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snuci
October 18th, 2018, 03:01 PM
I'd love to know if there are any other collectors out there with an MCM/70. Just thought some of you guys might appreciate a quick picture or two after unpacking it. It needs a good cleaning still. It is non-functional but I was expecting that. They are just not easy to find.

48860488614886248863

Slob
October 18th, 2018, 04:33 PM
Congrats! Super, Super rare.

If I may ask, did you luck into it or did you pay dearly :)

Do you intend to get it running. It's so old it probably has 1702/A UVEPROMS.

Chuck(G)
October 18th, 2018, 04:58 PM
Congratulations on owning a historic system! I envy you. How's your APL?

snuci
October 18th, 2018, 05:25 PM
Congratulations on owning a historic system! I envy you. How's your APL?

If I could get her working again, I'll let you know. I was worried there was a leaky battery in there that may have leaked all over the boards (for the battery backed shutdown) but it looks fine. They will have to go but it was a good design choice to separate them. I just didn't think there would be that many. Clearly, leaky batteries is not the issue.

48866

Chuck(G)
October 18th, 2018, 05:36 PM
I doubt that many of those made it out of Canada.

Looks like a Gates SLA battery made up of X cells 6x2V = 12V. The ones in your unit are probably dried out and sulfated beyond redemption. Fortunately, they're still made:

https://www.osibatteries.com/images/product/large/853.jpg

snuci
October 18th, 2018, 05:50 PM
I doubt that many of those made it out of Canada.

I actually got this from the original owner from Pennsylvania and it looks like he sold them given the documentation I have.


Looks like a Gates SLA battery made up of X cells 6x2V = 12V. The ones in your unit are probably dried out and sulfated beyond redemption. Fortunately, they're still made:


Thank you. I think you nailed it. Here's a quick pic. They are larger than D cells but not too much, in case anyone is wondering about the size.

48867

Chuck(G)
October 18th, 2018, 06:10 PM
On the previous photo you posted: here (http://www.vcfed.org/forum/attachment.php?attachmentid=48866&d=1539912285), you can just see the "Gates" peeking out from the rightmost cell. From the second photo of the battery you posted, I can see that you're in luck--many of these SLA packs used welded connections between the cells; yours are the "slip on" terminals.

falter
October 18th, 2018, 08:48 PM
Okay, now I'm jealous. Congrats!

How did you even begin to find someone with one of these, in the US no less?

snuci
October 19th, 2018, 03:49 AM
Congrats! Super, Super rare.

If I may ask, did you luck into it or did you pay dearly :)

Thanks Slob. Sorry, I didn't see your post initially. The thread notifications are slow and sometimes I don't get them if I am browsing the forum as was the case with your post.

Anyway, I lucked into it AND paid a reasonable sum. I was fortunate enough that the original owner reached out to me via email with two pictures and a description of the system and asked me for an offer. A little research showed that his name was on a list of original known installations (http://www.cse.yorku.ca/~zbigniew/mcm_add.html) noted under Chapter 8. In the past, I have made offers that would be "favorable to me" in terms of existing established value and sometimes they would work while other times it would result in no further communication. I did a little research but I could find no evidence of past sales for the MCM/70 so I didn't know what to offer. I could guess at a lower value and hope the original owner would bite but I could lose the opportunity and I didn't want to lose this opportunity as I am certain it would not come up again (I think the ones remaining are counted with single digits). So I made, what I thought, was a good reasonable offer based on 8008 computers. It was not low but was a little lower than what the seller originally wanted so we settled in the middle. Both of us are happy which is the key.



Do you intend to get it running. It's so old it probably has 1702/A UVEPROMS.

I will make every effort to get it running. Zbigniew Stachniak who wrote the book on the MCM/70 and presented at VCF doesn't know any that are working so it will be a tall task and if I fail, it won't be be disappointing to me. Just having it is good enough for me. Last night I started to take the cover off but I couldn't see the main board. To be honest, I'm not sure where it is as it needs to be further taken apart. I couldn't find the Intel 8008, the upgraded 8k of memory (originally 4k) or the ROMs but the ICs I did see had 1973 dates. I would be certain the ROMs are 1702As as there was some discussion of this "new technology" in Stachniak's book that helped with this project. I just couldn't see the main board when I opened it up and it was late so I just opened the top cover an peaked in. I was shocked at how modern it looked on first impression but it was just a glance. I'll certainly take pics.

snuci
October 19th, 2018, 03:57 AM
Okay, now I'm jealous. Congrats!

