04-02-2021, 12:54 AM (This post was last modified: 04-02-2021, 12:56 AM by Ernest.)
Hi Lew,
Nice collection you have there
"Number 3 is a B-ETI Serial Terminal. I have found this morning that it does seem to work, but seems to be sensitive to supply voltage, and only seems to boot up when I change that (between about 10 and 12 V DC). This might just be a capacitor issue. I'm not sure how much use a maximum 4800 baud terminal would be these days - it's seems so much easier to use a program on a PC. Maybe this unit could be reprogrammed as a Microbee?"
Haven't seen one of these in a very long time. The unique aspect of this terminal was it's ability to be used for the old Baudot RTTY as was common in the day among radio Amature's as the old Teleprinters that were cheep to pickups from disposal stores in the early eighties became more and more rare.
As you have a couple of the normal MB1248 Mainboards on your other machines, if you are going to make the change to the B-ETI, I would look at doing any changes as a two step approach.
1) Do the Coreboard first and get it working on one of your other MB1248's.
2) Then look at tackling the Mainboard. The great advantage you have is having a number of systems to work with.
The Coreboard should be straight forward (just missing quite a number of parts) as only 2K of SRAM was fitted to those units, this was in the memory space of D800h - DFFFh .
As most of the board is empty, this would give you a reasonably blank canvas to work with, I would look at fitting a 256K SRAM single chip to give you a 32K unit with minimal changes.
If my understanding of the B-ETI is correct, the Mainboard also had differences from the normal Microbee.
It didn't have the PGC RAM or supporting chips and also I think IC16 the Bipolar PROM 82S123 was different, most likely all to do with the lack of PGC RAM.
As there was no program memory as such, they also omitted the cassette interface parts around and including IC35 (nowhere to load a program too anyway).
It's nice to meet you. Thanks very much for your very helpful message. The background information about the B-ETI is very interesting.
Your suggestions about a possible modification path make a lot of sense to me. I was well aware of all the missing chips on the coreboard, but I haven't looked at the baseboard yet, and I imagined that it was complete.
I haven't decided what to do with the B-ETI yet, but if I did modify it, the idea of adding a single 256K SRAM would be very appealing. Hypothetically, would a viable alternative be to use a 128K coreboard? Or is that not compatible? I see that there are kits for these, although not currently available.
(04-02-2021, 09:19 AM)Lew Wrote: I haven't decided what to do with the B-ETI yet,
Personally I would not "upgrade" the B-ETI, it is such a rare variety of Microbee I would leave it alone as it would be worth much more as a B-ETI than if converted to a 32k or 128k bee.
(04-02-2021, 12:54 AM)Ernest Wrote: If my understanding of the B-ETI is correct, the Mainboard also had differences from the normal Microbee.
It didn't have the PGC RAM or supporting chips and also I think IC16 the Bipolar PROM 82S123 was different, most likely all to do with the lack of PGC RAM.
Hi Ernest.
I just thought I would comment on the PCG RAM
I would have thought that PCG ram was still needed to include inverse characters ("reverse video").
There is no hardware support on the standard Microbee for inverse - it was provided by copying and inverting the
FONT ROM data to PCG ram.
I have a B-ETI here, although I cannot be certain that the baseboard & coreboard were a matching set as produced in
the factory. The baseboard that I have does have PCG ram in it and is stock standard.
The only thing missing from mine is the top case & badge. It wasn't working when I got it, but I've repaired it & works well.
Thanks Ewan for the confirmation that the BETI will work OK on a Standard Mainboard.
A long while back I looked into the BETI myself and after a long while of pouring through lots of Technical documentation I made some notes for myself and then put the idea aside.
My notes appear to have been based primarily on an Engineering Correction Notice Max Maughan put out back in 1984.
The notes refer to ECN 4-50684 "Modifications for BETI Main and Core boards".
Under the Coreboard mods it says "Parts not fitted IC14, 18, 32, 35, D14. Special Part IC16 82S123 along with a number of other minor parts links and track cuts complete with a diagram showing a resistor, diode capacitor fitting at IC15.
Once I read the part about special part IC16 82S123, that is where I stopped my investigating. I noted that the configuration for a 56k Coreboard looked straight forward.
Anyway as ChickenMan said "this would be worth much more as a B-ETI" it really depends on what Lew is wanting with this unit long term, especially as he has other more conventional units to play with.
Thank you all for the helpful and interesting discussion and information in reply to my suggestion of perhaps turning a B-ETI into a regular bee.
I have now realised that it's not good idea, and I think it makes much more sense to keep the B-ETI just as it is for the sake of posterity. And, as Ernest so rightly says, I have enough regular bees to play with.
I have learnt a lot, and really enjoyed the discussion.
Thanks Ewan for giving me the confidence that the B-ETI would work on a unmodified Mainboard, I decided to drop a BETI PROM into my MB1656 Coreboard in place of the v54 D-Boot ROM and fired it up.
Bingo and operational B-ETI looks like all the extra SRAM is not causing any issues (certainly on the bit of testing I did today)
Would have been great to have the old mechanical Teletype I had back in the eighties and give the BAUDOT a go. Oh well, I'm sure I'm not alone in ruing the throwing out of something that was taking up too much space
So looks like it would be very simple to set up with a 8k EPROM and switch between a normal 56k Disk based system and a B-ETI.
Today I learnt something new, now to update my notes for future just in case I revisit the idea way down the track.
(05-02-2021, 01:27 AM)Ernest Wrote: Thanks Ewan for giving me the confidence that the B-ETI would work on a unmodified Mainboard, I decided to drop a BETI PROM into my MB1656 Coreboard in place of the v54 D-Boot ROM and fired it up.
Bingo and operational B-ETI looks like all the extra SRAM is not causing any issues (certainly on the bit of testing I did today)
Hi Ernest.
I hadn't thought of trying the ROM in a 56K coreboard. Good thinking.
The one I have has a MB1732 (32k rom based) coreboard in it as per the pic attached.
If I'm recalling correctly, the one my mate had was the same configuration.
I must look up the ECN you mentioned. Thanks for that.
