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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 5th Nov 2016, 10:55 pm   #1
JohnBHanson
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Default MDS 800 Interlec computers

I am trying to write an emulator for one of these machines. It is an 8080 based multibus machine with a co-processor on the mutlibus for floppy acess.

It runs an operating system called ISIS. In additon it was the machine that cp/m was written for initially. I have ISIS booting at the moment, but cannot get the hard disk interface quite right.

I don't have a hard disk image with isis on it and I don't have details about how to drive the io interface from the 8080 processor. It appears to be similar to the floppy disk but there are some differences I have not worked out.

Does anyone have details of this system that could help me.

Thanks.
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Old 6th Nov 2016, 7:15 am   #2
TonyDuell
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Default Re: MDS 800 Interlec computers

I am not sure whether to post this or not as I don't really have any useful information.

I am pretty sure the name of the machine is 'Intellec', it was made by Intel and is, I believe the second generation 8080 development system. The first was the MCS8i, which has a lights-and-switches front panel and no disks (just paper tape), although you can see some things on that machine (like IOBYTE at location 3) that carried over to CP/M

ISIS is something like 'Intel Systems Integrated Supervisor'.


I have an MDS800, but mine just has twin 8" floppy drives. These are connected to Intel's double density controller, which is incompatible with just about everything else. A normal disk controller can't read/write such disks. It is based on Intel's bit-slice chips (3001 and 3002). I don't know if that's what you mean by a 'coprocessor', certainly it is not user-programmable (the software for the bit-slice ICs is in ROMs). I have, alas, never seen a hard disk for one of these. What was the controller, was it Intel's own?

I assume you have looked on bitsavers for manuals.
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Old 6th Nov 2016, 11:06 am   #3
JohnBHanson
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Default Re: MDS 800 Interlec computers

I believe the controller was an intel one. It appears to work in a similar manner to the floppy controllers but having an address of 68h rather than 78h or 88h. However there are some slight differences to the bits in the registers that appear to be different.

Thanks for the information about the bitslice chips, those I assumed to be the co-processor. My guess was that what happened was that when floppy controller was driven by the 8080/8085 that a coprocessor would access the ram via multibus to read the parameter block and decide what function to perform.

I have searched all around the web and got what infomation I can, including that on bitsavers. I have actually found how to initialise the drive using dinit. The procedure is involves booting isis with the hard disk not ready. At this point the boot floppy is :F4: and the hard disk is :F0:. Then ready the hard disk and use disk init to copy the files. The problem is I don't know what the controller returns for a not ready hard disk that will allow isis to boot in this manner. When I try various options I get an error from isis and no prompt or the floppy starts as :f0:.

I have a scanned copy of the source for ISIS and I have looked at the PLM source for the bootstrap, but this is an older version of ISIS that does not support the hard disk.

At the moment it is frustrating because I feel I am inches away from getting a working system which would be nice.

Love to hear you still have one of the blue boxes. Do you still use it?
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Old 6th Nov 2016, 4:53 pm   #4
TonyDuell
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Default Re: MDS 800 Interlec computers

I suspect arguing whether the disk controller is a coprocessor ultimately comes down to semantics. It's a long time since I looked at the schematics (I have the original Intel diagrams) but it may well access the main memory (using DMA) to read the parameter block and read/write data. I will have another look sometime. I have never seen the hard disk for one, though.

As far as I know mine still works, but I don't use it much (no I am not selling it!). I do have the UPP (Univeral PROM Programmer) too, but only with the 2708 and 2716 programming boards. Making the other boards for it is almost impossible, the thing uses a 4040 CPU to control it, the 4001 ROMs for that are on the option boards (containing the actual programming algorithms for the device in question). So to make more boards you (a) have to get the right firmware and (b) have to emulate 4001s. Neither is easy.

I actually use the MSC8i (the older one) more, if only because it has an internal programmer for 1702As and this is the easiest way to read out such an IC.
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Old 17th Nov 2016, 11:11 pm   #5
JohnBHanson
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Default Re: MDS 800 Interlec computers

Success - I have an ISIS system booting from hard disk. The io parameter block has a couple of extra bits of addressing
Bit 3 of the Disk Instruction iopb entry is used for head select ( 1=head0, 0=head1)
Bit 7 of the Sector iopb entry contains the top bit of the track number
This gives 400 tracks (200 Cylinders), 2 sides and 36 sectors of 128 bytes each

Disk instruction bits 6 and bit 4 are the unit select bits. Bit 6 being the least significant unit select and also this bit is inverted. Bit 4 is the most significant unit select bit and is not inverted.

ISIS reports the disk as 100 tracks, 144 sectors per track.

Other than these addressing differences it appears to be the same as a floppy controller.

It must be 40 years ago I last run ISIS(!).

Thanks for the help.
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Old 18th Dec 2016, 1:41 pm   #6
JohnBHanson
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Default Re: MDS 800 Interlec computers

If anyone wants an 80bus (Z80) emulator for GNOME/Linux this one. It will also emulate a nascom. MDS 800 Floppy disk controllers are also implemented so it can run ISIS to (Useful to those programming in PLM). Use the command MDSBOOT ? for infomation on how.
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