Commodore PET 4016

[SiriusHardware]

And that also has me beginning to suspect a dodgy connection to UA6; most probably pin 14 (Data out) or pin 15 (CAS).

Hmm, looks to me as though Colin is testing known faulty RAMs in the UA6 position, as a way of testing the various test utilities he has.

If he puts a known good device in that position, no fault is reported? (As below). That doesn't fit with there being a problem with the UA6 socket or its connections or signals.

OK - UA18, UA16, UA14, UA12, UA6 and UA4 all replaced and the PET now reports 32k if I boot it up, the Commodore Diagnostic Clip reports 32K as does PETTESTER v4. The Tynemouth Diagnostic board now reports no errors.

[ScottishColin]

Just to clarify, I was testing probable duff memory chips in UA6 and they all failed with the same PETTESTER message.

Once a good chip was inserted, PETTESTER passed with no errors.

A test with no chip at all in UA6 gave exactly the same error message.

Toepost v12 screenshots to follow later on.

Colin.

[julie_m]

For anyone interested (maybe you followed a link here from vcfed.org) and for GPL compliance purposes, here are the changes I made to PETTESTER to force it to test the full 32KB:

--- PETTESTE2KV04.a65	2025-06-03 19:54:12.026772340 +0100
+++ PETTESTE2KV04-JM1.a65	2025-06-03 20:05:24.140965178 +0100
@@ -1030,6 +1030,8 @@
 	
 	lda	#$00		; Start of DRAM to test (low byte).
 	sta	STARTLL		; "   "   "   "   "   "   "   "   "
+
+	; Bodged to force 32K
 	
 	; Find top of ram. Assume either 4K, 8K, 16K or 32K.
 	
@@ -1037,11 +1039,13 @@
 	sta	ENDLL
 	sta	ADDRLL
 	
-	lda	#$0F		; Assume 4K (0FFF) to start with.
+	; lda	#$0F		; Assume 4K (0FFF) to start with.
+	lda	#$7F		; Assume 32K (7FFF) to start with.
 	sta	ENDHH
 	sta	ADDRHH
 	
-	lda	#$04		; 4K
+	; lda	#$04		; 4K
+	lda	#$32		; 32K
 	sta	MEMSIZE
 	sta	TEMPA
 	
@@ -1051,8 +1055,13 @@
 MEMSIZEa:
 
 	lda	#tst55
-	sta	(ADDRLL),y
-	cmp	(ADDRLL),y
+	; sta	(ADDRLL),y
+	nop			; Occupy same space as deleted instruction
+	nop
+	; cmp	(ADDRLL),y
+	nop			; Occupy same space as deleted instruction
+	nop
+	; Now this will definitely branch
 	bne	MEMSIZEb
 	
 	lda	#tstAA

It initialises the relevant locations with a fixed memory size of 32K bytes, NOPs out the first part of the test to preserve the code size and forces a branch to MEMSIZEb as though it had found the first non-existent location at &FFFF, with the last good address being &3FFF and the memory size set to 32K.

[ScottishColin]

Couple of screenshots from v12 attached with the failed RAM chip back in UA6.

I'm interested in the second screenshot showing (to me at least unless I've misunderstood it) that there are failures in D0 and D6? This doesn't match up to my vie that only D6 (UA6) is at fault?

Also, are there any other screenshots that may be of use for documentation purposes?

Colin.

[julie_m]

The second picture is saying that it wrote &01 but read back &41, so bit 0 is actually working correctly and bit 6 is faulty -- I think I need to tweak the error reporting to make the "write-and-read-back" test highlight only the difference between what it actually read and what it was expecting to read, in the bit map.

By the way, i tried to sign up on vcfed.org, but never received the confirmation e-mail.

[SiriusHardware]

By the way, i tried to sign up on vcfed.org, but never received the confirmation e-mail.

Has it possibly just been diverted into your 'junk' folder?

It's a useful forum, quite USA-centric but with quite a few European, including British, members.

[ScottishColin]

I had assumed that the inverse 76543210 in the attached screenshot reversed for each individual bit that is in error?

This one shows that 6 and 0 are not inverse so I assumed that they were both showing as faulty?

Colin.

Originally Posted by julie_m

The second picture is saying that it wrote &01 but read back &41, so bit 0 is actually working correctly and bit 6 is faulty -- I think I need to tweak the error reporting to make the "write-and-read-back" test highlight only the difference between what it actually read and what it was expecting to read, in the bit map.

By the way, i tried to sign up on vcfed.org, but never received the confirmation e-mail.

[julie_m]

Originally Posted by SiriusHardware

Has it possibly just been diverted into your 'junk' folder?

No -- I have full ssh access to the IMAP / SMTP server (perk of the job ), and nothing was received addressed to the address I supplied.

I tried again this morning with a different username and a Gmail address and got nothing there, either. Nor in the spam folder.

[julie_m]

Originally Posted by ScottishColin

I had assumed that the inverse 76543210 in the attached screenshot reversed for each individual bit that is in error?

This one shows that 6 and 0 are not inverse so I assumed that they were both showing as faulty?

Yes, I can see how that might be a bit misleading. That's indicating all the bits within the value that was read back, including the one that was actually working.

