System 80 - EPROM in the spare socket not working

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System 80 - EPROM in the spare socket not working

Postby SirMorris on Sat May 21, 2016 10:44 pm

Hi all,

My system 80 (Black label, early model) Is getting some upgrades. First off I have fitted a socket for the fourth ROM position - Z13. To test this I hacked some code together which puts a few characters on screen, blew a 4K (2732) EPROM and popped it in. The machine is fine with no EPROM in the socket but crashes on power up with the EPROM fitted. The EPROM verifies and otherwise checks out electrically.

I read in a technical document that the socket is compatible with standard 27 series EPROMS, and human nature being what it is I have not bothered to question this.

Is there piece of information missing? Some systems require their utility ROMS to have a special header, for instance.

Any help gratefully received.

C
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Re: System 80 - EPROM in the spare socket not working

Postby SirMorris on Sat May 21, 2016 10:46 pm

I have just glanced over at the processor board layout diagram and the spare socket is labelled 2716, a 2K device.

:oops: :oops: :oops: :oops: :oops: :oops: :oops: :oops: :oops:

I'll lift an address pin (I don't have any 2k EPROMS) and see if this helps.
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Re: System 80 - EPROM in the spare socket not working

Postby SirMorris on Sat May 21, 2016 11:21 pm

Despite the obvious stupidity it hasn't helped. Looking at the solder side of the board I see that pins 20 & 21 are shorted. This isn't right according to a 2716 pinout.
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Re: System 80 - EPROM in the spare socket not working

Postby Audronic on Sun May 22, 2016 11:38 am

@SirMorris

20 /Output enable
21 Vpp

Shorted together is correct as per Circuit

Ray
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Re: System 80 - EPROM in the spare socket not working

Postby tezza on Sun May 22, 2016 3:49 pm

Yes, that 4th optional ROM is a standard 2716. I've added a similar EPROM in mine and it's straightforward. No track cutting/changes should be needed.
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Re: System 80 - EPROM in the spare socket not working

Postby SirMorris on Tue May 24, 2016 6:39 am

Thanks guys. I wonder if there is a problem elsewhere with this system. No amount of fiddling is allowing the machine to boot when the EPROM is present :(
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Re: System 80 - EPROM in the spare socket not working

Postby tezza on Tue May 24, 2016 9:13 am

I'm assuming it boots when the EPROM isn't present? If that's the case, I wonder if it's a short in the EPROM somewhere. Have you tried a different one?
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Re: System 80 - EPROM in the spare socket not working

Postby Audronic on Tue May 24, 2016 12:58 pm

@SirMorris

Are you putting the Eprom in the correct way ?

Recheck that the Eprom has been programmed .

What brand Eprom , can you tell me what is written on the Eprom all of the info please

Good luck Ray
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Re: System 80 - EPROM in the spare socket not working

Postby SirMorris on Wed May 25, 2016 3:20 am

The machine is fine without the EPROM. It's a modified 2732, an AMD device. I've lifted pin 21 (A11) and tied it to ground.

It verifies fine in my programmer, but I realise that doesn't eliminate the possibility of malfunctions. I will try another device later tonight in case.

The eprom is inserted with notch facing west, same as the ROMs.

My machine is an early version 1 Genie, if that makes any difference.

The schematic in the technical manual isn't much help, Z13 is missing from the diagram (or hiding!) The decoding logic table indicates pin 5 of Z22 is the enable, I'll trace that out later too.

Thanks for taking the time to help!
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Re: System 80 - EPROM in the spare socket not working

Postby SirMorris on Sun May 29, 2016 11:47 pm

So after a second look it's apparent that the EPROM pinout for the '16 and '32 are roughly compatible, the active low select signal from pin 5 of Z22 joins pins 20 & 21, resulting in A11 being driven low along with the !CE pin. After fitting the missing pull-up R15 the machine boots with the EPROM in place woohoo!

The EPROM contains a tiny routine which preserves registers then fills the screen with the letter A. Registers are restored and a return executed.

I tried executing the code:

SYSTEM
*? /12288

but the machine hung. PEEKing address 12288 gives 255. As does peeking the remaining 2048 addresses :cry: I removed the chip and once again verified it in a programmer. I also replaced the chip with a different brand - a Fujitsu this time - with the same symptoms.

I've looked at the chip select for the EPROM and it pulses as expected if I execute a BASIC loop to PEEK from it, so I think that part of the equation is sound. I'll stick the tickler on there this afternoon and see what it reveals.
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Re: System 80 - EPROM in the spare socket not working

Postby Audronic on Mon May 30, 2016 11:39 am

@SirMorris

can you get a 2716 , Program it with a known code, and test that to confirm that the machine is capable of working ?
Ebay has them for about $1.75 NZ from china.

A late addition :- This is an example of what i had to do with an unknown 2716 Eprom i had, which a standard eprom Programmer/ reader could not read.
Andrew Quinn suggested i use this method (Thanks Andrew).

ROM4S80_CIM
Is a dump of the Z13 Rom as installed on my System-80 Educator Model.
Under Newdos you would use the DUMP Command :-
DUMP ROM4S80/CIM :0 3000H, 37FFH, FFFFH
This will give you a dump of the Rom Plus the entry address of the dump.
at the beginning of the file.
These 2 bytes have to be removed to dump it back to an Eprom.
I use a Apple Mac and use a program called " HexEdit " which has the ability to do a destructive backspace to remove the 2 bytes.
ROM4S80F.CIM Is the fixed version ready for Burning to a 2716 Eprom
This has a CheckSum of E240h (on G540 Eprom programmer)
Good luck
Ray
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Re: System 80 - EPROM in the spare socket not working

Postby SirMorris on Sat Jun 11, 2016 3:53 am

The EPROM socket Z13 requires some re-wiring on early model 1 motherboards. I've compared schematics from issue 1 (black label/no vu meter) to blue label issue boards and it's apparent where the problem lies. The bus buffers are not being enabled for read accesses to the address range served by the EPROM. It can be made to work though, so I'll persist at my leisure. Right now however I have other neat things to distract me..
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