Help for a WE RAM / ROM board not working

[Kazzie]

So the Beeb is sending plenty of (active low) S21 requests to the board, but the board isn't sending any back. That's in keeping with the behaviour you've described so far.

(BTW, is S21E connected to IC16 pin 5 or pin 6? Your last two posts mention both pins.)

Now that you've got a scope to hand, it might be interesting to look at what else is happening on IC16. Particular candidates would be pin 7 (output for pins 5/6), pin 1 (select pin 5 or 6) and pin 15 (strobe/enable output).

[aotta]

So the Beeb is sending plenty of (active low) S21 requests to the board, but the board isn't sending any back. That's in keeping with the behaviour you've described so far.

(BTW, is S21E connected to IC16 pin 5 or pin 6? Your last two posts mention both pins.)

Now that you've got a scope to hand, it might be interesting to look at what else is happening on IC16. Particular candidates would be pin 7 (output for pins 5/6), pin 1 (select pin 5 or 6) and pin 15 (strobe/enable output).

Kazzie, that's the scope output (pins name are corrected now, sorry for previous mistake):

So, pin 7 give the correct answer after 0.4s?

[KenLowe]

Does the board need it's own 5v supply? Where does it pick that up from?

[aotta]

Does the board need it's own 5v supply? Where does it pick that up from?

it keep power from vcc2 psu spades and return it to board with its wires...

But.. S21 West is connected to Pin 1 of IC1 (74ls00), that is an input to the first nand gate of ICs... and pin 3 in fact is not flat and waves...
So, is W the input and E the output, or viceversa? i am confused...

[KenLowe]

But.. S21 West is connected to Pin 1 of IC1 (74ls00), that is an input to the first nand gate of ICs... and pin 3 in fact is not flat and waves...
So, is W the input and E the output, or viceversa? i am confused...

That does sound like it's the wrong way round! On the beeb motherboard S21 SE is normally fed from the output of a NAND gate (IC21) and S21 SW normally feeds an input to IC20. What happens if you try connecting the S21 lead from the RAM/ROM board to the beeb S21 SE / SW pins the opposite way round?

[aotta]

But.. S21 West is connected to Pin 1 of IC1 (74ls00), that is an input to the first nand gate of ICs... and pin 3 in fact is not flat and waves...
So, is W the input and E the output, or viceversa? i am confused...

That does sound like it's the wrong way round! On the beeb motherboard S21 SE is normally fed from the output of a NAND gate (IC21) and S21 SW normally feeds an input to IC20. What happens if you try connecting the S21 lead from the RAM/ROM board to the beeb S21 SE / SW pins the opposite way round?

first test i made, as the manual indicate the inversion of S21 wires as cause of "language?" error... no way.
And, from the wires "siluette", it seems they stay 30 years in that position...
What seems strange to me is that SW is connected to pin 1 of a 74sl00b1, that looks like an input...
maybe who own same card could test it on his card..

[KenLowe]

I don't have exactly the same card. However, I do have a WE 12 ROM board that has a similar S21 that has flying leads that connect to the Beeb motherboard. In the case of this ROM board, S21W is connected to IC4 pin 3, which is a 74LS32 output. S21E is connected to IC4 pin 2, which is a 74LS32 input. That seems correct, and aligns with the jumper block on the beeb motherboard. I would definitely try swapping your flying lead around.

Edit: Sorry, I see you have tried swapping the wires around, but it still didn't work.

[aotta]

Thank you Kenlowe for the info, but in my case it seems i have two inputs... it's not the wiring the fault, but i'd like to understand the board to perform further test.

[KenLowe]

in my case it seems i have two inputs... it's not the wiring the fault

Ah. Ok. Is it possible that S21W is actually connected to more than one IC on the ROM / RAM board, or to more than one pin on IC1?

[aotta]

in my case it seems i have two inputs... it's not the wiring the fault

Ah. Ok. Is it possible that S21W is actually connected to more than one IC on the ROM / RAM board, or to more than one pin on IC1?

That's what i thought too and... yes! s23w is connected to IC19 pin 8 too (74ls20)... an output!
Further investigation on its inputs..

[Kazzie]

Now that you've got a scope to hand, it might be interesting to look at what else is happening on IC16. Particular candidates would be pin 7 (output for pins 5/6), pin 1 (select pin 5 or 6) and pin 15 (strobe/enable output).

Kazzie, that's the scope output (pins name are corrected now, sorry for previous mistake):

S21WE2.png

So, pin 7 give the correct answer after 0.4s?

