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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 13th May 2020, 9:34 pm   #101
Timbucus
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Default Re: Recreating the Bywood Scrumpi

It is a mistake (I think I mentioned above to ignore that bit) I drew an extra wire in but have not erased it yet on that older diagram.

Built two of your little adapters with some sockets and wire wrap wire and powered up the board and it is locked in Tri State - it will not budge as NHOLD is always held high so something is wrong. It is also unstable as I realised I forget any decoupling capacitors so the CMOS buffers are triggering. I also realise without a +Ve on the Data switches you will not be able to write a 1 into RAM as the bus will be Tri State from the processor so the bottom of the switches will all need +Ve I think.
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Old 13th May 2020, 9:41 pm   #102
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Default Re: Recreating the Bywood Scrumpi

I wondered about the switches not having pullups which is what I would have done. Putting +5v on the other side of the switch won't work because the ram and cpu will then never be able to drive the bus. I wonder how Mr Kirkpatrick solved the problem - unless there were pullups on the underside?!
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Old 13th May 2020, 9:52 pm   #103
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Default Re: Recreating the Bywood Scrumpi

Good point unless they are three position switches not changeover... that would be a pain though for me as I would have to swap them.
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Old 13th May 2020, 9:54 pm   #104
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Default Re: Recreating the Bywood Scrumpi

I don’t understand your remarks
corrected my plan to have a unique base I start the pcb and I do understand your piece of plan slothie a raison protect does nothing in your diagram have you looked at both pdf?
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Old 13th May 2020, 9:56 pm   #105
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Default Re: Recreating the Bywood Scrumpi

We have been wondering what R8 does - assuming that it is something to do with the steppe - I wonder if it is a single pullup for the switches - cant see how that would work though as all would get pulled up?
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Old 13th May 2020, 9:58 pm   #106
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Default Re: Recreating the Bywood Scrumpi

for the data pulup it is possible with a resistance on each data between swich but the original sense of scrumpi n is more
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Old 13th May 2020, 10:02 pm   #107
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Default Re: Recreating the Bywood Scrumpi

R8 is the pulup I think a direct draw on vcc of R/H R11 that of the reset (can like the PIC mclr)
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Old 13th May 2020, 10:03 pm   #108
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STEP is fired direct vcc without pulup
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Old 13th May 2020, 10:14 pm   #109
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Default Re: Recreating the Bywood Scrumpi

Quote:
Originally Posted by philoupat83 View Post
I don’t understand your remarks
corrected my plan to have a unique base I start the pcb and I do understand your piece of plan slothie a raison protect does nothing in your diagram have you looked at both pdf?
It is not correct - I made the change in error please ignore - I have it wired as in the original in the article and it works fine.

The problem we are discussing is how to make a 1 on the data bus to write into RAM as all we can do as it is now is make them a zero.
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Old 13th May 2020, 10:16 pm   #110
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Default Re: Recreating the Bywood Scrumpi

Sadly one pull up won't work because it would short all the data lines (see sketch).
In fact since the instructions state the switches should be all '1' when running the computer that clearly implies that the data lines 'float' when in the '1' position, which would require some kind of {weak} pull up - is anything connected to the bus likely to do this (e.g. the LED buffers)?
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Old 13th May 2020, 10:19 pm   #111
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Default Re: Recreating the Bywood Scrumpi

Oops forgot the sketch!

EDIT: I can't upload it because the image is too big and I have no way to reduce the resolution but I'm sure you get the idea.

Last edited by Slothie; 13th May 2020 at 10:28 pm.
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Old 13th May 2020, 10:27 pm   #112
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Default Re: Recreating the Bywood Scrumpi

Quote:
Originally Posted by Slothie View Post
Sadly one pull up won't work because it would short all the data lines (see sketch).
In fact since the instructions state the switches should be all '1' when running the computer that clearly implies that the data lines 'float' when in the '1' position, which would require some kind of {weak} pull up - is anything connected to the bus likely to do this (e.g. the LED buffers)?
Agreed.