Thanks falter. It really is my "end game" computer and oddly enough, I was already in the process of acquiring an MCM/900 while this email popped up. The MCM/900 is a later model with dual 8" drives and a printer. I was excited just to find this. I was shocked at the MCM/70.

snuci
October 19th, 2018, 04:05 AM
On the previous photo you posted: here (http://www.vcfed.org/forum/attachment.php?attachmentid=48866&d=1539912285), you can just see the "Gates" peeking out from the rightmost cell. From the second photo of the battery you posted, I can see that you're in luck--many of these SLA packs used welded connections between the cells; yours are the "slip on" terminals.

According to "The Making of the MCM/70 Microcomputer" (http://www.8008chron.com/MCM-01203059.pdf) (section below) the batteries are integral to the shutdown procedure. I wonder if the batteries are required for the start-up circuit too? Could I just put 12V DC on the two wires that go to the battery pack and try to start it? I have not attempted to plug it in yet. Before even thinking of doing this, I still have to look it over but it was just a thought I had last night.


Indeed, an on/off switch was nowhere to be found on the MCM/70. To start it, the user had
only to press the start key (on the keyboard) and the computer responded with “MCM/APL” on the screen to
indicate that it was ready for use. To switch the computer off, one typed ■ OFF and pressed the
return key. However, before the computer would be deactivated, the entire contents of
the workspace and the workspace status were preserved in the current state on the cassette.
The MCM/70 user’s guide explains:

"This is to insure that nothing is accidentally destroyed. In order to have the system restore
them [i.e. the workspace and the workspace status] back in the computer at some later date, the
cassette must be mounted in the tape drive before the start key is pressed. If this is done the
computer will automatically reconstruct the saved items in memory to appear as though the
■ OFF function had never been executed."

A unique feature of the computer’s power supply was a power failure protection system,
designed by Edwards. It allowed continuous operation under battery power in the event of
power failure; for extended power loss, the computer initiated an orderly shutdown: it
automatically provided system backup by copying the RAM content to a cassette before it shut
the computer down. The system was automatically reinstated when the power was restored
and batteries were recharged.

Chuck(G)
October 19th, 2018, 04:51 AM
According to "The Making of the MCM/70 Microcomputer" (http://www.8008chron.com/MCM-01203059.pdf) (section below) the batteries are integral to the shutdown procedure. I wonder if the batteries are required for the start-up circuit too? Could I just put 12V DC on the two wires that go to the battery pack and try to start it? I have not attempted to plug it in yet. Before even thinking of doing this, I still have to look it over but it was just a thought I had last night.

It might, but with something this rare, you have a bit of homework to do. The first thing is to disconnect the AC-fed power supply from the main board and check its voltages. Since it's most likely a linear supply, you can apply the "bring the line voltage up slowly to re-form the capacitors" procedure. Then you need to check the resistance of the DC feed to the main board to determine if you're looking into a dead short--if so, you need to diagnose and repair that first.

It could well be the case that simply replacing the batteries and plugging the AC cord in that it'll spring to life. But I wouldn't bet my life on it. Take your time with this gem.

snuci
October 19th, 2018, 05:11 AM
It might, but with something this rare, you have a bit of homework to do. The first thing is to disconnect the AC-fed power supply from the main board and check its voltages. Since it's most likely a linear supply, you can apply the "bring the line voltage up slowly to re-form the capacitors" procedure. Then you need to check the resistance of the DC feed to the main board to determine if you're looking into a dead short--if so, you need to diagnose and repair that first.

It could well be the case that simply replacing the batteries and plugging the AC cord in that it'll spring to life. But I wouldn't bet my life on it. Take your time with this gem.

