PC85 Core Board on 1248 Rev I Base board?

[CheshireNoir]

Hi Folks,
Since I upgraded my Microbee (1248-4) from being a PC85(b) to a 128k DRAM Board, I have missed having the PC85, especially as it has all the additional ROM sockets that I can do evil things to.
Now I have a Personal Communicator 32k just sitting there. It's a 1248 Rev I base board. 
I know there is some compatibility between base boards, but clearly not enough for this to be a drop in replacement, as it doesn't work reliably with the PC85 core board. 

Am I attempting the impossible? Are there some mods I need to do to the Rev I board to bring it up to spec to work with the PC85 core board?

I had a good look through the Hardware Notebook for anything obvious, but no dice. (It could be in one of the other Engineering Change notices, but I have no idea where to start in those :-D )
There are mods that need to be done to the PC85 board mentioned, but they've all already been made.

If it can't be done, that's not the end of the world. I'll just need to live with my 32k Personal Communicator.

Cheers!

John

[Ernest]

Hi Folks,
Since I upgraded my Microbee (1248-4) from being a PC85(b) to a 128k DRAM Board, I have missed having the PC85, especially as it has all the additional ROM sockets that I can do evil things to.
Now I have a Personal Communicator 32k just sitting there. It's a 1248 Rev I base board. 
I know there is some compatibility between base boards, but clearly not enough for this to be a drop in replacement, as it doesn't work reliably with the PC85 core board. 

Am I attempting the impossible? Are there some mods I need to do to the Rev I board to bring it up to spec to work with the PC85 core board?

I had a good look through the Hardware Notebook for anything obvious, but no dice. (It could be in one of the other Engineering Change notices, but I have no idea where to start in those :-D )
There are mods that need to be done to the PC85 board mentioned, but they've all already been made.

If it can't be done, that's not the end of the world. I'll just need to live with my 32k Personal Communicator.

Cheers!

John

Had John,
Totally doable.what symptoms are you getting?
I assume the mainboard was functioning ok with the 32k Coreboard.
I would start with removing all The ROMs but the two Basic Roms. A & B from memory.
And see it was it will boot into Basic.
Ernest

[CheshireNoir]

Hi Ernest.
Some fiddling around and everything seems fine until I put ROM I in (IC 27, the one that contains the menu ROM)
With ROM I in, it seems... erratic? It'll sometimes boot me to the menu but no keys work. Sometimes I get the top row filled with characters.
Without ROM I, it boots fine to the Applied Technology BASIC Prompt.

This is a long shot, but could it be something to do with C29? I'm using a switchmode power supply and it still has the normal 4700uF capacitor in there.
(Not sure what impact it would have. It's just a smoothing cap)

Cheers!

John

[someone]

Have you not seen someone's posts on this forum for details to make baseboard compatible with both SRAM and DRAM coreboards via 3 jumpers?
(Be aware that some late baseboard revisions do have some extra features such as video access arbitration but that's not important for coreboard compatibility).

The elephant's that in the room is the use of different sized coreboard connectors between SRAM and DRAM coreboards.
The original Utililux socket connectors to support the larger sized pins are are no longer manufactured in 25 way configurations.
The part number now refers to a replacement connector that only accommodates the smaller sized pins.

[Ernest]

Hi John,
I had a look back on your original topic with your PC85 Coreboard "Upgrading a Microbee PC85 to a PC85b and adding extra Game ROMs" and you originally had a problem with the ROM I which disappeared. So I suspect you have a mechanical issue around there associated with ROM I.
I have not reread the whole post, but did you change the 25 way headers on the PC85 Coreboard? If so then as someone says you could have a compatibility issue with the pins causing poor contact on either of these headers.

Unless you made alterations to the MB8328 Coreboard and the other (working) MB1248-6 board to get them running, then changing from a MB1632 or a MB1732 across to the MB8328 should work fine, as both are SRAM boards.
The big difference between them is the use of !PORT A signal. The older 32K boards didn't use this and it is critical to the correct selection of the ROM PAKs. The other big difference is the power drawn by by the MB8328 is a lot higher and if there is some high resistance in the connections, this will show up more in the PC85 configuration. Are you using the same power input you were when it was working fine before? I do know you made up an adapter to drop the power down to 10V, was this after you had the system running before? If so try taking the power in back to 12V and give that a try.

Hopefully that will give you a few ideas to try out.

Ernest

[CheshireNoir]

Thanks Someone and Ernest.
Someone, clearly I have not read your post with 3 jumpers. I will endeavour to find it.
Ernest, I am using the same PSU as always. 
Someone and Ernest, I notice the main board board is using red header sockets. I might swap them out for headers that are a better match for my core board.
Ernest, The !PORT A signal. Is that the one on pin 9 of X4 on the main board? Because it seems to be connected to pin 10 on IC34. I couldn't see it anywhere else, but I haven't poured over the schematic either.

Cheers!

John

[Ernest]

Thanks Someone and Ernest.
Someone, clearly I have not read your post with 3 jumpers. I will endeavour to find it.
Ernest, I am using the same PSU as always. 
Someone and Ernest, I notice the main board board is using red header sockets. I might swap them out for headers that are a better match for my core board.
Ernest, The !PORT A signal. Is that the one on pin 9 of X4 on the main board? Because it seems to be connected to pin 10 on IC34. I couldn't see it anywhere else, but I haven't poured over the schematic either.

Cheers!

John

Yes and No.
Yes !PORT A is pin 9 of X4
No it should go to Pins 2 & 8 of IC34 on the Coreboard, but yes it comes from Pin 10 of IC34 on the Mainboard (how confusing) .This is the trigger with either read or write to latch the address for ROM Pak selection and NET ROM (Portion).
To remove one more variable, try removing TELCOM from the NET ROM location and see if the MENU comes back to normal operation.

I have had a bit of trouble with the RED headers some are fine some are a headache and best replaced.

Also Make sure that your Mainboard is fitted with a Z80 rated at 4 Mhz some of the early ones were on 2 Mhz and then modded to 3.75 by changing amoung other things, the 12 Mhz crystal to 13.5 Mhz however some CPU's and PIO's could mostly work but not always so you need a Z80A and same for the PIO.

Ernest.

[CheshireNoir]

No it should go to Pins 2 & 8 of IC34.This is the rigger with either read or write to latch the address for ROM Pak selection and NET ROM (Portion).

Hi Ernest. Are you sure of that? Later boards have it to pin 10. Pin 8 is GND. (Looking at the 1248-5-6 Schematic as a reference point)


Cheers!

John

[Ernest]

Sorry John,
You must have been watching the thread as I posted.
As soon as I posted I realised that you were referring to the Mainboard IC numbering not the Coreboard and amended my post accordingly, however you must have already read it and gone to investigate before I had time to correct it.

Not that it makes any difference in this case, but would be helpful to use the MB1248-1 schematic as that is much closer to the Rev I board you have there.

Ernest.

[CheshireNoir]

Ah yes. :-D
A bit quick off the bat there. I decided this morning to start with the edge connectors. I replaced both of them and it's working fine now. :-)

Easy fix for the win.

A related question: Can I drop the C29 4700uF capacitor on the main board to something smaller? If I change between programs, currently I need to walk away for a few minutes to let that bad boy discharge. Smoothing is good, but this is ridiculous.

John