Stuff inside an issue 4 Model B

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adrm
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Stuff inside an issue 4 Model B

Post by adrm » Wed Oct 24, 2018 10:45 am

Still looking at the machine with the Watford Electronics 13 ROM board fitted.

(I have removed the WE board for now, because it might cause damage when the machine is stored away.)

I have found other stuff inside, and I'm wondering if anyone has thoughts on these:

1)
The standard ROM sockets had 2x NEC D4364C-20L fitted. As far as I can tell, these are "Static CMOS RAM" devices.

Does anyone know how these are used in a Beeb?
Any software or hardware that uses them?

2)
This machine is fitted with an EPROM (type 27128) marked "NOS" in the leftmost socket.
When booting, the machine displays "SkoleData AS" (loosely translated as "SchoolData Ltd") which I vaguely remember as a company working which national schools to adapt computer solutions in the 1980s. The EPROM clearly has some Norwegian text programmed into it.
Pin 1 (Vpp) has been cut off, and pins 28(Vcc) and 27 (~Pgm) have been soldered together, likely to protect against tampering.

There was also an EPROM marked DNFS - iss.3, which is the network filing system, I assume.

On power up (not soft/hard break) the machine seems to be seeking for something, but there is no Econet fitted and the machine tends to hang until soft or hard break is hit. After that it works fine.

I'm not sure it it this "NOS" chip causing issues on power up, but I'd like to remove it completely. Problem is: the machine just "long beeps" if it's removed. I did a quick comparison with a machine that was not fitted with the NOS-chip, and jumpers seem normal.

Any ideas what has been modified for the NOS, so that I can undo it?


Edit:
There is a BASIC ROM fitted, and I also have another Acorn branded ROM from the Watford board which I assumed is the regular MOS. Marked: 00201685.
This seems to be wrong, and this ROM is probably a "HighBASIC" chip, which possibly explains why the machine won't boot if the NOS is removed.

I'm starting to suspect that this machine used to be connected to one of the Second Processors (Z80 and 6502) I got in the deal.
I'm not sure if there is a connection between a 2nd processor and a NOS, though ... ?
Last edited by adrm on Wed Oct 24, 2018 11:01 am, edited 1 time in total.
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Re: Stuff inside an issue 4 Model B

Post by tricky » Wed Oct 24, 2018 11:45 am

I don't know about the rest, but with a couple of extra signals, you can put 16K sideways ram in a rom socket.
If there is nothing obvious, it might be worth checking the underside of the board for mods.
Last edited by tricky on Wed Oct 24, 2018 11:45 am, edited 1 time in total.

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Re: Stuff inside an issue 4 Model B

Post by Kazzie » Wed Oct 24, 2018 12:04 pm

adrm wrote:
Wed Oct 24, 2018 10:45 am
2)
This machine is fitted with an EPROM (type 27128) marked "NOS" in the leftmost socket.
When booting, the machine displays "SkoleData AS" (loosely translated as "SchoolData Ltd") which I vaguely remember as a company working which national schools to adapt computer solutions in the 1980s. The EPROM clearly has some Norwegian text programmed into it.
Pin 1 (Vpp) has been cut off, and pins 28(Vcc) and 27 (~Pgm) have been soldered together, likely to protect against tampering.

There was also an EPROM marked DNFS - iss.3, which is the network filing system, I assume.

On power up (not soft/hard break) the machine seems to be seeking for something, but there is no Econet fitted and the machine tends to hang until soft or hard break is hit. After that it works fine.
DNFS is a ROM that provides both the Disc Filing System and Network Filing System, so it could be perfectly reasonable to find it in a machine with no Econet interface but with a disc interface.

With regard to the "NOS" in the left-most socket, is that IC51 or IC52? IC51 is where the machine operating system (MOS) ROM is placed, so if your NOS is there it has replaced the usual Acorn operating system. Or is it actually labelled "MOS"?