From what I read, it has a switching power supply but I have to confirm that. The power supply in this computer was controversial in terms of getting it to work when it was first built and could be the case today. I will not rush and am just looking at options right now. In the meantime, I am checking if power supply schematics (or any schematics) are available.

Chuck(G)
October 19th, 2018, 05:21 AM
A full-blown switching power supply would indeed be unusual in something of this vintage. When this thing was made, I suspect that Bob Boschert was still tinkering with his stuff. Switching regulators, on the other hand were not uncommon.

snuci
October 19th, 2018, 06:52 AM
A full-blown switching power supply would indeed be unusual in something of this vintage. When this thing was made, I suspect that Bob Boschert was still tinkering with his stuff. Switching regulators, on the other hand were not uncommon.

I'll take some pictures but the original design was to try to include a switching power supply (link to part of "Inventing the PC: The MCM/70 Story") (https://books.google.ca/books?id=RY2DDczsJ50C&pg=PT121&lpg=PT121&dq=mcm/70+switching+power+supply&source=bl&ots=iDe4kkGh5W&sig=2d5hoyzop53F59aHj-fS9LBL7TI&hl=en&sa=X&ved=2ahUKEwi43drC3ZLeAhXi8YMKHQH4BmUQ6AEwCHoECAAQA Q#v=onepage&q=mcm%2F70%20switching%20power%20supply&f=false). Seems that Google books doesn't show all pages in a specific chapter so I'll have to check with the paper copy when I get home.

snuci
October 19th, 2018, 08:13 AM
Here is a link to my MCM/70 document. It is also impossible to find :)

Al, if you could put it on Bitsavers, that would be great.

https://vintagecomputer.ca/files/MCM/

Chuck(G)
October 19th, 2018, 08:28 AM
Interesting in that the system won't start without the internal battery being fully charged. Well, you know where you have to begin. :) I'd probably initially test using a DC (14.5V) supply. If the thing really does have a SMPS, it's quite likely that issues will show up there first.

The printer shown in the brochure is clearly a Diablo Hitype. You still may be able to find APL typewheels.

snuci
October 19th, 2018, 08:48 AM
Interesting in that the system won't start without the internal battery being fully charged.

Turns out that isn't the case. The only thing you will see is a battery Low indicator light. Also, no schematics so that's a bummer but I wasn't really expecting to find any.

MikeS
October 19th, 2018, 10:55 AM
Turns out that isn't the case. The only thing you will see is a battery Low indicator light. Also, no schematics so that's a bummer but I wasn't really expecting to find any.

If you haven't already, you might want to get in touch with Josh Bensadon; I think you know each other and you're practically neighbours.

He reverse-engineered and rebuilt the MCM-800 at York U (even an emulator), including a lot of time spent on the tape drive and format; I would think that some of his experience and expertise might be useful for the MCM-70 as well, and he might even be able to help you get yours going.

Good luck!!

snuci
October 19th, 2018, 11:18 AM
If you haven't already, you might want to get in touch with Josh Bensadon; I think you know each other and you're practically neighbours.

He reverse-engineered and rebuilt the MCM-800 at York U (even an emulator), including a lot of time spent on the tape drive and format; I would think that some of his experience and expertise might be useful for the MCM-70 as well, and he might even be able to help you get yours going.

Good luck!!

Ah, so it was Josh who did that. I wasn't aware of who helped out there so thanks for the tip. They added a modern switching power supply to the MCM/800 to get it to work. Zbigniew at York U suggests that I go that route if it cannot be fixed and it is non-destructive so perhaps Josh could help with that if he helped them. I am going to do a little exploration first but I will certainly get in touch with Josh. I was going to send my two tapes to York U to decode but maybe Josh can do it, if he would like.

Thanks again. I want to see it run one way or another but hopefully it can be fixed as is. Good to know there are other options.

falter
October 20th, 2018, 08:46 AM
Well if you ever decide to teach a course on acquisition outside of ebay please let me know. :) I hate being reliant on that place for the majority of my 'finds'.

Really hope you do get it powered up. Would love to see what it does.

