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PET 2001 - No cursor, questionable IEEE

P

Phu

Member
Joined
Apr 24, 2009
Messages
21
Hi,

Back again with another PET, this time a 2001 model with caculator keyboard and built-in tape deck.

For the most part, it seems to be working. When it first arrived there were RAM issues, but a few replacement 2114s sorted that out. But, I have no cursor. The keyboard works, however... and if you could see the cursor it would be moving around the screen in response to the arrow keys... it's just not on screen.

At the same (and possibly related) the IEEE port is "questionable". In so far as trying to load from the disk gives "Searching", then a pause, then "Loading", then.... nothing. The loading stage never ends.

I read elsewhere here that the 6520s might be to blame for this, so I tried cycling the chips with a third working chip. All possible combinations made no difference. I also tried a replacement 6502 CPU and 6522 VIA.

Can any of the PET gurus here offer some tips as to what to check next?

-- Richard
 
Richard,

I had this problem with a 6520. It wasn't the chip it was socket. The latter was damaged. See the second part of this article, especially the paragraph before figure 4.

Tez
 
The cursor problem is interesting. So the keyboard works and scans normally but there's no visible cursor? If you arrow it around is *anything* drawn over the location where the cursor should be, like a black square instead of a white one?

(Does the PET have a hardware cursor or does the CPU update the cursor location in memory with a white box? I sort of thought it was the latter.)

Just for fun, what happens when you run this?

10 for X=0 to 255
20 poke 32768+X,X
30 next X

Is the second set of alphanumeric characters that appears in reverse video?
 
Nothing appears over existing characters when moving the "cursor" around.

I like your thinking on the inverse character effect... if I remember right that could be a faulty 7486 XOR gate?

I'll run that program when I get in tonight, and let you know the results.

Thanks

-- Richard
 
tezza said:
Richard,

I had this problem with a 6520. It wasn't the chip it was socket. The latter was damaged. See the second part of this article, especially the paragraph before figure 4.

Tez
Click to expand...

I read through that, but I don't think this is the case, as the keyboard is working fine. However, I will keep it in mind and will do a basic voltage check later.

-- Richard
 
Phu said:
I like your thinking on the inverse character effect... if I remember right that could be a faulty 7486 XOR gate?

I'll run that program when I get in tonight, and let you know the results.
Click to expand...

Yeah, for some reason I'd brain-locked and couldn't remember if the PET had a flashing square cursor or just a reverse video block when I posted earlier. Looks like it is reverse video, so... sort of like the chances it being something wrong there. I guess we'll find out.
 
What version of BASIC do you have, i.e. what are the numbers on the ROMs? The IEEE routines did not work very well in the first version (*** COMMODORE BASIC ***).
 
Is the tape drive actually functioning/motors moving? I had a rather simple brain fart on an 8032 with an external drive by not remembering the ground wire needing to be connected to the case somewhere for the drive to function, but that was the motor starting then stopping.
 
barythrin said:
Is the tape drive actually functioning/motors moving? I had a rather simple brain fart on an 8032 with an external drive by not remembering the ground wire needing to be connected to the case somewhere for the drive to function, but that was the motor starting then stopping.
Click to expand...
Something wrong then; the external 'ground' is just for shielding and whether it's connected or not shouldn't affect tape operation. But hey, if it works...
 
Ok, the results are in:

Eudimorphodon: that program shows two characeter sets, no inversion. So look like the inverter circuit is screwed. I looked at the circuit diagram, but I can't quite work out how this is achieved.

MikeS: Can't find a definitive version, but yes it shows the *** COMMODORE BASIC *** banner on startup. The roms are 901447, if that's any use to you.

barythrin: not sure where you're going with that one? But to answer your question, worn drive belt aside, yes the tape deck works. But it was accessing a disk drive over the IEEE port that was an issue, not the tape deck.

-- Richard
 
Phu said:
Eudimorphodon: that program shows two characeter sets, no inversion. So look like the inverter circuit is screwed. I looked at the circuit diagram, but I can't quite work out how this is achieved.
Click to expand...

I'm not really great at this, but looking at this schematic (I'm not sure what board you have, but it looks like the SRAM PETs are mostly the same):

http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/320137-3.gif

It looks like the Data 7 output from the VRAM chips feeds into 74LS74, C1, pin 2, and... that IC is wired so it produces two complementary pulses which go into 74LS00 C2, and the state of those lines chooses whether the "Normal" or "Inverted" output from shift register B2, 74LS165 (output from pin 9 to input pin 2 or pin 7 to pin 5 respectively) is used. So... I'd trace all the lines and otherwise check out H1, C1, C2, and E2.

