Joe's Intel MDS web page

Created 10/20/2000

This site still very much under construction

Pssst! MDS = Microprocessor Development System
ISIS = Intel System Implementation Supervisor

CP/M and a brief history lesson

Click here and then click on the interview with Gary Kidall to read what he had to say about the Intel MDS and the development of CP/M. Briefly, Gary said that he borrowed one of these machines from Intel and wrote both PLM and CP/M on that machine. He said that Intel bought PLM but not CPM. If you look at the code examples in the CP/M manuals you'll see that the headers state that they were configured for the Intel MDS machine. Furthermore I've been told that the BDOS and BIOS on the **GENERIC** CPM OS disks were already configured for the Intel MDS (I think for the MDS 800). If that's true then you should be able to put a **GENERIC** CPM boot disk in the MDS and boot it with little or no changes. If (when?) I ever get a generic CPM disk I'd like to try it. Also by using the code examples in the CPM manuals you should be able to reconfigure just about any machine-varity of CPM to work on the MDS."

What's a Multi Bus?

All the plug-in cards in the MDS are a standard type card called MultiBus. All MultiBus cards are readily identifiable because they all have two connectors on the bottom edge. One connector has 80 contacts spaced .156" on center and the other has 60 contacts spaced .100 on center. The standard card size is 12 x 6.75 inches. Click here to see a drawing of the MultiBus connector.

What's the operating system?

The operating system for these is ISIS (Intel System Implementation Supervisor). The MDS 800 used ISIS. The Series II MDSs used ISIS-II but many were later upgraded to ISIS-III. That's what I have on mine. All of them have a monitor ROM built into them and you can do things like move files from one device to another, edit and move things in memory, etc. There was also an option card that had an 8080 assembler and an editor in ROM. This card came standard on the model 210 since didn't have any disks drives or other storage medium. The later machines all ran XENIX or iRMX as far as I know.

What Languages could they use?

PL/M (aka PL/M-80), a Macro Assembler, BASIC-80, iCIS-Cobol, Pascal and Fortran-80 were all available for them. The "-80" languages compiled into code for the 8080 CPU. There were also "-86" versions of the languages that compiled to operate on the 8086 (and 8088) CPUs. Of course, the "-80" language could be used and the 8080 code would run on the more powerful CPUs but it wasn't as efferent. There were probably more versions for other CPUs that I'm not aware of. FWIW I've also heard that Forth was available for them but I've never seen it and I haven't found it listed in any of the Intel catalogs so perhaps someone else sold it.

What are the general specifications for the MDSs?

Here are the specs for the 220 model, that's about as generic as they came.

4 K ROM (2K in monitor and 2K for the test and diagnostics), 32K RAM expandable to 64K, a 2.6 MHz 8080A, eight priority level maskable interrupts, built in interfaces for paper tape reader, paper tape reader/punch, printer and Universal EPROM programmer and two built-in serial ports. Compatible with standard iSBC (MultiBus) expansion modules, integral CRT 2000 upper and lower case character display and a detachable full ASCII keyboard. One 250K 8" floppy disk drive. Standard MultiBus with multi-processor and DMA capability. The case has six slots and five are available unless an option is installed.

What kind of manuals are/were available for the MDSs?

I don't know what was available for the others but I have SOME of the docs for the 200 series systems and they can only be called MASSIVE! I have seven 8 1/2 x 11" binders and each one is about 5" thick. Click here to see the list of what I have. This is only a fraction of the stuff that was available from Intel! There's a long list of the stuff that was available in 1979 in McCraken's book. I'm sure Intel released more later.

Tell me more about how they were built.

Most models of the 200 series MDSs had an 8" floppy drive standing vertically on the right side of the CRT.There was also an option to add two drives in a separate box. The earliest ones were mounted vertically in a narrow (relatively!) box. The later ones were mounted horizontally in a shorter but wider box. The later box is the same width as the main chassis. The ordinal drives were single sided and, I think, single density. They held 250K. Mine has been upgraded to Double sided Double density (the 503 mod). The main chassis had a row of interrupt switch on the front of it. There was also a similar box that does not have the interrupt switches. That is an optional four slot expansion chassis. I have the separate Intel EPROM burner and Intel paper tape reader with mine. The EPROM burners are common but this is the first paper tape reader that I've seen. I also found an Intel printer that's made to go with the unit but it was incomplete so I passed it up.

What else?

Then the great Eric Smith saithif "AFAIK, the internal drive only supports single density FM, since it is wired to the 8271 FDC on the IOC board. External drives can either be FM or double density M2FM "

Correct. There are a couple of ways you can go with this. (1) Option 730 just adds a pair of external drives but they're connected to the existing controller and it determine what density you get so you'll end up with all of the drives being single density. (2) OR you install the 503 mod along with option 720. The 503 mod replaces the original controller to a double density one so now all the drives are DD. (3) OR you can add option 720 which gives you the external drives along with a second drive controller uses a 3001 conntroller. In this case your internal drive will be single density and the external ones will be double density. (4) Again you can add the 503 mod and all the drives will then be DD but you'll have two controllers installed instead of only one. The addition of a pair of external drives changes a 22x model to a 23x model. I just found out something interesting, the factory built 23x models did not include the built-in disk drives, they only had the two external drives. I guess that all of the ones that I've seen were upgraded from 22x models since they had the built-in drive.