I've already found a simple change to make so only the bits that are actually faulty will get marked, so I'll be able to put up a new version tonight.

[ScottishColin]

Thanks. And this will be my misunderstanding again - could you explain why there are two sets of high/low 7654210s next to each other?

Colin.

Originally Posted by julie_m

Yes, I can see how that might be a bit misleading. That's indicating all the bits within the value that was read back, including the one that was actually working.

I've already found a simple change to make so only the bits that are actually faulty will get marked, so I'll be able to put up a new version tonight.

[julie_m]

Originally Posted by ScottishColin

Thanks. And this will be my misunderstanding again - could you explain why there are two sets of high/low 7654210s next to each other?

Don't worry about asking questions! I'll be able to copy the answers I'm giving you straight into the ToePost documentation anyway

The left-hand set are for Write/read back errors (i.e., the value read back from the written location differed from what was written there) and reFresh errors (i.e., a non-zero value was read from a previously-zeroed location after a delay long enough to cause errors in memory not being regularly refreshed). The right-hand set are for Read errors (i.e., a non-zero value was read from a location that should have contained zero).

The thinking behind this was, it's possible for different things to be failing in different ways; and separating out the failure indications makes one fault less likely to end up disguising another.

[ScottishColin]

3 out of the 5 ROMs had to be replaced but once that was done it's now up and running under its own steam.

I replaced the BC337s with a B325d and the datasette ports work fine now too.

I'd list what I have replaced but it's a long list. All 40 pin chips (with the exception of the 46505), 3 out of 5 ROMs, most of the RAM, most of the 244s and both video RAMs. Strangely the 3446s which control the IEEE port are fine which doesn't happen often.

We've discovered more about PETTESTER and how to force it to run a 32k check when you know the memory is installed but it can't see it. We're well on the way to some brand new testing firmware which is great. I've discovered that while this motherboard broadly fits the schematics that are publicaly available, it doesn't in terms of the factory mods made to the board which are documented nowhere.

Continue to test toepost and also work on the cosmetics is what remains.

This one needs a new label applied, the base stripped down, sanded, de-rusted and resprayed. The main cream case is not metal so a thorough wash of that should do fine, and the same for the monitor case. There is a metal 'base' between the monitor and the case which needs stripping down, de-rusting and respraying though.

Once that's done, a small rest and onto the advanced graphics board and the 4040 which came with it.

Colin.

[julie_m]

Here's my altered version of ToePost which now indicates the bits that actually read incorrectly, as opposed to the bits that read "1" (one of which should read "1", because we deliberately set it).

testrom_13.zip

I think this behaviour actually feels more correct. It's good to have an extra pair of eyes on something, because knowing exactly how something works can lead you to miss things that aren't obvious to everyone .....

[ScottishColin]

I'm still here - the PET is in bits though for cleaning/spraying so it'll be a few days before I get power back into it.

Colin.

[ScottishColin]

Before and after.

The 4016 label isn't as long as the original, but it's the only one I could find so I'm OK with that.

Onto v13 of toepost now.

Colin.

[ScottishColin]

Julie - screenhots in the link below showing a successful test on for 4016 with 32k of known good RAM.

https://drive.google.com/drive/folders/15LFqvkULoA55doVwUv_gqqErgiGtjx-4?usp=drive_link

Colin.

[SiriusHardware]

Great job - it would be interesting to know what the price difference is between a shabby, broken PET and a working, clean looking PET.

I suspect probably not enough to pay you back for the parts and especially the time you spend on each one. I was thinking initially that you are now so confident with these that you could conceivably make a little bit of money buying them, fixing them up and selling them on - but thinking about it, I'm not so sure.

[ScottishColin]

Julie - screenshots in the link below showing a failed test on the 4016 (UA6 failed).

https://drive.google.com/drive/folders/1Bp2_VcDRyijteFXDit_hV4lk1JAa2tgJ?usp=sharing

Please let me know if there's anything else you would like.

Colin.

[ScottishColin]

I guess ebay is a reasonable place to start with for broken PETs, although I have had a few of mine for free via Mastodon.

Generally, I haven't had to pay more than £100 for a PET so far (this one's the seventh....). I view them like my Dad does with his O Gauge railway engines - he has plenty of them but if he sees one that he likes, then he buys that to add to the collection.

Even so, I recognise that seven is quite a few to have.

Colin.

Originally Posted by SiriusHardware

Great job - it would be interesting to know what the price difference is between a shabby, broken PET and a working, clean looking PET.

I suspect probably not enough to pay you back for the parts and especially the time you spend on each one. I was thinking initially that you are now so confident with these that you could conceivably make a little bit of money buying them, fixing them up and selling them on - but thinking about it, I'm not so sure.

[julie_m]

Originally Posted by ScottishColin

Julie - screenshots in the link below showing a failed test on the 4016 (UA6 failed).

https://drive.google.com/drive/folders/1Bp2_VcDRyijteFXDit_hV4lk1JAa2tgJ?usp=sharing

Please let me know if there's anything else you would like.

Thanks very much -- those will be enough to get going for now. Though if you get a chance, it would be great to see what happens if you deliberately cause a refresh fault by removing UE6.

Also, did you manage to test out the reprogramming trick I built into ToePost?