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IMSAI 8080 on eBay

If you've got the cash, and are looking for that particular model, I don't see any big red flags. I'm by no means an IMSAI expert, though. Heck, the auction lists a 30-day return policy, so you could always return it if it's not as represented! :) The seller has a high rating with a lot of auctions to their credit, so that's a plus.
 
From the pictures, it appears to have an 8080 processor board, a few memory cards, a bunch of I/O boards (that may or may not be useful to you), and also has some modifications made to the power supply. They do not talk about providing any documentation, and they show the obligatory picture with the power turned on, where you see all address lines except A2 and all data lines lit up. So, the LEDs work, but there is no indication that anything else is actually working.

I think that $2100 (BIN plus shipping) is a bit steep for this one. I have purchased similar machines in the past for $1000-$1500, but that was in the past.

I would be more sympathetic if this was an individual Seller who you could get into a conversation with and find out what he knows and what he has actually done with the machine. I would be more inclined to try and contact this institutional Seller and see if they might be open to take an offer of something like $1000 or $1500, or re-list it with a more reasonable BIN, but that's me.

My IMSAI has a Z-80 CPU board, a 64K RAM board with one of the RAMs in high memory replaced with an EPROM with a system monitor installed, and a S100computers.com SIO board, and it is working. Using the system monitor with a laptop connected to the SIO for a terminal, you can load HEX files to get programs into the machine. The blemishes are that 3 or 4 switch paddles are missing (I have spares but have not had the guts to try and install them), and it has a clear plexiglass cover rather than the blue metal original cover. If I were to list my machine, I would like to list it for $2500, but I do not know if I would actually get it.

Good luck!

smp
 
The price does not seem low to me, but also not outrageous. Given that it comes with a decent complement of memory, etc., and that the front panel at least lights up, I'd say there's a good chance it works or can be made to work. There's nothing unique about this unit that would make it risky to pass up.

I would expect it to sell for somewhere in the $1500-$2000 range on auction, so it's at the upper end of what I would expect for this unit. I'm sure a similar one will come along and sell for less, if you're not in too much of a hurry.

It does look like a nice unit though.
 
Definitely a nice computer, C8080A date code 7604 tells its age.
 
g4ugm said:
Checking the "sold" listings this rather newer model:-

http://www.ebay.com/itm/191537608541

went for a wopping (and it says sold) US $2,669.99...

the last one at auction, which was really scruffy

http://www.ebay.com/itm/301544773064

went for US $1,525.00 and that had only one card in it. So I would ay for what you are getting its a "fairish" price. I would expect it to sell...
Click to expand...

I feel the price of the IMSAI 8080 is neutral. It has full fledged configuration and complete port on back.

I have couple of S-100 computers in collection, so I will not buy it this time.
 
New member here. I "cut my teeth" on Apple II's (//c, //e, and later IIgs) in high school, and am an inveterate collector of "old stuff" (e.g. 1920's era radios). I've always wanted an IMSAI since seeing ads for them in old computer magazines when I was in college. I am the one who purchased the system in this thread.


My initial goal is to get this system working to the point to where I can key in a simple program via the front panel and have it execute (e.g. the test program described in the IMSAI assembly manual). I used to build my own boxes for running OS/2 in the early '90s and know my way around 80's and 90's Intel and Apple equipment fairly well, but this is my first S-100 box.


The system as I received it contains the following:


IMS MPU 4 Rev A
MITS 88-16MCS 16K Static RAM (configured for 0-3FFF)
California Computer Systems - 2016B 16K RAM Board (configured for 4000-7FFF)
(2) Solid State Music 4K Static Ram Cards (configured for C000-CFFF, D000-DFFF)
IMSAI - RAM 4A Card (configured for F000-FFFF)
IMSAI SIO Rev 3 (Connected to 2 DB25 ports on the rear panel marked 44H & 45H Serial and 42H & 43H Serial)
Northstar MDS Disk Controller (no cabling / connections present)
Cromemco D+7A I/O (Connected to a number of individual I/O jacks on top of the back panel, and a couple of DB25 ports marked 18H parallel in and 18H parallel out)
Bus Extender


I'm guessing there was once another 16K RAM card for 8000-8FFF as well as another 4K RAM card for E000 that are no longer present (or are there other reasons why the system would be deliberately set up with these gaps?). In addition there are ports on the back (marked Lear Seigler and Sperry -- I'm guessing the former at least was for a terminal) that are connected to a couple of loose edge connecters -- presumably there was at least 1 other i/o board present that has since been removed.


I have downloaded all the 8080 manuals from imsai.net, and found manuals for most of the 3rd party cards on s100computers.com.




I have NOT attempted to power the system on since receiving it, and would appreciate pointers on how to proceed. I have a basic multimeter but no oscilloscope. Based on smp's comments above I'm not assuming that anything works.


My initial thinking is start by pulling all the cards and testing the voltages on the power supply first (I just had to recap the power supply in an Apple IIe that gave up its magic smoke after powering it on for the first time in 15 years - would rather avoid that here). The IMSAI manual describes testing the power supply -- but with the front panel card in place. Is this safe, given the age of the electronics? If not, can an "empty" system with just the backplane (appears to be a fully populated EXP22) be safely powered on and used to get reliable voltage readings?


Thanks,
Jason
 
So I ended up pulling the cards (except for the CP-A), testing voltages, and adding just the MPU and 1st RAM card back, testing, then re-adding the remaining cards one at a time - each time reentering the small test program from the assembly guide. After the cards were all added back I entered and ran a few other programs I found on on the net, including the "cylon effect" program for the programmed output lights I found on Lawrence Woodman's site.

