PET 4016 Upgrade options?

[Eudimorphodon]

ETA: Frustration! There aren't any unpopulated patterns on the board, drilled or not. So it looks like I'm limited to a piggyback hack (ugly, I agree) unless I want to buy a 3rd party upgrade board. So that kinda sucks.. and is likely why I've never done anything with it previously.

Any chance you could take a close-up picture of the RAM chip banks? Like I said earlier, I'm kind of surprised Commodore actually made machines with two banks of 4108 chips. (The 4108 isn't actually a "real" part, they were a way to sell 4116 16K chips that had an error in one half of the memory matrix but otherwise worked. There were even two SKUs for them depending on which half was bad, because it affected the RAS/CAS signal generation.)

(Just to sanity check, your machine actually says it's a 16K machine on the BASIC banner, right? It's not possible that someone upgraded or swapped the board for a 32K one already?)

As noted, if it is two banks of 4108s your options are an expansion board or unsoldering all the 4108s and replacing them with 4116s. The latter is straightforward enough (you just need to change a couple solder jumpers to let the multiplexing circuitry know about the change), but understandably not a thing you might be enthusiastic about doing. If you wanted to DIY your own expansion board it actually would be really trivial to make a 16K expansion board that hangs off those internal pin jumpers. Looking at the schematics I think it'd be easy to do with just two chips; so far as I can see all you'd need is a 16K or bigger SRAM, like these dirt common 32K ones, and something to decode it; a single 74LS00 NAND gate would do the trick. This would simply expand your RAM to 32K while continuing to use the onboard RAM for the lower 16K, not replace it like those fancier CPU socket boards.

 

[WimWalther]

I won't be able to get any photos until I have help. The machine is on a high shelf and I'm too disabled to safely move it..

But IIRC, there's only a single bank of 8 RAM chips in there. Did I lead you to believe otherwise?

 

[Eudimorphodon]

But IIRC, there's only a single bank of 8 RAM chips in there. Did I lead you to believe otherwise?

You said there "are not any unpopulated patterns on the board, drilled or not". All PET motherboards have 16 sockets for RAM chips, so if your machine has 16k in 8 chips it should have a big blank spot? (Drilled or not.)

 

[Hugo Holden]

Looking at the drilled out boards I think there is one way that does not involve having to repair the board at all.

Although if I was fixing those damaged pcb's, I would make custom pcb inserts (discs) to fill the holes and replace the tracks, but it would be costly.

Or make a thin pcb overlay on both sides of the board to fix it that way, replicating the original track-work and conceal the damage.

The repair with the kynar or wire wrap wire is ok, but it is not as tidy as I would prefer.

The other way that will work is to leave the drilled pcb just as it is and call it a "historical record" of very bad judgement and stupidity by Commodore (somewhat analogous to a reject stamp I found inside a PET VDU) and instead electrically deactivate the entire DRAM bank , for reads alone, over the address range $0000 to $7FFF, by simply pulling pin 11 of IC B2 low (TP3) from an address decoder. Then from the same address decoder that does that, activate SRAM on a card that plugs onto the expansion connector. I am making a pcb that does this which will be finished in the next few months.

 

[Hutch]

But IIRC, there's only a single bank of 8 RAM chips in there. Did I lead you to believe otherwise?

You did. By saying "There aren't any unpopulated patterns on the board" you indicated you had looked at the board to make that determination.

FYI, There are ALWAYS 16 RAM chips on all versions of the PET, or at least places on the PCB for them, as shown in the previous photos.
Early boards had two rows of 8 chips and later universal boards have a single row of 16 interleaved.
These are usually 16K by 1 bit each for a total of 32K.
The early 4016 & 2001-N can have one row populated with 16K chips leaving one row empty,
or it can have both rows populated with 8K chips leaving no empty slots.

 

[Hutch]

Sorry, what I meant to say was that it's not possible to have only a single bank of 8 RAM chips AND have no unpopulated patterns on the board.
There has to be PCB patterns for 16 RAM chips total.

 

[Eudimorphodon]

Sorry, what I meant to say was that it's not possible to have only a single bank of 8 RAM chips AND have no unpopulated patterns on the board.

Which, yeah, is what I said as well. If the board doesn’t have a set of unpopulated footprints then it’s either 16k in two banks of 8k 4108s (which, like I said, I’d be interested in seeing a photo of) or maybe your 4016 already has 32k in it from a previous upgrade or board swap.

 

[WimWalther]

Due to the machine's present location I can't shoot any pics of the board. Can't tell you how frustrating this is.. not being able to move it!

Because yeah, I've peeked at the board several times and all I can see is one line of 8 identical chips which I take to be the RAM bank. But I can't even read the markings on them, so who knows.

 

[WimWalther]

The PET is finally somewhere that it can be looked at. The board must be a later version, it's marked UNIVERSAL DYNAMIC PET ASSY. 8032089. On the left of the board, running next to the TO-3 heatsink are (8) 4116 DRAM chips dated 2181 (May 1981?) giving 16K RAM.

The row of 4116s actually has 16 chip patterns, with every-other one populated. So it appears that another (8) 4116 can be installed, giving a total of 32K RAM.

I haven't powered it yet. Need to get a Variac hooked up, as it's been at least a decade.

