6502 and IC's on daughter board

[Colday]

Good afternoon.

I acquired a couple of beebs today as well as some other bits and bobs which I will get to in another post.

One of the beebs, an issue 7, has the 6502 processor removed and a daughter board with the 6502 processor and some other IC's plugged into it.

I'm happy to note that the board is one of bob's, but I'm curious about the daughter board.

Picture to follow.... **edit - actually it has been so long since I was last here, how do we do pics again - sorted**

What is this please?

[1024MAK]

Dunno, but the four smaller chips are all buffers/line drivers. They are used to boost the amount of current available to drive the address, control and data busses.

Mark

[Colday]

Thanks Mark.

The guy said that he thought one of the machine's processors was upgraded. Would that be classed as an upgrade?

[1024MAK]

The 6502 chip is a standard 2MHz NMOS 6502 (Rockwell R6502AP). So the same as what I would expect to find in a factory standard Beeb.

Mark

[1024MAK]

If a ROM board (as shown in your other post) is used, the address, control and data busses would need to be buffered...

Mark

[Colday]

Ok, what would be the point of the ribbon cable? It’s a 40 pin. At the moment the machine it is in does not boot.

Long low tone is emitted.

[Colday]

I presume this is some sort of disc controller

[johnkenyon]

I presume this is some sort of disc controller8DC2E9E6-DE8C-4830-9493-C9E9DBA68AB7.jpeg

Microware DDFS?

Looks like the item pictured on page 9 of Beebug Jan/Feb 1984
available at http://www.computinghistory.org.uk/downloads/14544

The "8272" marking on the board seems to tally with a double density controller.

[Pernod]

I presume this is some sort of disc controller

This is interesting in that I'd always thought the Microware board used a WD2793. The United Disk Memories (UDM) board is virtually identical and whilst many photos are similar to yours with the markings scrubbed I do have a single photo showing a WD2793. So maybe the original Microware version used a 8272 and when UDM took over they updated it to a WD2793.

Do you know which version of the Microware DDFS came with it? The only known dumped version reports MDDFS 0.90 with *HELP, if yours is anything different then I'm interested in a dump of it.

[Coeus]

I presume this is some sort of disc controller8DC2E9E6-DE8C-4830-9493-C9E9DBA68AB7.jpeg

The "8272" marking on the board seems to tally with a double density controller.

So if this an 8272 why does the chip have what looks like a covered window as one might expect on an EPROM? Is the 8272 actually a micocontroller with some specialist circuits and thus has firmware on board such that an erasable, re-programmable one exists?

[Coeus]

Do you know which version of the Microware DDFS came with it? The only known dumped version reports MDDFS 0.90 with *HELP, if yours is anything different then I'm interested in a dump of it.

That's interesting. 0.90 was the version reviewed. Does that mean they were taken over before producing the next version, I wonder?

[CMcDougall]

8272 what looks like a covered window as one might expect on an EPROM? Is the 8272 actually a micocontroller

it's probably just an earlier version of same FDC chip, but made prior to 1986, as the ones on two of my bare board 286 51/4" controllers has no 'window' & has Intel P8272A on them.

[CMcDougall]

I tell a lie, other board has :

[Colday]

Unfortunately the machine it came in has yet to boot. I’ve got the psu caps on order as the magic smoke was released from one of them.

Had a solid tone and was testing psu power outs when it went. I’ve got a couple of other psus and will swap one in tomorrow. Sadly the second machine I acquired appears to have been in a puddle at some point.

Will clean the board up and do some testing as otherwise it seems to be a reasonable condition.

Is still be interested in what people think about the cpu daughter board. Has that been seen before?

[Pernod]

Is still be interested in what people think about the cpu daughter board. Has that been seen before?

There is a similar board for the Aries B12, see http://chrisacorns.computinghistory.org ... s_B12.html.

I think yours would've been for a similar purpose but haven't seen it before. How long is the cable? Wondering whether it's long enough to connect something externally, or more likely to attach to something stuck to the inside upper case.

Edit: Isn't it to attach to your RamRom 15 board externally?

[johnkenyon]

I presume this is some sort of disc controller8DC2E9E6-DE8C-4830-9493-C9E9DBA68AB7.jpeg

The "8272" marking on the board seems to tally with a double density controller.

So if this an 8272 why does the chip have what looks like a covered window as one might expect on an EPROM? Is the 8272 actually a micocontroller with some specialist circuits and thus has firmware on board such that an erasable, re-programmable one exists?

The chip is also painted black. At first glance I thought 8748 or 8751 microcontroller, but there's no crystal on board, and the Google search lead me to the microwave device. I get the impression it's a bluff to make wannabe copiers believe that the board has a custom device on board. The reality is a 8272 or a upd765 (both used in the IBM PC) were two a penny compared to the 8271.

[Colday]

Isn't it to attach to your RamRom 15 board externally?

You could very well be right. The cable is plenty long enough to reach outside the casing and in fact when you release the cable it does seem to want to 'lay' out the back of the case.

I'll swap the 6502 out of a known good machine later and try hooking one of these RamRom-15's up (yes, there are 2x of them)

Does Mark's comment now make more sense if that is the cards purpose?

Dunno, but the four smaller chips are all buffers/line drivers. They are used to boost the amount of current available to drive the address, control and data busses.

Would that 'improve' the performance of talking to the RamRom-15?

[1024MAK]

Because microprocessors like the 6502 were designed originally for use in embedded applications, they did not have particularly good output drivers on their outputs (it is also due to the limits of the technology they were made out of).

Hence if you want a system that has lots of chips connected to the busses, you need to use buffer/line driver chips. Otherwise the load on the bus (called fan out) will drag down the voltages to outside the defined logic levels. It’s even worse if part of the bus is carried by ribbon cable, as then crosstalk between signals increases.

Think of a thin washing line. It can support say twenty items of wet clothing. But if you try to double the number of items, it will either sag and the clothes will drag in the dirt or it will break...

In fact, the design of the BBC B is apparently close to, or over the recommended bus loading for an NMOS 6502 (note that I have not actually investigated this myself).

Mark