That behaviour is a little unexpected, as according to the 74ls157 datasheet, when the Strobe/Enable input (pin 15) of IC16 is high, all the outputs (including pin 7) should be forced low. But that doesn't appear to be the case on your readout. (Your readout is labelled IC18: a mistake?)

Ignoring the Strobe issue, when Select (pin 1) is high, the value of output pin 7 depends on the input on pin 6: they should be equal. (Worth checking.) Without knowing more about how the chips are connected together, though, it's hard to know what should be happening overall.

I'd suggest one of two strategies now:

1. Use a multimeter's continuity test to work out which chips are connected to which, so we can build a circuit diagram. (Working "inward" from S21E and S21W, as well as outward from the Chip Select pins on the ROM (pin 20) and RAM (pin 1) chips, would be a good idea. The ROM and RAM should be connected to IC17 and IC18. Note also that each bank of RAM will share the same connection on pin 1.)

2.. Use your probe to measure the behaviour of each chip individually, comparing against their datasheets, to try to identify any faulty behaviour. (There may also be dry solder joints or broken tracks inbetween chips, which wouldn't be identified in a chip-by-chip test.)

[aotta]

Kazzle, now I realized that there was a misunderstanding: S21E goes to IC18 (the sl138), and I referred test and probing to it, while you was talking about ic16! Really sorry for it, I will have to sleep a bit more in the night instead of playing with old computers..
Anyway I'll follow your guidelines, starting to a resoldering of ic from 15 to 20 or, better, a desoldering and socketing of them, so I can do a check with the ic logic test function of my eprom programmer.

About following tracks on the board and replicate schematics, it's of course a good idea but really hard on this board: they run from on side to others and up and down like a jo-jo, and tends to hide themselves under ics sockets.. May be it was an intentional "copy protection" technical..

[Kazzie]

IC 18 makes a lot more sense. That means S21E goes to an active-low enable pin on the '138.

On the topic of replicating schematics, I know it can be a pain to do: I've tried it on a floppy drive circuit board with a large mix of ICs and discrete components, which was slow work, following tracks by eye and poking components with probes.

On an IC only board like yours, I'd probably work from one IC, hold the first probe to one pin, and sweep the other probe along all the other ICs (with board power off), noting the connections in a list. After that, I'd use the list to start drawing a diagram, with reference to datasheets.

I don't have a USB scope or logic tester, though, whereas you do.

Soldering in sockets and testing all the ICs should find any fault other than physical track damage, and that should be easier to spot with the ICs removed (or while you work on the board).

[aotta]

Thanks Kazzle, your hints are very helpful! I will use the weekend for resocketing and getting pics of board when "nude".
A little OT, about instruments, I bought from aliexpress the scope (LA1010 usb) for 25£ shipped, after some years of a Diy kit that worked in a very random way. And, from aliexpress too, I bought Tl866 II programmers, that include the logic test for most 74xx and some GAL, for under 30£. Items cone in a pair of months, but price is good. Think about..

[Kazzie]

Thanks for the suggestion, I note that some have had success in using the TL866 under Linux (which is my main OS), so that may well find its way on to a shopping list.

[1024MAK]

On an IC only board like yours, I'd probably work from one IC, hold the first probe to one pin, and sweep the other probe along all the other ICs (with board power off), noting the connections in a list. After that, I'd use the list to start drawing a diagram, with reference to datasheets.

If you have a multimeter that is very fast on the continuity mode, this method can be quite effective, especially when you consider that educated guesses will help. Use chip pin-outs and data sheets to help. For example, with the ROM sockets, all the D0 lines will be connected together, same for D1, D2... A0, A1...
Again for the ROMs, each /CS (/CE) pin will come likely from a decoder, such as a 74LS138 output.

On the DRAM, all the DRAM address lines will be in parallel with one another, which should also go to two of the 74LS157 multiplexer chips. Each data pin will of course go to the boards data bus. Note that the DRAM address pins may not be in order.

You already know the data bus pins on the 6502 socket, so the data bus should go to the 74LS245, then onwards to the rest of the board.

Again, the address bus on the 6502 socket most likely goes to the 74LS244’s, which then feed the board’s own address bus to the two of the 74LS157 multiplexers. And to the ROM address lines.

Once this lot is mapped, that’s two thirds of it done! And this method, can actually be a lot faster than hand tracing for this stage. The next stage can be a bit more time consuming.

As said above, the address decoding for the chip used as the ROM latch (ROMSEL) is likely to be the 74LS133. Easiest is to start from 74LS133 inputs and work your way back to the address bus. Then use the single output and work your way around looking to see where it goes next. Which is likely to be another gate or two, the output of which will go to one (or both) of the 74LS75 chips.

Good luck

Mark