BBC B+ vs ABC

for bbc micro/electron hardware, peripherals & programming issues (NOT emulators!)
Post Reply
firthmj
Posts: 235
Joined: Tue May 26, 2009 8:37 am
Location: Ipswich, UK
Contact:

BBC B+ vs ABC

Post by firthmj » Sat Jan 04, 2020 2:50 pm

Hi,

A question occurred to me while answering a Facebook posting about the B+.

The B+ and ABC/ACW share a motherboard. But is it known which came first?

Was the motherboard designed because some changes were needed from the BBC B one for the ABC, and then reused for the B+, was it designed for both uses, or something else?

I’m assuming that because of the ABC/ACW specific features (e.g. internal video connectors) that it wasn’t designed just as a “new BBC B” layout.

It may be this is something that’s been discussed before, but I haven’t seen it if so.

Thanks in advance for any wisdom on this...

Michael
Had fun at the
Image
Meeting 21st September 2019

acorndave
Posts: 29
Joined: Wed Jul 10, 2019 9:26 pm
Location: East Yorkshire
Contact:

Re: BBC B+ vs ABC

Post by acorndave » Sat Jan 04, 2020 10:21 pm

There's some info here:

http://chrisacorns.computinghistory.org ... s/ABC.html

http://chrisacorns.computinghistory.org ... s/ACW.html

According to dates I can find the first ABC came out in 1984, so that would seem to suggest that Acorn wanted to make a 'Business' computer and tweaked the BBC B to that end. The B+64K came out in 1985 ( I think). So perhaps they fed the design back to rejuvenate the domestic market demand for the B

Boydie
Posts: 414
Joined: Sat Oct 24, 2015 8:25 am
Location: Sunny Wigan
Contact:

Re: BBC B+ vs ABC

Post by Boydie » Sat Jan 04, 2020 10:43 pm

The ABC/ACW definitely came first, by quite a considerable margin.
I’m not aware of any significant differences between the ABC and B+ boards. IIRC the ACW board looks pretty much like a B+ board cobbled into an all-in-one case, just with slightly different writing on it.
Functionally, it’s hard to tell why they redesigned the motherboard, at least for the ACW (or the ABC 200 or 300 series). To all intents and purposes, it may as well be a bog standard Model B inside. Perhaps the shadow ram made the lower ABC models more usable when fitted with ADFS and HDD; shame they never saw the light of day.
Certainly they didn’t seem to make much use of the improved ROM sockets in the standard ACW; the
only improvement they capitalise on is the 1770.

firthmj
Posts: 235
Joined: Tue May 26, 2009 8:37 am
Location: Ipswich, UK
Contact:

Re: BBC B+ vs ABC

Post by firthmj » Sun Jan 05, 2020 6:06 pm

Boydie wrote:
Sat Jan 04, 2020 10:43 pm
The ABC/ACW definitely came first, by quite a considerable margin.
I’m not aware of any significant differences between the ABC and B+ boards. IIRC the ACW board looks pretty much like a B+ board cobbled into an all-in-one case, just with slightly different writing on it.
Functionally, it’s hard to tell why they redesigned the motherboard, at least for the ACW (or the ABC 200 or 300 series). To all intents and purposes, it may as well be a bog standard Model B inside. Perhaps the shadow ram made the lower ABC models more usable when fitted with ADFS and HDD; shame they never saw the light of day.
Certainly they didn’t seem to make much use of the improved ROM sockets in the standard ACW; the
only improvement they capitalise on is the 1770.
Thanks - I should have made the connection on the timing. Does that mean there is a possibility that they had a batch of motherboards built that they refused to make the B+ when the ABC was scrapped as a project?

There are 4 differences that I’m aware of that the ABC/ACW takes advantage of:
1) The 1770 FDC, for ADFS
2) An internal RGB and CVBS header, for connecting to the integrated monitor (this could have been bodged, but it is more elegant as a header
3) Slightly different speaker volume control arrangement, that allowed a flying lead to the case volume knob.

It is a little odd that they didn’t integrate the Tube enable/disable functionality into the motherboard, but left it as a weird bodge on the keyboard and Tube cable.
Had fun at the
Image
Meeting 21st September 2019

Boydie
Posts: 414
Joined: Sat Oct 24, 2015 8:25 am
Location: Sunny Wigan
Contact:

Re: BBC B+ vs ABC

Post by Boydie » Mon Jan 06, 2020 11:28 am

I've just double-checked on a couple of ACWs, and the model code for the motherboard is the same as for the B+ (0233,000).
There are, however, some important differences in how the motherboards are assembled - the ACW has vertically-mounted connectors for the 1MHz bus, user port, etc etc. It also has Econet fitted (soldered in) as standard. It is also lacking the circuitry for colour composite output - ICs 79, 86, 87, and 90 are all unpopulated (and the holes closed off with solder), as are a lot of resistors and other gubbins in that area.

So, it's not impossible to turn an unused ACW board into a B+, but it's quite a bit of effort.

The simplest way to make an ACW would be to take a Model B, develop the 1770 upgrade a year earlier, solder the internal video connections onto the RGB socket, and solder a different volume control in place of VR1 (unless that's the one that make Kenneth Kendall go all squeaky, in which case VR2).

The whole thing only makes sense if the aim (at least on the part of the designer) was to develop the B+ and ACW (ABC) simultaneously. There's a lot of extra capability that the ABC simply doesn't use, and which becomes largely superfluous in a machine intended mainly to use a built-in second processor, but which makes a lot of sense in a B+.

That goes some way to explain why the ACW's Tube enable/disable solution is a bit kludgy - anything motherboard mounted may be inappropriate for a B+. Besides, I've always found the keyboard switch to be quite an elegant solution in actual use; it only looks a bit messy when you look under the lid.

As to why the B+ came out so much later than the ABC/ACW (indeed the Beebug review of the B+ concluded with "why not a year or two ago?"), I don't know; it may have been that they had to wait for component prices to drop before a more mass-market machine was economically viable whereas a high-end ABC could be "reassuringly expensive". However I'd be willing to bet that the real reason was in 1983/4 Acorn had over-ordered Issue 7 motherboards, and demand had dropped off more than they expected, so they had to delay releasing the B+ until the warehouse full of Model Bs had been shifted (nobody mention the Electron).

Post Reply