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tezza
December 31st, 2007, 02:51 PM
I’ve been trying to get some software from BBC disk images from my PC to my BBC b via the serial port. For this I’ve been using Jon Welsh’s DXFSfer software, and made up a serial cable to specifications.

Sadly, this exercise revealed that my BBC (b) serial port is broken. It can receive data, but cannot transmit it? Anyone experience this or (even better) know how to fix it?

It definitely IS the BBC serial port. This was proved by a loopback test, the program and details of which were given to me by Jon himself as he helped me with this problem. The cable wiring and the PC side of things are fine.

Druid6900
December 31st, 2007, 09:01 PM
The transmit and receive functions of the RS-232 are controlled by two different chips, the 1488 (which I think is the transmit) and the 1489 (which I think is the receive). AFAIK, it's the same setup for the 432.

Those suckers often blew up and you can salvage the parts out of most old modems, either external or internal.

If I'm incorrect in any of my details, I'm sure someone will correct them.

nige the hippy
January 1st, 2008, 05:07 AM
Also known as 75188 and 75189,

!!!!! However, the beeb uses a couple of different chips, a DS3691 for transmit, and a DS88LS120N for receive, both are still pretty available new. (do a google) !!!!!!

It might pay you to download the schematic/service manual.

tezza
January 1st, 2008, 12:50 PM
Yes, Jon Welsh has been quite helpful and sent through some further information and schematics by email. I've now identified the chips I need to look at.

I'll wait until I get a lot more skilled at removing and re-soldering chips though. Given my low level of skill with a soldering iron at the moment, I'm more likely to break the Beeb entirely. In the meantime I'll explore some alternative way of getting a BBC disk image from a PC to the BBC's single density DS, 80 track drives.

If anyone knows how, please advise! I've tried OmniXfer, but (as expected) it seems my PC disk controller just can't do single density.

tezza
February 5th, 2008, 11:45 AM
Well, as I'm on the boards, in bed (hooray for laptops!) and relucant to get out of it on this bank holiday, I might as well post a BBC update here.

Last night I replaced the DS3691 chip. Unfortunately, I've still got the same symptom. I'm down to three possibilities...

1. The new chip was DOA
2. When I was desoldering on the underside, the iron slipped and ran across one of the tracks near the chip on the board. It could have actually cut this track (there is a mark there). I didn't think it went deep enough but you never know.

3. The ULA Serial processor chip is faulty.

1. is always a possibilty I guess. 2. is also a possibility but I'm not sure the scratch has actually cut the track. It looks superficial. Its a track that continues through the board then under an adjacent chip so I'm not actually sure where it goes. I'll have to study some scematics if I ever try to bridge the connection.

Looks like I might have to get a waveform analyser and try to follow the signal through to get to the bottom of this problem.

A question for those who might know though. The serial processor chip also controls the cassette port and this seems to work just fine. Can these chips develop partial faults? In other words, could just the transmit function of the RS-432 be affected and everything else be ok? The port seems to be able to receive but not transmit.

If anyone has any more diagnostic ideas, please feel free to share them.

nige the hippy
February 5th, 2008, 02:25 PM
look at the i/o parts first before you go delving deeper. there's a reason that there are so many dentists; ear,nose & throat specialists, gynaecologists & proctologists in the world!

It would probably pay you to get a cheap scope, or even better, to get a good scope cheap ((you should) see my recent dump finds!) I feel blind without one.

yes it is possible for a part to go partially faulty!

check for continuity between the chip & the rs423 socket, often sockets get rocked & pushed about until either the soldered joints or an internal part fractures. this beeb could have had a serial connection plugged in & been pushed against a wall, or dropped onto it.

check continuity of plated-through holes nearby (and on the chip you replaced) it's really easy to turn a tube of copper between the top and bottom sides of the board into "just a tube" when you're reworking.

good luck !

tezza
February 6th, 2008, 04:05 PM
Nige, I think I will need good luck.

Last night I checked out the connections as you advised. All pathways seem to be connected from the pins on the sockets themselves right through to the smaller ICs, then onto the pins on the ULA chip itself. All the wiring seems ok, and resistors in the circuit seem to have the right resistance. There are a couple of small ceramic capacitors involved. I'm not sure how to test these, but they have no surface damage I can see.

I now strongly suspect the serial ULA chip itself, even though the cassette part works just fine. It does run quite hot, although it's a large chip and it's really no hotter than some of the other large chips.

Fortunately it is socketed. Unfortunately I suspect these chips are not common. Does anyone know where to get a Serial ULA 2C199 chip as used in the BBC (b) models. A google search doesn't reveal much. Maybe there is a modern compatible equivalent?

I'll also post a note on the "wanted" section of this forum, someone might have an old one they are willing to part with.

I'm wondering why I'm spending so much time (and now money) trying to repair this? It must be something in my Psyche. I just HATE having a machine where I KNOW there is something wrong, even if it is just a small thing. Also, as my PC floppy drive doesn't do single density, having a working RS-423 port is the only way I can see to get some disk software off the Internet for the unit. Thirdly, the more time I invest in the fault, the more I'm determined to actually fix it!

alexr
August 15th, 2012, 08:47 AM
Hello Tezza,

I picked up a ULA 2C199E among some bits at the bring and buy at the Kings Lynn Radio Rally. Also on the foam LM324 DP 8440 (or S440). In the course of searching for info I came on this site and found your search from 2008. You have probably binned the micro but if not the chip is here. Obviously I've no idea whether it is in working order. In the event of difficulty in reply I'm to found at alexr0433 at gmail.com or Tel 01508521155.

Rgds

Alex

tezza
August 15th, 2012, 05:21 PM
Hello Tezza,

I picked up a ULA 2C199E among some bits at the bring and buy at the Kings Lynn Radio Rally. Also on the foam LM324 DP 8440 (or S440). In the course of searching for info I came on this site and found your search from 2008. You have probably binned the micro but if not the chip is here. Obviously I've no idea whether it is in working order. In the event of difficulty in reply I'm to found at alexr0433 at gmail.com or Tel 01508521155.

Rgds

Alex

Thanks for the offer Alex. The problem's fixed now, but you still might be able to help. There is a bit of a story to it. Let me check a few things on the weekend and get back to you.

Tez