I'm curious if the serial tells us anything about how many were likely produced.

nigwil
October 20th, 2018, 02:35 PM
He reverse-engineered and rebuilt the MCM-800 at York U (even an emulator)...
Good luck!!
Did the emulator ever get released somewhere?

If the MCM/70 EPROMs could be captured then it would be great to see it running on a 8008 emulator.

snuci
October 20th, 2018, 04:07 PM
Well if you ever decide to teach a course on acquisition outside of ebay please let me know. :) I hate being reliant on that place for the majority of my 'finds'.

I would say my best finds are NOT on eBay but it is a major source. There are a few things one can do but if I gave that away, i would never find anything anymore ;)


Really hope you do get it powered up. Would love to see what it does.

Me too! I did learn from the original owner that when he said it wasn't working, he meant that if powered up the last tie be tried 10-15 years ago but it showed random moving dots. That sounds promising but it's like a Chinese puzzle box to take apart. Not being around this weekend, I can do anything so it waits for now.


I'm curious if the serial tells us anything about how many were likely produced.

No, I don't think so. Serial number is D509118. Could there have been 9118 produced? I don't know but I think that number is lower.

48903

snuci
October 20th, 2018, 04:15 PM
Did the emulator ever get released somewhere?

If the MCM/70 EPROMs could be captured then it would be great to see it running on a 8008 emulator.

There is an emulator for the MCM/800 but I don't think that has been released. It is different from the MCM/70 as it does not have an Intel 8008 but has something similar built from discrete logic. I will see if I can read the ROMs if I ever get to taking it apart. The boards are not easy to access at this point. I'l have to try again next week.

snuci
October 21st, 2018, 03:43 PM
I was able to take a quick pic of the card cage/power supply of the MCM/70. The card cage is located in the back thick part of the wedge. The black back metal panel is a large heat sync. There is no fan in the computer. I think this is the whole power supply shown. Below that board is a series of other boards that would make up the rest of the board set. I still have to figure out how to take those boards out. I was expecting a few large filter caps but there are none. 120v is plugged into the bottom right directly. The boards are wired for power individually from the top left red/green wires. There is no back plane from what I can see.

48921

Bigger picture here (http://vintagecomputer.ca/wp-content/uploads/2018/10/MCM70-Power-supply-and-card-cage.jpg).

Chuck(G)
October 21st, 2018, 04:28 PM
Love those stud-mount diodes! If you're interested in prophylactic measures before powering things, check the electrolytic capacitors; remove the socketed ICs and give them and the sockets a spritz of contact cleaner. You could also test the diodes. Otherwise, the power supply looks pretty simple.

snuci
October 22nd, 2018, 05:05 PM
Here's the money shot. Still in the process of taking it apart.

48935

snuci
October 23rd, 2018, 02:39 PM
I put some some more internal card pics as I take it apar and photograph the MCM/70. Being commercial, there is very little I can remove without desoldering. I can't read the ROMs as they are not removeable 1702A EPROMs. I am already seeing local help from Josh.

These are pics that will go away once I complete a decent page or two on the MCM/70 but it's where I am putting pics as I take them.

http://vintagecomputer.ca/mcm-70-pic-dump/

snuci
October 24th, 2018, 04:31 PM
How's your APL?

Maybe this will answer your question.


https://www.youtube.com/watch?v=kOhJkH9SWoQ

I can't believe it works. The screen is a little glitchy so I'll have to look into that but now I'll have to find a program sample to try it out. This was more than I was expecting.

nigwil
October 24th, 2018, 06:34 PM
thank you for the video.

Wonderful to finally see a live MCM/70 after all these years wondering what they looked like working.

I hope the EPROMs can be read so we can keep it alive forever on an 8008 emulator.

nigwil
October 24th, 2018, 06:37 PM
Obviously there are lots of APL examples to be found, but you could try the simple ones via this website:

https://tryapl.org/

Then you can compare results easily.

Slob
October 25th, 2018, 08:48 AM
thank you for the video.

Wonderful to finally see a live MCM/70 after all these years wondering what they looked like working.

I hope the EPROMs can be read so we can keep it alive forever on an 8008 emulator.