If it's got an Acorn MOS fitted, one reason it may not boot is it it's configured to boot from a floppy diskette. In its default configuration, a BBC will search for a !boot file if shift+break are pressed, but boot to ROM on break or on power-on. This functionality can be reversed using one of the optional links of the right hand side of the keyboard circuit board, specifically switch 5 (bit 3 of OSBYTE &FF). If there's a wire link here, then your BBC is probably waiting for the floppy controller to get an answer from the (non-existent) floppy drive.
Last edited by Kazzie on Wed Oct 24, 2018 12:05 pm, edited 2 times in total.
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Re: Stuff inside an issue 4 Model B

Post by adrm » Wed Oct 24, 2018 12:36 pm

Good ideas, Kazzie.

Unfortunately, the switch was not the issue. I now suspect this machine has one or more HW issues, including RAM (sometimes garbled screen or random pixels when switching to graphics modes)

I now strongly suspect that NOS = Norwegian OS.
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Re: Stuff inside an issue 4 Model B

Post by 1024MAK » Wed Oct 24, 2018 2:08 pm

adrm wrote:
Wed Oct 24, 2018 10:45 am
1)
The standard ROM sockets had 2x NEC D4364C-20L fitted. As far as I can tell, these are "Static CMOS RAM" devices.

Does anyone know how these are used in a Beeb?
Any software or hardware that uses them?
These are indeed static RAM (or SRAM) chips. Each being 8K bytes each. So two are required to make 16K bytes. They are often found in ROM expansion boards (which have special sockets reserved just for these SRAM chips only). And these boards are often designed so that this RAM appears in sideways ROM slot 15. Hence this RAM is often referred to as sideways RAM.

As supplied by Acorn, the BBC Micro cannot use these chips. But it is possible to modify the PCB so that it can take SRAM chips. Although normally either a larger capacity SRAM chip would be used or the two D4364C chips would be stacked on top of one other (“piggy back style”).

There are a limited number of games and applications that will use sideways RAM.
adrm wrote:
Wed Oct 24, 2018 10:45 am
2)
This machine is fitted with an EPROM (type 27128) marked "NOS" in the leftmost socket.
When booting, the machine displays "SkoleData AS" (loosely translated as "SchoolData Ltd") which I vaguely remember as a company working which national schools to adapt computer solutions in the 1980s. The EPROM clearly has some Norwegian text programmed into it.
Pin 1 (Vpp) has been cut off, and pins 28(Vcc) and 27 (~Pgm) have been soldered together, likely to protect against tampering.
If this EPROM is fitted in the socket marked IC51, then the modifications to the pins of the EPROM chip are required so that it will work in this socket. As the connections for this socket are not compatible with a EPROM.

Socket IC51 is reserved for the OS / MOS chip only. The 6502 CPU Reset vectors are in the address space of the OS / MOS chip, so if it is missing, or the chip that is fitted is not a OS / MOS chip, then the machine will not start up.

Mark
Last edited by 1024MAK on Wed Oct 24, 2018 2:10 pm, edited 1 time in total.

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Re: Stuff inside an issue 4 Model B

Post by Kazzie » Wed Oct 24, 2018 2:55 pm

Given 1024MAK's observations, it may be a good idea to remove the main board and check if there are any modifications underneath.

Edit: If your Norwegian OS is attempting to load something from a floppy, there should be some activity on the floppy connector even if you don't have a drive connected. Pin 10 (drive select 1) would be a good one to watch, it should be active low IIRC.
Last edited by Kazzie on Wed Oct 24, 2018 4:14 pm, edited 1 time in total.
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Re: Stuff inside an issue 4 Model B

Post by adrm » Wed Oct 24, 2018 3:54 pm

1024MAK wrote:
Wed Oct 24, 2018 2:08 pm

If this EPROM is fitted in the socket marked IC51, then the modifications to the pins of the EPROM chip are required so that it will work in this socket. As the connections for this socket are not compatible with a EPROM.

Socket IC51 is reserved for the OS / MOS chip only. The 6502 CPU Reset vectors are in the address space of the OS / MOS chip, so if it is missing, or the chip that is fitted is not a OS / MOS chip, then the machine will not start up.

