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help with Xerox 820

N

norwestrzh

Experienced Member
Joined
Jan 11, 2021
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186
Location
mount vernon, wa
With all the recent interest in revisiting the Big Board, it got me curious about digging out my old Xerox 820's (I have several of them) to see if they would still run. The Big Board and the 820 are supposed to be of a very similar design? I used to use them with a mechanical keyboard and a very nice Panasonic CRT that had separate video, H and V sync inputs (i.e. with the stock EPROMS labeled U-63 and U-64). I don't have any of that stuff anymore, so I figured it might be easy to get the 820 working over a serial port. Boy, was that ever a BAD idea! I have been unable to get any serial output from any of these guys. I've checked the boards, and they have good power, the Z80 clock (pin 6) is being driven, the chip selects and output enables on the EPROMs are active, and the Tx and Rx clocks to the SIO/0 appear good. I think my problem is with the serial connection and its initialization? I'm using the DB-25 that has the label "printer" on it. I think that is the SIO port "B"? Is a null modem required? Looking at the disassembly of the EPROMs, the serial initialization bits that are being sent out seem quite odd to me. I've had a lot of experience with the SIO/0 and I've always used something like: 4,44h,3,c1h,5,68h,1,0. That gives N/8/1 at various baud rates depending on the SIO/0 clock. Is there something inherent in the 820 hardware that would preclude the use of this sort of initialization? This morning, I tried using the CRTRAM.BIN utility that is supposed to be good to diagnose these boards. I can't get any serial output from it at all. I think (but don't actually know) that it is supposed to use port "B" for a console?

So -- any 820 experts out there? What am I doing wrong, and is it even possible to configure the 820 serial port "B" to operate at 9,600 N/8/1 and with a 3-wire interface (Tx,Rx, and G) without all the flow control B.S.?? I must say that in lots of my projects that involve serial I/O, the serial I/O is often the most troublesome thing to work out. I'm going to back-burner this effort unless somebody can offer up some pointers about how to get a serial console working on the 820.

Thanks in advance for any suggestions!

Roger
 
Possible, Chuck. I'll have to try to find my 232 breakout jig? I had it right at my fingertips not 20 years ago. *grin*

I was hoping that somebody familiar with the Xerox 820 could tell me if it was possible to run the serial ports without that handshake B.S. It's just an added complication, IMO. I've tried to initialize the SIO/0 to run without it -- just run 8/N/1 at 9600 or 19,200, but it didn't work. I wonder if there is something about the 820 hardware that makes that impossible?

Roger
 
My understanding is unlike the BigBoard which will route terminal output to either the CRT/keyboard or serial port depending on which one receives input first, you can can't redirect terminal output to the serial port on the 820 and it expects to always have a keyboard and monitor. I seem to recall someone replacing the ROM(s) to allow for BigBoard type operation, but can't seem to find that thread so maybe I'm imaging that. :)

Here's my thread about how I got my bare 820 board up and running. https://forum.vcfed.org/index.php?threads/xerox-820-project-time.1251315/

Some good info here also, https://emergent.unpythonic.net/01683808046
 
Good News!!! I managed to hack the "diagnostic" code -- CRTRAM.BIN -- so that it runs a console on the SIO/0 B port. Trouble is, it comes up at 300 baud!! Ugh. I tried to fix that in the firmware, but it didn't work (for some reason -- still trying to figure that one out). BUT, using my monitor program (in ROM) I was able to send the proper code to the 820 to get it to run at 19,200. I have to use both 2716's because the whole thing won't fit in one. The hacked code is a *real* mess. Lots of wasted space and dead code in there! I'll be working on cleaning it up, and maybe it will fit in one 2716? However, somehow I doubt it.

I bypassed all the funky interrupt stuff, and the buffered I/O. Seems like a waste on the 820? Not using the CRT stuff either.

Once I get things cleaned up, I'm going to try to get a Tilmann Reh GIDE to work with the Xerox using a compact flash module. I think it will work OK with an IDE to CF adapter? Who knows? It would be nice to have sixteen 8 MB "disks" within CP/M running on the 820.

Roger
 
OK, so I got the serial port sorted out, and the Xerox 820 now comes up with the B port running at 19.2k. The hack of CRTRAM.BIN and the old SDSystems monitor now fits in one 2716. That leaves the other 2716 available for user options except for a small area that will be reserved for a loader that loads CP/M 2.2 from the compact flash on the GIDE interface. Yes, that's right -- the Xerox 820 runs the classic GIDE, and now boots up a sixteen 8 MB "disk" version of CP/M 2.2. I'm still trying to streamline the CP/M boot process, and figure out the RTC clock that comes on the classic GIDE. The GIDE works very well, and I feel that it is probably faster than floppy I/O?
 
As a follow-up to the above post, I've managed to repair all 5(!) of my 820's, and got them all working -- without a CRT and keyboard. I've had a copy of Tilmann Reh's GIDE made (without the clock stuff), and it works as a daughter board plugged into the Z80 socket. I have a few extras, so if anybody wants to get a Xerox 820 working on a terminal emulator with a knock off of the GIDE, give me a ping.

Roger
 
Good job! My neighbor had a Xerox 820 when I was growing up, and I got a Ferguson Big Board II with two 8" drives. It was a pain just getting it all working, I remember I had to do lots of coding to get xmodem to communicate with the serial port so I could connect it to my TRS-80 Model I (that had an external modem) to download CP/M files. I also had to rig up a custom keyboard that interfaced some how (I don't remember) and I built a printer on the breadboard area to print to the TRS-80 printer (or did I have a Okidata Microline 80 by then?).
 
I have an 820 without a keyboard Id love to restore. I do have the CRT and dual 8inch drives. Any tips for powering up.
 
The 820's that I have sat in a closet for many years. No problem powering them up. However, the CRT and 8" drives are a different story. I gave up on floppies years ago, and use compact flash on them. I avoided the CRT issue by using the port marked "printer" for the console. I wired up a converter pigtail to go from the "printer" DB connector to an RS-232 to TTL adapter and then to a TTL to USB adapter. Of course I had to re-work the monitor in the EPROMs. I interact with the 820 via USB at 9600 baud (and Linux minicom).

Roger
 
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