So do I. I'd like to add it to my emulators. This one( https://hackaday.io/project/161404-videops2-keyboard-8008-system-emulator) does 24x80 NTSC video and PS/2 in addition to the 8008 emulation, all from a $3.00 ESP8266 and junkbox parts.

I wonder if the display is made of dumb MAN-2's or some smart display or subsytem. That, and scanning the keyboard, might make emulation trickier. The tapes, I could probably do in flash. Hopefully some schematics exist somewhere.

That MCM-70 was really ahead of its time. I wonder what the designers went off to do...

daver2
October 25th, 2018, 09:40 AM
I would like to second making the ROMS available if possible (although we may not be able to do much with them unless we can get hold of a schematic due to the memory 'tricks' that were pulled.

I did try to get hold of a copy of the ROMS and the manuals - or the emulator - but to no avail.

I did a "Hello world" (or more correctly "Hello Dave") APL example for the Commodore SuperPET that I can dig out for you if you like...

The download website is here http://mikenaberezny.com/hardware/superpet/waterloo-languages/ but I can hunt out the specific file if that would help?

Good going to getting the unit working though!

Dave

snuci
October 25th, 2018, 02:07 PM
Thanks guys. I had already offered up my computer to have the ROMs dumped by someone who knows how to do them without removing them and there are two versions that have already been dumped so it is not needed.

There is also an emulator that is almost done for the MCM/70 that has not yet been released. To help out, I have been asked to type in a game written in APL and take a video of the output so that the emulator output can be confirmed to match the real MCM/70 or adjusted. There is some glitching of the screen that is normal because the firmware uses display memory for temporary variables; allowing more RAM to be used for the APL system. I wonder if that will be emulated? The display is a 32 character Burroughs Panaplex self-scan display (http://ferretronix.com/tech/nixie/pdf/ss_theory.pdf).

There are no existing schematics for the MCM/70 that are known to exist. The manuals I have and have scanned them to this directory (http://vintagecomputer.ca/files/MCM/MCM70/) but there is not much technical information there.

falter
October 25th, 2018, 04:12 PM
I wonder, as a theoretical exercise, if you could build a replica/clone of an MCM-70, assuming you had the schematics. I've had some success taking photos like the ones snuci was kind enough to post and then using Photoshop and schematics, converting them into PCB artwork. I realize this is a fairly complex machine of course.

You can still find the burroughs display, you can make the PCBs and probably find all of the support ICs. The tape decks would be probably too difficult (but you could probably find a workaround). The ROMs would be the one thing I imagine would be really hard if not impossible to recreate in their original form?

daver2
October 26th, 2018, 08:04 AM
I was hoping to have a go and put the ROMs into my JavaScript 8008 emulator (with the expected tweaks).

Snuci, you say there are two versions of the ROMs that have already been dumped. Are they online anywhere by any chance and, if so, are you able to point me at them?

Cheers,

Dave

snuci
October 26th, 2018, 10:57 AM
Snuci, you say there are two versions of the ROMs that have already been dumped. Are they online anywhere by any chance and, if so, are you able to point me at them?

I don't believe they are public. I will ask the person who has them if he will give me a copy and see if he has any issues sharing them. I, personally, would share them if I had them as more emulators could only strengthen the knowledge of these computers out there.

daver2
October 26th, 2018, 11:55 AM
Thanks,

I am disassembling your boards at the moment from your photographs...

It's starting to get late in the UK though so I may have to give it up shortly.

Dave

snuci
October 28th, 2018, 07:57 AM
An now a proper running APL program running on the MCM/70. It's a game called Horse.


https://www.youtube.com/watch?v=YitUfJySYz4

daver2
October 28th, 2018, 08:28 AM
Nice :-).

I have uploaded my first attempt at disassembling the RAM card to https://drive.google.com/open?id=1ajHsD5Jpz_6Ih92aSb7xKXcUheC-wjgy.

I am going away on a business trip this week - so I will take the photographs of the remaining cards with me and see how far I get...

I will use the J1 pinout from the RAM card to help with the ROM card. I suspect that the CPU/RAM and ROM cards all communicate via a shared bus - so the function of each pin on these connectors should be the same.