Your mileage may vary, that's all just a guess.
 
Eudimorphodon: Ah! But you point out what I could not see! In other words, part of that schematic wasn't drawn very well and confused the hell out of me. Once you explained it, I saw what I couldn't see before, and the likely chip stood out like a sore thumb.

Piggy backing a spare 7474 (or in fact in this case a 5474) onto C1 made the cursor appear! One quick desolder and replacement later, and we now have a flashing cursor :) Thank you :)

That just leaves the issue of the IEEE bus. Unfortunately, I don't have the disk drive with me to test any ideas, but if anyone comes out with some good hunches I'd love to hear them :)

-- Richard
 
Phu said:
MikeS: Can't find a definitive version, but yes it shows the *** COMMODORE BASIC *** banner on startup. The roms are 901447, if that's any use to you.
Click to expand...
That's version 1; IEEE disks weren't really supported until version 2, so if you plan to use the disk drive you'll have to upgrade. Fortunately you have the almost-standard 901447 (2316) ROMs instead of the more commonly used but harder to replace 901439 (6540) ROMs, so an upgrade should be fairly straightforward, and of course you could also use a PETvet to replace them.
http://en.wikipedia.org/wiki/Commodore_BASIC
 
Hi,

Just resurrecting an old post... it's been a while since I worked on this PET (it's still sitting on the floor as it was) because of other projects, but I have a bit of a head-scratch on the "Simply Upgrade BASIC" part. Replacing 6-odd 2316s with an EPROM and doing the rewiring I'm fine with... I can even read the schematic to work out which chip is which block of memory, it's the "wtf do I put in there" part I have issue with.

I'm aware that Commodores basically divide into Kernal and BASIC. But as I understand it, the IEEE functions are in the Kernal, correct? And the Kernal needs to be right for the machine.... so which ROMs do I need to combine to do this upgrade, and in what order?

I'm looking at the files here: http://www.zimmers.net/anonftp/pub/cbm/firmware/computers/pet/index.html and scratching my head in confusion.

-- Richard
 
MikeS said:
Need to know a bit more: which version BASIC are you planning to put into what kind/size EPROM?
And where are you?
Click to expand...

Not too bothered on the version, but according to earlier replies I have BASIC 1 which doesn't like IEEE disk drives. So whatever will run that satisfactorily. I have a preference to keep it as early as possible (for that retro feel) but otherwise I'm not bothered.

I have 16K EPROMs (27C128s) I can use, but I don't mind using multiple EPROMs or if necessary a bigger one.

I'm in Portsmouth, United Kingdom.

-- Richard
 
Phu said:
Not too bothered on the version, but according to earlier replies I have BASIC 1 which doesn't like IEEE disk drives. So whatever will run that satisfactorily. I have a preference to keep it as early as possible (for that retro feel) but otherwise I'm not bothered.

I have 16K EPROMs (27C128s) I can use, but I don't mind using multiple EPROMs or if necessary a bigger one.

I'm in Portsmouth, United Kingdom.

-- Richard
Click to expand...
I believe BASIC 4 causes some display instabilities on an original 2001 (and takes up 160kB instead of 128 ), so I'd probably stick with version 2; you could even use a '256 and have both version 1 and 2 BASIC selectable with a jumper or a switch. One of Jim Brain's adapters is great for this sort of thing; ISTR that all it took was about 7 jumpers and a couple of diodes to replace both sets with one '256.

BASIC2 (4kB images):
Cxxx:901465-01
Dxxx:901465-02
Exxx:901447-24 (only 2kB)
Fxxx:901465-03

BASIC1 (2kB images)
C000:901447-09
C800:901447-02
D000:901447-03
D800:901447-04
E000:901447-05
E800:
F000:901447-06
F800:901447-07

If you have the old CG chip (901439-08/901447-08 ) you may have to replace it with 901447-10

Double-check; I've been known to make the occasional error ;-)
 
Just to let you know, your sage advice was spot on :)

I had some fun with the SEL C - SEL F lines (4 diodes and a resistor to CE) and permitting the I/O "hole" at E800-EFFF (creatively wired up NAND gate to OE) but it all worked :)

I left the CG ROM in place as it seemed to be running fine as it was. (901447-08). IEEE port working fine now :)

-- Richard
 
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