View Full Version : Intel 8085 on the S100 Bus

March 6th, 2007, 05:57 PM
I have found various specifications for using the S100 bus for a single-board computer running the Intel 8080 or Zilog Z80 processors (think Altair/Imsai), but I would like to know if there is a specification for the extra signals on the Intel 8085 processor, namely Restart 5.5, Restart 6.5, Restart 7.5 and TRAP.

There are vectored interrupt lines in the S100 spec, but I've only seen them used to interface to an 8214 Priority Interrupt Control IC.

Should I stop being picky and just use four vacant lines, or is there an established standard for this?

Also, any great sources for a few S100 backplanes? I don't need a 20 slot, but a 4 slot would probably not be enough--so somewhere in the middle would be good. I intend to have Protocase build me the cases for this project.

Input is appreciated, thanks!

April 7th, 2007, 07:10 PM

I do not have any great suggestions at this time but I do have some S-100 books on order. When they arrive I will look them up and see what they say on the subject. If you find the answer to your question, I would certainly like to hear what it is so please post it.

Have you tried looking at the IEEE-696 standard? Maybe they address it there?

Best of luck!

Andrew Lynch

Terry Yager
April 7th, 2007, 07:35 PM
I'd agree. If 696 does not specify, make do with what's available, but at the possible expense of compatibility with other devices/systems/software/etc. Might be better to not use the extra signals unless you absolutely have to.

BTW, I've just scanned thru Z-100 manual, but nothing useful there.

You might try poking around here for some ideas/schematics, etc. http://www.pestingers.net/HZ100_man.htm

It's a problem that may have been solved already, as the Z-100 interfaces an 8085 to the S-100 bus.


Terry Yager
April 7th, 2007, 08:10 PM
Here's a link to Herb Johnson's page comparing various flavors of S-100:



April 9th, 2007, 10:02 AM
Thanks for your input on this...

I'll check out that 696 standard to see if it offers any insights.

I've knocked around Herb's website before, and he's got a lot of information. That other site is new to me, however, and I think I'll be spending some time there today...


July 26th, 2009, 12:27 PM
There was once a s-100 board that used the 8085, but I have seen NO information on it, I am looking for the Bios loader from it...It is/was spacebyte , if I remember correctly.

Intel 8085 Test Engineer
September 29th, 2009, 03:16 PM
Just saw your information about the 8085.

Back in 1974-1978 I worked at Intel in the Test Engineering group. A new test system from Megatest, the Q8000 was just purchased by Intel. I was assigned to develop the Testing for Intel for the 8085 using this system.

While every opcode was used and the 8085 had the hardware to allow every command, the software group was just too overloaded to add all these new instructions to the compiler. So the decision was made to drop 12 of the opcodes from software support. The manuals and support software from Intel did not reference these 12 instructions -- i.e. they were "hidden" commands and they were not discovered for many years (I think a German company was the first to find most of them).

I was instructed to re-write the test code so these 12 instructions were not tested. However, I was way to busy so I never got around to removing references to them in the Test Program.

So, every 8085 manufactured by Intel was actually tested to verify these hidden instructions, and of course all the instructions, worked.

I still have the original sheet showing what these 12 instructions are and how they work...a collectors item for sure.

good luck, great chip. Cort Allen

nige the hippy
September 30th, 2009, 02:23 AM
I hope someday someone/something datamines this forum. the number of posts from "horses mouths" (sorry Cort, not an insult by any means, I hope this phrase is used in the US), is just incredible.
So many bits of folklore confirmed, denied or explained by the people who worked on it. Thanks for sharing that!

October 19th, 2009, 07:11 PM
CompuPro made an 8088/8085 card for it's S-100 system in the early 80's


October 19th, 2009, 07:33 PM
An interesting twist to the undocumented 8085 instructions.

Calmos, a late seller of the 80c85 fully documented these in their regular datasheet for the CA80c85B. I suspect they got the original artwork for the silicon when they acquired Siltronics in the late 80s. Calmos, was, in turn, acquired by Newbridge Networks, who was then acquired by Tundra Semiconductor (all Canadian outfits). So you can find the datasheet for the CA80C85B and its instructions under three different manufacturers--Calmos, Newbridge and Tundra. It's all the same chip.

When we were working with the 8085 in 1976-77, I sure wish that one of those Intel applications engineers had told us about the added instructions! Sigh, we could have used them.

Wandering around in one of the 1980's hobbyist mags is a description of how to devise an NSC800 replacement for the 8085 on the Godbout 85-88 board, so you get the Z80 instruction set instead of what amounts to the 8080 one. It's not much, just the NSC800 and a couple of "glue" chips.

October 8th, 2010, 05:12 PM
I have Located a SpaceByte 8085 CPU S-100 Card... I don't have a Schematic for it yet, But I hope to ohm out the Board and make one soon... If anyone is interested....

October 8th, 2010, 05:27 PM
I'm interested...I'd considered wire-wrapping a S-100 8085 board before starting on my current 8085 projects!

October 8th, 2010, 07:32 PM
Hi! Let me know what you come up with. We could make an N8VEM manufactured PCB if you are interested. Then everyone could enjoy your home brew 8085 CPU board.

Thanks and have a nice day!

Andrew Lynch

October 10th, 2010, 03:54 AM
My Godbout 8/16 board has jumpers to allow RST5.5 to be connected to VI0, RST6.5 to VI1, and RST7.5 to VI2. The inputs are buffered, not direct.

July 3rd, 2012, 07:51 PM
Hi All;
I have found the schematics and the one of the origional bios eproms, It has taken a long time to get it all... I assume they had variations depending on customer order information... I also have another Bios that is written for an operating system that this board was used with.. I have the full listing, But I would need permission to give it all out... If anyone is interested, send me an email at geist1122@juno.com and I will forward what I can..

July 4th, 2012, 09:44 AM
Info/manual on CompuPro 8088/8085 S-100 Board:-

July 4th, 2012, 09:58 AM
Hi All;
The information I have is for the SpaceByte 8085 and not for the Compupro 8088/8085.. I want to interface my SpaceByte 8085 to a 3-1/4 " drive IF possible...