There are a few things I can't determine from the photographs - so I have documented these on the schematic(s) and will revisit them in dour course once I have disassembled the CPU card (if I can that is).

Dave

daver2
October 28th, 2018, 08:42 AM
Today horse racing - tomorrow STARTREK on one line!

It would be good to get some photographs of how the cards are cabled together at some point when you have a spare minute or ten. I think I have worked it out from the cards though as follows:

The RAM and ROM cards are connected together and to CPU/J3 via 2*20 way ribbon cable.

The I/O card is connected to CPU/J2 via 2*20 way ribbon cable.

The two 2*13 way connectors on the I/O card are connected to the two tape drives - one to each.

The connector 'on its own' on the I/O card is connected to the outside world via the rear of the case.

The two 2*10 way ribbon cables on the CPU card (J1 and J4) are connected to the keyboard and the display (not sure which way round though) - but I am going to hazard a guess that J1 is the keyboard and J4 is the display (based on the number of connections to the connectors).

Dave

daver2
October 28th, 2018, 08:50 AM
Time to make some inroads on the ROM card before tea...

I can see 3 * 74155 dual 2 to 4 decoders and a hex latch. I am going to start with these decoders first. I suspect the 74175 hex latch is something to do with the memory paging.

Dave

daver2
October 28th, 2018, 09:01 AM
Correction - the 74175 is a quadruple latch with active high and low outputs. Not a hex latch (that is its 174 cousin).

Dave

snuci
October 28th, 2018, 02:43 PM
The two 2*10 way ribbon cables on the CPU card (J1 and J4) are connected to the keyboard and the display (not sure which way round though) - but I am going to hazard a guess that J1 is the keyboard and J4 is the display (based on the number of connections to the connectors).

I think you have it right. When the cards are stacked, the connectors line up as do the power connector terminals. The cassette drives are on parallel connectors. Drive one is closest to the edge of the board.

The keyboard goes to the connector marked "158". This goes off to a small board that then goes to the keyboard. Newer MCM/800s have circuitry on the keyboard. I am thinking this small board may be that circuitry as the MCM/70 keyboard has no ICs. The lone connector on the other side of the board goes to the display. The display runs on 250V that is fed from the green tape "2" from the power supply board. "1" and "3" go to the batteries.

I asked about the ROM dumps and since I have been working with the University, making them available is a bit of a liability. I will see if I can get the ROMs read. I think I know who did them for the university. I will be happy to make them available if I am able. Unfortunately, the MCM/70 emulator that is being worked on seems like it's a couple of months away. Perhaps it's release will make the ROMs available. I don't have any knowledge of how it is being done so I don't know for sure. I wish I had better news.

daver2
October 29th, 2018, 02:29 AM
>>> The keyboard goes to the connector marked "158". This goes off to a small board that then goes to the keyboard.

That's why I am not getting much joy resolving the interface between CPU/J1 and the keyboard - there is a 'bodge board' in the way...

The ROM board is being a little hesitant to give up it's secrets! I think I have identified the ROM manufacturer and type - and an associated data sheet. This gives me a starting point for the power supply pins, address, data and chip enables to start from.

It would appear as though the ROMs have an address latch on the front end. There are two (active high) chip enables - two separate lots of 4 bits - that appear to be connected together in this application. The 74155 provides active low decodes - so the 7404 inverters appear to act (primarily) to turn the active low chip enables into active high. I am sure 1 or 2 have other uses though. 19 ROMS. 4*6 7404 inverter gates = 24 inverter gates.

The 7405 (open collector inverters) buffer the data bus from the ROMs to the data out pins of the bus to the CPU card.

My initial guess would be that I have DOUT0..7 swapped on my schematic for the RAM card...

There are quite a lot of traces that are on the component layer of the PCB and are hidden by the ICs. Extracting a definitive schematic for the ROM board will be difficult as a result.

I will have a look this week at the 74175 and 7400 logic and see if I can deduce what is going on.

I will also have a look at the CPU board for a little light entertainment :-)!

>>> I asked about the ROM dumps and since I have been working with the University, making them available is a bit of a liability.