 

[WimWalther]

 

[Eudimorphodon]

Backtracking to earlier discussion… now that we see this is a Universal board, is this in a case with a 12” or 9” monitor? The latter would be very rare, probably only to be found in a refurbished/repaired machine.

This is also the board that should make a “chirpchirpchirp” sound on power-up. (That disk-shaped thing in the lower right on the board is a piezo speaker.) You said earlier it didn’t have sound? This is why we were assuming you had a 9” one with the older motherboard.

 

[WimWalther]

I'd only assumed that it lacked a speaker being that's the case with the older boards. Again, it's been a very long time since I've looked at this machine.

So upping the RAM to 32K is just a matter of filling the empty patterns in the bank + configuring jumpers? Any 4116 that are 150nS or faster should do?

And yes, I believe it has the 12" screen. The monitor section takes up the whole width of the case.

 

[Hutch]

I'd only assumed that it lacked a speaker being that's the case with the older boards. Again, it's been a very long time since I've looked at this machine.

So upping the RAM to 32K is just a matter of filling the empty patterns in the bank + configuring jumpers? Any 4116 that are 150nS or faster should do?

And yes, I believe it has the 12" screen. The monitor section takes up the whole width of the case.

Just fill in the missing chips. It will auto detect. no jumpers required. Can confirm 150ns chips works. Not sure if slower chips will work.
It has to have a 12 inch screen. The universal boards are all CRTC based with 12 inch screens at 20Khz

 

[Hutch]

As for upgrade options, ...
The universal board can be converted to 80 columns by adding video RAM and changing a few jumpers.
There's a project that allows you to easily switch between 40 and 80 column mode.

CBM 40/80 Switcher Project

 

[WimWalther]

As for upgrade options, ...
The universal board can be converted to 80 columns by adding video RAM and changing a few jumpers.
There's a project that allows you to easily switch between 40 and 80 column mode.

CBM 40/80 Switcher Project

That's interesting, thanks for the link. Is the monitor hardware on the 4016 the same as the 80XX models? I mean, it's up to the task of 80 col. display?

Why exactly did they produce both 40 & 80 col. versions? Was it only a matter of cost savings of a few RAM chips?

ETA: Right now the best deal on eBay for 4116 150ns chips is item no. 224949888401

Genuine NOS TI parts, still in the original TI sticks - 10 pcs. for $16.00 shipped to the USA. About $1.60/ea. Seller has very solid FB.

 

[Eudimorphodon]

The universal boards are all CRTC based with 12 inch screens at 20Khz

There is a special EDIT ROM available for using a Universal board with a 9” display. (The CRTC is capable of being programmed appropriately.) There are images of it in the usual places and it has an official Commodore part number. But like I said, so far as I know it was only for repairs/upgrades, I don’t think they actually sold new 9” PETs with Universal boards.

(But Commodore was also notorious for slapping together machines out of whatever parts they had around, especially for education and other bulk orders, so it’s hard to say it ‘never’ happened.)

Why exactly did they produce both 40 & 80 col. versions? Was it only a matter of cost savings of a few RAM chips?

There are significant compatibility issues between the 40 and 80 column PETs. (in addition to the screen mapping being different the keyboard matrix is completely different, breaking compatibility with a lot of the non-BASIC software base.) So Commodore mostly treated them as two separate lines. (The 80 column machines were targeted as “business” computers while the 40 column ones became more the home/educational models.)

 

[WimWalther]

There are significant compatibility issues between the 40 and 80 column PETs. (in addition to the screen mapping being different the keyboard matrix is completely different, breaking compatibility with a lot of the non-BASIC software base.)

Screen mapping I can mostly understand, due to the 80 col. requiring twice the video memory.. but the keyboard matrix? I'm sure it makes some sense if you know the architecture, but otherwise that just seems weird.

So I have the 40/80 Switcher board Gerbers uploaded to JLC. Guess I'll order up a small batch unless someone here feels like sending me one. ;-)

 

[Hutch]

That's interesting, thanks for the link. Is the monitor hardware on the 4016 the same as the 80XX models? I mean, it's up to the task of 80 col. display?

Yes. The video frequencies and sync signals are the same. The 12" monitor is identical on both versions.

Why exactly did they produce both 40 & 80 col. versions? Was it only a matter of cost savings of a few RAM chips?

One board was cheaper to make than two,

ETA: Right now the best deal on eBay for 4116 150ns chips is item no. 224949888401
Genuine NOS TI parts, still in the original TI sticks - 10 pcs. for $16.00 shipped to the USA. About $1.60/ea. Seller has very solid FB.

I usually get mine from Jameco

 

[WimWalther]

I usually get mine from Jameco

Jameco.. It's one of those funny things. I've dealt with them in the past, at a previous employer.. but for some odd reason I always overlook them when I'm parts shopping for my own stuff.

Next time, I'll try to remember!

 

[WimWalther]

There's a project that allows you to easily switch between 40 and 80 column mode.

CBM 40/80 Switcher Project

Something is escaping me here. I'm looking over this mod, but I cant find anything about the need to increase the video RAM on a 40 col. machine. I suppose that it's not a requirement if you're modifying an 80 col. machine - but in the case of a 40 col. it's a different story.

I'd initially assumed that the additional RAM would be on the daughterboard, but that only holds logic chips. Could someone please supply additional details on this?