Mark
Looking at the BBC schematic, I notice (for the first time) that IC51 is, indeed, connected differently.

I distinctly recall my first Beeb, back in the eighties, being equipped with unmodified EPROM(s) instead of the later ROM.
Am I correct in assuming that these came fitted into sockets 52 and 53, then?

I'm asking because the 3rd Beeb I recapped today was missing the OS ROM entirely, so I'll need to either buy one on eBay OR burn an EPROM myself.
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Re: Stuff inside an issue 4 Model B

Post by Kazzie » Wed Oct 24, 2018 4:24 pm

adrm wrote:
Wed Oct 24, 2018 3:54 pm
I distinctly recall my first Beeb, back in the eighties, being equipped with unmodified EPROM(s) instead of the later ROM.
Am I correct in assuming that these came fitted into sockets 52 and 53, then?
The very earliest machines came with OS version 0.1, which was on EPROMs in the four right-most sockets, with the BASIC ROM fitted to IC51. Upgrading to OS 1.x required modifications to the board to switch things around.
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Re: Stuff inside an issue 4 Model B

Post by adrm » Wed Oct 24, 2018 4:41 pm

Kazzie wrote:
Wed Oct 24, 2018 4:24 pm
adrm wrote:
Wed Oct 24, 2018 3:54 pm
I distinctly recall my first Beeb, back in the eighties, being equipped with unmodified EPROM(s) instead of the later ROM.
Am I correct in assuming that these came fitted into sockets 52 and 53, then?
The very earliest machines came with OS version 0.1, which was on EPROMs in the four right-most sockets, with the BASIC ROM fitted to IC51. Upgrading to OS 1.x required modifications to the board to switch things around.
It's very hazy now, but I do seem to recall modding the PCB. I don't remember 4 EPROMs, only two. Of course, I may be remembering it all wrong #-o

Question, can I burn an OS EPROM and fit it into socket 52, or will the OS only work in 51? Meaning I'll have to locate a ROM somewhere?
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Re: Stuff inside an issue 4 Model B

Post by 1024MAK » Wed Oct 24, 2018 4:50 pm

The OS/MOS will only work in IC51 position. If using an EPROM, isolate pin 1 from the socket (bend it out is easiest). Do the same for pin 27. Then, connect pin 1 and pin 27 to pin 28 using some fine solid core wire. Pin 28 is the +5V supply and pin 1 and pin 27 should be logic high for normal use.

You can see what someone did to an issue 7 board in one of my Beebs in this thread.

Mark
Last edited by 1024MAK on Wed Oct 24, 2018 4:55 pm, edited 1 time in total.

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Re: Stuff inside an issue 4 Model B

Post by adrm » Wed Oct 24, 2018 4:59 pm

1024MAK wrote:
Wed Oct 24, 2018 4:50 pm
The OS/MOS will only work in IC51 position. If using an EPROM, isolate pin 1 from the socket (bend it out is easiest). Do the same for pin 27. Then, connect pin 1 and pin 27 to pin 28 using some fine solid core wire. Pin 28 is the +5V supply and pin 1 and pin 27 should be logic high for normal use.

You can see what someone did to an issue 7 board in one of my Beebs in this thread.

Mark
I'll do that.
I'm just averse to bending / soldering on brand new components :cry:
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Re: Stuff inside an issue 4 Model B

Post by tingo » Wed Oct 31, 2018 9:06 pm

Well, if there is space, you can do all the bending and soldering on a socket, and then just put the socket between the chip and the onboard socket. Of course, if components are soldered directly to the board you can't avoid some soldering either way.
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Re: Stuff inside an issue 4 Model B

Post by Coeus » Thu Nov 01, 2018 12:16 am

1024MAK wrote:
Wed Oct 24, 2018 2:08 pm
If this EPROM is fitted in the socket marked IC51, then the modifications to the pins of the EPROM chip are required so that it will work in this socket. As the connections for this socket are not compatible with a EPROM.
Interesting. BITD I had the idea that it would be good to have a version of the OS that did not respect the "clear memory on break" flag and du;y gave a patched version to my mate who had access to an EPROM programmer at work. I still have the resulting EPROM in my BBC B, and it still works and I don't remember even making these modifications. Here's a pic:
20160821_185034_9594.jpg
Is the mod only needed for Issue 4?