To net things out, the system seems to be working (at least the CPU and RAM -- haven't tried to do anything with the IO boards other than verify the system continues to behave with them in place).

I am having an issue though, with the deposit next switch not working -- but only on one of the RAM cards.

For some reason, deposit next is unreliable with the first memory card. It will always advance the address, but will usually not write to memory (it seems to always store if all 0s, but but for nonzero bit patterns it will usually store only a subset of the 1's). The deposit ("up position") switch always works, and I can enter programs by using deposit next to advance the address, then flipping the switch up to actually store. At first I thought there was an issue with the switch itself, or with the circuitry on the CPA, except that deposit next works flawlessly on the other 4 memory boards.

The problematic card is an MITS 16K at address 0000. The other cards consist of a CCS 16K, a couple of Music 4K cards, and an IMSAI RAM4A. All are static memory cards. I can deposit/deposit next and examine/examine next memory on the other cards without issue (some of them even have LEDs that light up when the memory on the card is accessed).

There's not much in the way of configuration on the MITS card -- just a single dip switch bank used to set the base address.
 
Last edited:
BubbaShakers said:
So I ended up pulling the cards (except for the CP-A), testing voltages, and adding just the MPU and 1st RAM card back, testing, then re-adding the remaining cards one at a time - each time reentering the small test program from the assembly guide. After the cards were all added back I entered and ran a few other programs I found on on the net, including the "cylon effect" program for the programmed output lights I found on Lawrence Woodman's site.

To net things out, the system seems to be working (at least the CPU and RAM -- haven't tried to do anything with the IO boards other than verify the system continues to behave with them in place).

I am having an issue though, with the deposit next switch not working -- but only on one of the RAM cards.

For some reason, deposit next is unreliable with the first memory card. It will always advance the address, but will usually not write to memory (it seems to always store if all 0s, but but for nonzero bit patterns it will usually store only a subset of the 1's). The deposit ("up position") switch always works, and I can enter programs by using deposit next to advance the address, then flipping the switch up to actually store. At first I thought there was an issue with the switch itself, or with the circuitry on the CPA, except that deposit next works flawlessly on the other 4 memory boards.

The problematic card is an MITS 16K at address 0000. The other cards consist of a CCS 16K, a couple of Music 4K cards, and an IMSAI RAM4A. All are static memory cards. I can deposit/deposit next and examine/examine next memory on the other cards without issue (some of them even have LEDs that light up when the memory on the card is accessed).

There's not much in the way of configuration on the MITS card -- just a single dip switch bank used to set the base address.
Click to expand...

The benefit of MITS 16K SRAM board is very low heat dissipation compared to other same era 16K SRAM card, you can try to change faulty chips on the MITS SRAM board.
 
Hi All;
Bubba, One thing to be aware of is that You have an Imsai and You are using a Mits Board in it, So, Mits has pins 20 and 70 for Protect and Unprotect.. If I remember correctly, Imsai Grounded one of those and did something else to the other pin with a diode..
So, the Mits board might be in Protect mode..
To check for sure put some scotch tape over pins 20 and 70 and Retest the Board, and see if You can Write to it..

THANK YOU Marty
 
Marty said:
Hi All;
Bubba, One thing to be aware of is that You have an Imsai and You are using a Mits Board in it, So, Mits has pins 20 and 70 for Protect and Unprotect.. If I remember correctly, Imsai Grounded one of those and did something else to the other pin with a diode..
So, the Mits board might be in Protect mode..
To check for sure put some scotch tape over pins 20 and 70 and Retest the Board, and see if You can Write to it..

THANK YOU Marty
Click to expand...

Hi, I have MITS 16K SRAM board on Altair 8800, but there has no protection mode on these card maybe.
 
Hi All;
Leiy, If you are using a Mits memory board in a Mits machine, then You have a Protect and Unprotect Switch on the Front Panel.. Which Sets or Unsets the the protect Flip-flop on the Memory Board..

THANK YOU Marty
 
Marty said:
Hi All;
Leiy, If you are using a Mits memory board in a Mits machine, then You have a Protect and Unprotect Switch on the Front Panel.. Which Sets or Unsets the the protect Flip-flop on the Memory Board..

THANK YOU Marty
Click to expand...

Hi, Marty, I can't toggle on protect mode on MITS 16K SRAM, but I can toggle to activate protect on MITS 1KB SRAM board.
 
Hi All;
Leiy, Without a schematic, to look at, I can't say if the protect mode is there or nor on the 16K SRAM..
But, one way to find out is to see If there are any connections to pins 20 and 70 on the bottom of the Board..
If there is a connection, then it might need further looking into, otherwise if there is no connection, then there is no Protect/Non-protect..
THANK YOU Marty
 
The manual for this card does not mention a write protect feature, nor does the schematic show pins 20 or 70 active. In any event, if this was a write protect issue, why would deposit work but deposit next fail (wouldn't they both fail)?

On a side note, I noticed the front panel did not work at all when I initially put the D+7A IO card back in. I found a note on Herb Johnson's site about pins 70 and 20 causing issues on front panel systems with other Cromemco cards, and sure enough a bit of scotch tape over those 2 pins allowed the front panel to start working again with this card inserted. The deposit next issue occurs, however, even if the MPU and the RAM cards are the only cards in the system.
 
Marty said:
Hi All;
Leiy, Without a schematic, to look at, I can't say if the protect mode is there or nor on the 16K SRAM..
But, one way to find out is to see If there are any connections to pins 20 and 70 on the bottom of the Board..
If there is a connection, then it might need further looking into, otherwise if there is no connection, then there is no Protect/Non-protect..
THANK YOU Marty
Click to expand...

Thanks for valuable advice.
 
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