external keyboard, and a 5V regulator.
I would like to install a keyboard beeper ( I have seen the circuit for this, but just can't seem to find it at the moment.)

Is there anything that has to be on this EP that someone could not put in a socket later?

[Math123]

My suggestion is to have a HRG logic on this exansion board. This is a simple OR-logic between /RD and /REFSH making a new /RD for a memory-pack.
Most people use a 2 diode plus pullup resistor logic. But a real gate would be better.

Has no negative effect on following IO-ports (do not use /REFSH) has no negative effect on following DRAM boards. But is needed for HiRes!
There are some examples to this subject on my website.http://www.swatosch.de/zx81
Matthias

[siggi]

IMHO: bus drivers.
Example:
http://www.fischerkai.de/zxteam/treibsch.gif

Siggi

[Littlejohn]

IMHO: bus drivers.
Example:
http://www.fischerkai.de/zxteam/treibsch.gif

Siggi

Excellent suggestion, consider it done.

On the HRG mod, I am not sure how to interface it for the fact i intend on using the 64K internal MOD. And, I am not sure that putting a dedicated RAM socket on the board is worth it. ( How many people use HRG ? )

[sirmorris]

>>i intend on using the 64K internal MOD.

If there was RAM on the board or at least a design for a RAM add-on there's no need to modify your precious '81. Many people don't want to, or aren't able to.

>And, I am not sure that putting a dedicated RAM socket on the board is worth it.

I agree. In fact I'd go further. A backplane should be only that. No JS interface, nothing extra in fact apart from the drivers. The hires mod, possibly - as this means a standard rampack wouldn't need to go under the iron.

>> ( How many people use HRG ? )

How many people DON'T?

[Math123]

Hi Littlejohn,
just have a close look at Kai Fischer's schematic and how he suggests to do the direction switching of IC4 (data driver). There it is: Or logic for the refresh reads. Made with diodes.
The only reason to do this is use of HRG with external RAM.

What do you mead by 64k internal MOD?
If you intend to use a Memopak 64k or a DK'tronics 64k RAM pack be aware that these have SCHMITT-Trigger input gates for the RD and REFSH signals. They are very hard to mod with internal or external diodes. (in fact the problems are not the diodes but the pull-up resitors and the resulting timing of the low to high transient)

If you build your own 64k RAM card then its an easy job.

If you are going to integrate 64k RAM into the zx81 then you have to make some more mods on Kai's data drivers. Has to be enabled only at IO cycles. So your extension board would not be usable for connecting any RAM at all.

Matthias

[siggi]

If you are going to integrate 64k RAM into the zx81 then you have to make some more mods on Kai's data drivers. Has to be enabled only at IO cycles. So your extension board would not be usable for connecting any RAM at all.

Internal expansions do NOT conflict with the bus drivers. The data bus driver is only active, while /BUSCS is low. So any EXTERNAL hardware using the driven bus must pull /BUSCS low to read/write data from/to the Z80 at the other side of the bus drivers. This is typically done by a WIRED AND of the /CE signals on the external boards(via a GE-diode):

/CE -----|<|----------/BUSCS

Siggi

[Math123]

I hope that nobody noticed. I was wrong. To combine /REFSH and /RD needs an AND not an OR!

[siggi]

There might be some additional parts necessary, if you want to connect standard ram packs to the external bus, because those ram packs don't have the output to enable the bus driver via the /BUSCS signal. But they indicate that they are connected, by pulling /ROMCS high when they are active. So a simple transistor inverter (resistor 10K and NPN transistor) may be used to generate the /BUSCS from of the /ROMCS signal.

If you want to have a mixture(!) of internal/external ram, then /ROMCS would also be driven the internal ram. Thus it is necessary to have an "external" /ROMCS at the external bus (which also activates /BUSCS) and an "internal" /ROMCS, which does not. The "external /ROMCS" could be fed (via a driver and) a diode to the "internal /ROMCS".

Siggi

[Littlejohn]

I missed my middle of August projected date, but I am getting close.
This is the back plane that I have gotten in. It is not perfect, but I am working on the design flaws right now. Somehow between sending the Gerber files and the product I received, 18 out of 22 of the pins got connected with a via. I might be able to drill out the via and make it work.