I have around 2V on the lines when in Tri State so something is creating a weak pull up - enough to activate the LED's - remember mine are inverted so come on when the line is 0 which is handy at the moment. D7 definitely goes high so I think the Processor is halting on the first 0 instruction. NHOLD can be released this way but of course it goes nowhere - I would expect the address lines to count but, I wonder if my Capacitor is not allowing the SC/MP to Oscillate.

I have tested the CPU and RAM in my MK14 so I know they are good.
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Old 13th May 2020, 10:34 pm   #113
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Default Re: Recreating the Bywood Scrumpi

Hmm perhaps the SC/MP 2 doesn't oscillate with just a capacitor? The Elektor article implies the oscillator circuit changed in the revision because the circuit for Crystal oscillator is more complex.
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Old 13th May 2020, 10:41 pm   #114
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Default Re: Recreating the Bywood Scrumpi

Well I am certainly getting the correct state of NHOLD low with D7 at 1 which indicates a BreakPoint has occurred so the CPU must be doing something - maybe I have an error in my wiring for the stepper and the run/halt which means I cannot move onto the next address which is also likely to be zero and be a halt but, at least I should see the address lines count in binary.
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Old 13th May 2020, 10:43 pm   #115
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Default Re: Recreating the Bywood Scrumpi

I have corrected the plan for the 74c173 (my error)
for the data made of the tests we note a single rail in the center of the switches (gnd)
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Old 13th May 2020, 10:47 pm   #116
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Default Re: Recreating the Bywood Scrumpi

ok bravo super finalised your tests to have a definitive plan we get there thank you we will bring back this piece of history of the pioneers
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Old 13th May 2020, 10:58 pm   #117
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Default Re: Recreating the Bywood Scrumpi

Yes - I am getting tired now so will look again tomorrow morning and in the evening for a wiring error. I think we are close to having a running machine to prove the wiring and therefore the layout. I had tested the board without a CPU using resistors and wires to simulate the signals like NADS, D7 etc. so I am not sure where the error is - maybe a short or something. I also need to finish wiring up A10-A6 LED's as the last CMOS buffer is going nuts with all the floating inputs... A11 looks pretty but, it does not tell me much...

I note a RED line on your IMG021 - that must not be there as we have established - how we are going to make a 1 on the bus is to be determined - if needed I will add 7 pullups to my board until we discover how it was done!
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Old 13th May 2020, 10:59 pm   #118
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Default Re: Recreating the Bywood Scrumpi

By the way nice drawing and neat provision of small links to allow use of SC/MP I or II!
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Old 13th May 2020, 11:03 pm   #119
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thank you good night I work tomorrow
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Old 14th May 2020, 9:39 pm   #120
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Default Re: Recreating the Bywood Scrumpi

Well - I think after a whole day of fiddling - finding a faulty 74LS00 and then having the next one in the tube also faulty without me thinking it would be - so I tested the tube and now have three dead from the 10!

Anyway that may be why the RC network I built did not get the machine running but, I had already unsoldered it. The II needs a pair of resistors and caps not just a single one - we will have to add some pads for that and a crystal / rc net which is what I used in the end and now have a clean 4Meg clock (only crystal I had and perhaps a tad fast for such a lash up board).

Anyway now it seems to run through every address - that is after I realised that the RUN/HALT switch was faulty and had to swap it - breaking another in the process...

You can get it to count with the address lines showing a binary count up 000001 000010 000011 etc! I have not wired the higher address lines to LED yet. The scope shows the CE on the RAM oscillating nicely and if you zero all the switches and hold LOAD then reset the count is clean as I assume sometimes something other 0 could be in RAM.

Now we just need to work out how to write a 1 into memory so I can see if I can program instructions in. Also you need a good earth or it jitters!
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