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Re: Stuff inside an issue 4 Model B

Post by 1024MAK » Thu Nov 01, 2018 10:07 am

Coeus wrote:
Thu Nov 01, 2018 12:16 am
BITD I had the idea that it would be good to have a version of the OS that did not respect the "clear memory on break" flag and du;y gave a patched version to my mate who had access to an EPROM programmer at work. I still have the resulting EPROM in my BBC B, and it still works and I don't remember even making these modifications.
Is the mod only needed for Issue 4?
It may be that your BBC B was modified to use an EPROM at the factory. If you follow this link you can see pictures of a BBC B with an issue 7 board that looks like it was factory modified.

I don’t know for certain, but as the PCB tracks for IC51 socket on the issue 7 board are wired as per the schematic, I suspect that the PCB track wiring for this socket is the same on all boards. With the modification only being carried out when there was supply problems with the mask ROM and hence the need to use an EPROM instead.

It is also possible that some EPROM chips may not actually fully comply with the published data sheets, and hence work with pins 1 and 27 not tied to a logic high. We already know that most EPROM chips don’t care about pin 1 being left not connected.

If you want to find out, but don’t want to remove the board from the case, and do have a multimeter to hand:-
With the Beeb switched off, switch the multimeter to the 200 ohms resistance range. Put the red probe on IC51 socket pin 28. Then test first pin 1, then pin 27 using the black probe. If the meter reads less than 1 ohm for these, then the board has been wired for an EPROM.

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Re: Stuff inside an issue 4 Model B

Post by Kazzie » Thu Nov 01, 2018 10:50 am

adrm wrote:
Wed Oct 24, 2018 4:41 pm
Kazzie wrote:
Wed Oct 24, 2018 4:24 pm
adrm wrote:
Wed Oct 24, 2018 3:54 pm
I distinctly recall my first Beeb, back in the eighties, being equipped with unmodified EPROM(s) instead of the later ROM.
Am I correct in assuming that these came fitted into sockets 52 and 53, then?
The very earliest machines came with OS version 0.1, which was on EPROMs in the four right-most sockets, with the BASIC ROM fitted to IC51. Upgrading to OS 1.x required modifications to the board to switch things around.
It's very hazy now, but I do seem to recall modding the PCB. I don't remember 4 EPROMs, only two. Of course, I may be remembering it all wrong #-o
Apparently they did put the OS in two EPROMS on a header board, which was in turn plugged into the OS socket. See http://lists.cloud9.co.uk/pipermail/bbc ... 11986.html
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Re: Stuff inside an issue 4 Model B

Post by adrm » Thu Nov 01, 2018 1:31 pm

Kazzie wrote:
Thu Nov 01, 2018 10:50 am

Apparently they did put the OS in two EPROMS on a header board, which was in turn plugged into the OS socket. See http://lists.cloud9.co.uk/pipermail/bbc ... 11986.html
That sounds somewhat familiar :D
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Re: Stuff inside an issue 4 Model B

Post by Coeus » Thu Nov 01, 2018 4:01 pm

1024MAK wrote:
Thu Nov 01, 2018 10:07 am
It may be that your BBC B was modified to use an EPROM at the factory. If you follow this link you can see pictures of a BBC B with an issue 7 board that looks like it was factory modified.
I will check when I get the chance, but following from that thread to this message on the mailing list, it seems both pins need to be at +5V and both of the address lines just happen to be high when the OS ROM is being accessed. That does make it seem like whether it would work without a mod is down to the particular EPROM including whether it tries to draw too much current (in the case of VPP) and, on the assumption that /PGM controls the direction of a set of buffers on the data lines, whether these can switch direction fast enough.

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