I am currently looking at options for how and where to sell my products (suggestions welcome).
As part of that, I have been checking stock, and I seem to be running out of everything. All the main kits have been hit by parts being discontinued.
The Mini PET kits need dual port RAM chips and microcontrollers that have been discontinued. I now only have a few of those left.
The Minstrel 2/3/4th kits need Z80s, which have also been discontinued, so they will only last until I run out of Z80s.
Whenever I run out of anything I inevitably get emails along the line of "I just missed out on getting an X, when will they be back?" Don't be that person. I have been selling these for nearly 10 years now, don't say you haven't had a chance.
The Mini PET would need a radical redesign to get around missing parts, but I don't think there is much I can do with the Minstrel kits when the Z80s run out. Maybe a prebuilt eZ80 based version, although I don't think it will be compatible enough to work. I should investigate that at some point.
I am also running out of PCBs, but there doesn't seem any point in ordering any more as I don't have the parts to make more kits anyway.
I was looking around to see what I had left, and found another pack of boards.
Great, they are Mini PET B boards. There was only one of those left.
Oh, I must have ordered a new set with the new style logo. Don't remember doing that.
Oh right, so these are not Mini PET B boards, which are 40 column only, these are Mini PET 40/80 Internal.
I think I vaguely remember that.
(at this point I was planning to insert some history, but it's all a bit cloudy, and well, wait and see, I will put that in later)
Looking over the board, the left hand side, the PET address decoding and IO chips are the same (I am working on a post about PET address decoding, coming soon, lots of diagrams to draw).
The changes are on the right.
Things look the same, but there are some part changes and quite a few traces are re-routed.
The 74HC74 has been replaced by a 74HC257, and the 7130 1K dual port RAM is now a 7132 2K dual port RAM.
The contents of the system ROM, character ROM and the micrcontroller are different though. The microcontroller was completely rewritten for the 40/80 systems.
The character ROM is also different as the original was like the PET, 128 characters, with hardware inversion. To save a flip flop and because the character ROM is mostly empty, I changed to storing the inverted characters in ROM as well, so all 256 could be selected directly from ROM, which simplified the video output circuitry. (avoiding the need for the character invert flip flop and the pixel synchronisation flip flop, since all pixels were generated in the same way at the same time)
I also seem to have moved the DIP switches, Ready LED, piezo and monitor connector, and also relocated a 1K resistor. I can't resist tweaking things.
I think I had better build one up.
I had to steal most of the parts from one of the other boards to try it out.
Time to give it a go.
Yay, it works!
Time to try out the party piece, 80 column mode. Unlike the stand alone 40/80, in this case I think 80 column mode is set via a DIP switch.
It didn't work.
I tried various ROM sets and DIP switch options, but it was firmly stuck in 40 column mode.
I dug out the schematic to find out what was going on.
The ROM images on the Mini PET are all 32K. There are 4, 8 or 16 of these are combined together to make the Mini PET ROM, with the appropriate image selected by the top address bits of the ROM chip.
On the normal Mini PET 40/80 with a built in normal / graphics keyboard, there are two DIP switches to select the ROM set, and the 40/80 mode is the third ROM select, giving a choice of 8 images. Seems sensible since 40 column ROM sets won't work in 80 column mode etc.
For the "B" models that fit in a PET case, there needs to be another 8 ROM images, the same as the first 8 but with the editor ROMs for business keyboards.
That extra address line means I would have to use a 27C080 ROM chip rather than the normal 27C040. (dollar signs spin around my eyes at this point, specially the prices these days).
However for testing I can use the normal ROM as the A18 pin is not connected on the normal 27C040 ROM, so will be ignored and I will just have to set switch 3 to match 40 or 80 column mode.
So why didn't it work?
Then I spotted the problem.
Can you see it?
Well, there are two sorts of switch arrangement used there. Look at switch #1, that is driving a ROM address line. When off, the resistor array pulls that down to 0V. When on, it is switched to 5V, giving the desired 0 or 1 for the ROM address.
The second type feed the microcontroller to set the video options. Look at switch #7 for example. The microcontroller inputs have an internal pullup, which pulls them high normally, and are switched to 0V.
Did you spot the problem yet?
It looks like I was in the process of changing things around. I suspect I may have been moving 40/80 column mode to switch #10, or to switch #6. Either way, I it is only half done. Switch #10 (video option 4) is pulled up to 5V, or can be switched to 5V, so will always read a 1. Switch #6 (40/80 column) is pulled down to 0V, or can be switched to 0V, so will always read a 0.
I probably had a phone call or a knock at the door at just the wrong moment and must have forgotten where I was up to when I got back to it.
A quick bodge and it's functional. I have disconnected the pull down on the 40/80 line and solder-blobbed it to ROM_A17 on the ROM, so switch 3 now controls ROM A17 and 40/80 mode, just as it does on the normal Mini PET 40/80.
I should also fit a wire from switch 10 to one of the spare pull down resistors.
And there we have functional 40/80 column mode, and it can also be switched back to 40 columns with the DIP switches.
Well, at least it working, although it means I can't sell these boards as a kit (not that I have the parts to do that anyway).
Maybe I can list this as a one off prototype - Anyone want to buy a Mini PET Internal 40/80 ?
Now, I promised a history section, here it is.
When the Mini PET started, there was only one version. The same board could be built for PET case or bench with a slightly different set of parts. Later I got some boards made in green to make the distinction clearer.
Later still the green board was expanded to fit better in the PET case and add in support for the PET power supply (which was original a separate board).
When I was looking for that photo (I had called it A+B, so it didn't come up in the search for Mini PET), I found this photo.
It was at this point that I noticed the writing on the larger board.
Hang on, that's one of the ones I had just built?
So I did build one of these before?
And then I found another photo, this time with the 40/80D, the pre-assembled version with the built in SD2PET.
I think that explains why I did not progress with these boards.
When TFW8b was selling the Mini PET 40/80 and the Mini PET 40/80D, I had agreed to stop selling the Mini PET kits so as not to compete with the TFW8b versions, and I only brought them back after TFW8b sold out and decided not to do anymore.
So I built one of those back in 2020 or 2021, although I have no idea what happened to it.
It worries me when I look through at photos from only a few years ago and find projects I had forgotten I had built.....
... oh, OK, yes, I guess I must have done an "A" version as well.
That I think is an earlier prototype as it has 5 chips on the right, so presumably still has the hardware character inversion I talked about earlier. (and the older logo)
I do remember doing a board with a slide power switch (I didn't like it).
I think that was what led to the soft power on functionality of the production 40/80 kits.
Seems like I am running out of everything at the moment, even memories.....
Great chunks of my past, detaching themselves like melting icebergs......
I am being diminished. Whittled away, piece by piece.
A man is the sum of his memories, you know. A Time Lord even more so.
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I think I still have everything in stock, but I am down to only few of each type of kit left, some are the last ones.
The SellMyRetro store is also on it's last days, everything is still listed there if you need more information, but best to use the contact me link about, tell me what you want and where you are and I will send a PayPal invoice. Sorry I have to keep saying that. I am working on an alternative.
All the links can be found here:
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