FWIW There is also a 740 option, it adds a 7.3 Mb hard drive to the system.

The 220 model and the 225 models come with a single density drive controller. The Intel book implies that all (most?) other models come with double density controllers but I don't think they do. For example, according to Intel, the 221 model is nothing but a 220 that was converted at the factory to operate from 230 VAC instead of 115 VAC so it should also have the SD controller.

Again Erik spoketh "Unless someone has put in the 503 mod which replaces the two drive controller cards. The 503 cards use a (which is unfortunately not compatible with anything else).You can even get a 8086 card for them if you look around. With the 8086 card (an RPB or RPC), it's called a Series III. Many units were field upgraded, so even a 225 might really be a Series III."

Righto. Paxton and I were trying to sort this out. I finally had to go dig out my Intel books and figure it out. The difference between a Series II and a Series III is the ADDITION of the iMDX 557 Resident Processor Card. The card is a RPB-86 and it has a 8086 CPU. Note that the original processor is left in the machine and is still used for some tasks.

But then Erik spake falsely "Earlier Series II have the 8080 IPB (Integrated Processor Board); later ones and all Series IIIs have the 8085 IPC (Integrated Processor Card)."

Not exactly. The models that I know of are the 220, 221, 222, 225, 226, 227, 230, 231, 232 and 240. Whew!

The 225, 226 and 227 models came with an IPC (Integrated Processor Card). It has a 4 MHz 8085A-2 CPU and 4 K of ROM and 64K of RAM. All other (22x) models came with the IPB (Integrated Processor Board). It has a 2.6 MHz 8080A-2 and 4K of ROM and 32K of RAM. The 23x models were nothing more than a 22x with the addition of the external dual disk drive. However, they also added a second 32K RAM card to 23x models that only had 32K originally (the ones with the IPB). There was also a 240 model, It was a 220 but came with a hard drive, I don't know if there were any other changes to it. I THINK a 225 was a 221 but with a IPC instead of an IPB. I think a 227 was a 222 with an IPC installed. But I don't know what a 222 was.

Confused yet? OK here's how it works in a nut-shell. You start with a 220. If the factory changes it to 230 VAC you have a 221. If you make another (unknown) change you get a 222. You take those models and pull out the IPB card and replace it with a IPC and you get the 225, 226 and 227. If you take any of those six and add the external drive box and you get the 230, 231 and 232. (Notice that a 230 may have a IPB or a IPC card in it. Same with the 231 and 232.) Add a hard drive to a 220 and you get a 240.

Add a Resident Processor Card and it becomes a Series III instead of a Series II. AFIK a Series III can be any configuration of RAM, drives and IPB or IPC processor.

You can see that there are a LOT of combinations that don't have a specific model number. That's makes it confusing enough but then you have to remember that Intel offered lots of other cards and accessories for these and many of them were upgraded/modified by their owners so the variations are endless. FWIW Intel wasn't the only one that made parts for these. I have a A/D card from Burr-Brown and I have a list from Intel that lists at least twenty manufactures of MultiBus cards that would work in the MDS.


Oops! I left one out. There was also a model 210. It was a bit different from the other Series 200 models. First it did not have a keyboard or monitor. Everything is done through a host computer or a terminal. It also did not have a disk drive or other storage medium therefore it has an 8080 Assembler and editor loaded in ROM on a spacial card. It still supported the EPROM burner and paper tape reader, paper tape reader/punch and printer so the source code and hex code could be loaded or saved on any of those devices or to or from the host computer or terminal (yes, some terminals had there own mass storage devices). Since it was expected to be used in minimal systems, it was mounted in the same four slot chassis as the expansion box instead of the normal 6 slot chassis. It must not have been very popular though because Intel later sold a kit to upgrade it to a standard 220 model. The upgrade kit included a six slot chassis (with I/O controller car and built-in disk drive), a monitor and keyboard, and disks with the ISIS operating system, assembler an editor. The upgrade instructions went like this; (1) remove the CPU from the 210 chassis (2) insert into the upgrade chassis (3) You're finished! The only bright spot was that the left over 210 chassis could be used as an expansion chassis.

What's the best way to get started with one of these?

The best book that I've seen for getting started with these is "A Guide to Intellec MicroComputer Development Systems" by Daniel D. McCraken. McCraken is better known for his PL/M book.

Here are a few more comments about the MDSs from Paxton Hoag.

I still have a working Intel Series III model 225 MDS. If anyone has another one or parts for them or just tall stories about them I'd like to here from you. e-mail me.

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