Never mind. Let's see how far I get in understanding the hardware first...

It looks as though the -9V supply is not used on the ROM board.

Dave

daver2
October 29th, 2018, 03:22 AM
My initial 'poking about' with the CPU board indicates a pretty good correspondance to the Intel SIM8-01 (not unsurprisingly)...

Dave

snuci
October 29th, 2018, 04:13 AM
My initial 'poking about' with the CPU board indicates a pretty good correspondance to the Intel SIM8-01 (not unsurprisingly)...

Yes. York University has the original Sim8 that was sent to MCM to test out the 8008 so it would make perfect sense.

snuci
October 30th, 2018, 08:47 AM
I asked Josh Bensadon to take a look at this thread. He is the one that can read my ROMs if needed. He did email some information because he has an expired login id but it's causing him issues to get back in.

Josh mapped out the MCM/800 which is very similar to the MCM/70 but the MCM/800 emulated the Intel 8008 with discrete logic so there are differences. This is a cut and paste of his email after looking at this thread. I have shared the pictures with him and will be happy to lend him my MCM/70 if he promises not to break it :)

<quote>
To answer some questions on there:
Yes, switching power supply in 1972. As we emailed earlier, while it's 1 board, it's actually 2 power supplies.
The Primary power supply.
-120VAC to 14VDC to charge the batteries and run the 2nd power supply.
-always ON
-outputs "AC Fail" and "Battery Low" signals to logic boards

Secondary power supply
-12 to 14VDC in
-multiple outputs +5V, -9V, +12 and +250VDC for burroughs selfscan (unless this is done by a 3rd power supply on the MCM/70)
-Soft ON/OFF input
-Start switch is only wired directly to this power supply to turn it ON.
-An output port latch will turn off this power supply on the []OFF command


The ROM's are MASK ROM's, 19 chips of 2K x 8. Organized to occupy the first 8K only as follows:
0-7FF M1 ROM
800-FFF M2 ROM
1000-17FF M3 ROM
1800-1FFF 16 Bank switched ROMs B0 to BF

An Output to port 0 selects the BANK... I think on the upper nibble.

Address 2000 to 3FFF are for the 8K of RAM.
3FFF is the highest address possible with 8008 cpu.

The I/O of this machine is:
System input, Power fail, Battery low.
System output, Power down the computer
Keyboard output, Scan row
Keyboard input, Column return
On the MCM/800, the key repeat is done in hardware (not sure if it's same for MCM/70)
Display Output (this is complicated).

Next are the I/O for the peripherals.
3 outputs are:
AOS (Address Output Strobe I think it stands for)
COS (Control output)
DOS (Data Output)

2 inputs are:
GSI (Status Input)
GDI (Data Input)


It is through AOS,COS, DOS, GSI, and GDI that the machine talks to the Cassette drive, Serial Port, Floppy drive (on the MCM800), CRT display, printer, etc.

Devices on the OMNI PORT, get addressed through AOS. Addresses C8 and C9 are to select the cassette drives 0 and 1.

About the display... well, that ties into memory usage.
address 2000 is the display buffer 222 bytes. The remainder is for temp/system use.
The page at 2100 holds a 13 word heap (or stack) and the remainder is for temp/system use.
The RAM at 2200 and up, is for the user and APL system and is allocated in blocks (more than this, I don't know as this is as far as I got in dissassembling the firmware).

Now, on the MCM800, there is an I/O instruction that was customized to read a hardware pointer, fetch that RAM content and output it to a port for display.
This instruction is executed in the loop that waits for keyboard input, thus refreshing the display.

The MCM/70 uses a similar instruction, but does not do all these custom actions as described since it's a real 8008 processor (Remember, the MCM800 is a custom built 8008 from discrete logic). I expect the CPU board dectects this I/O instruction (7F) and handles this DMA in hardware to refresh the screen. So, the 7F instruction just triggers this DMA to occur.
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Hope this helps.

daver2
October 31st, 2018, 09:22 AM
Very interesting.

I haven't managed to get around to looking at the CPU/ROM board any further yet on my business trip. Perhaps tonight or tomorrow now I have a bit of something to work with.

Dave