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Computer History Part Seven. The Xerox 820-II & CP/M

I really don't remember when I got my Xerox 820 system. It was sometime around winter of 1984. My particular machine was manufactured in August of 1984. The early eighties were full of different computers, and time has blurred some of the details. I do remember it came from some Mail Order retailer called BCE as part of a special deal. I paid $399.00 for it. Me and a friend actually both bought systems at the same time. I remember it arriving via UPS, in a couple of very heavy boxes. And that is the first thing you notice about the 820, it is heavy, very heavy. 54 pounds heavy, and that is just the disk drive. The monitor/CPU adds another 30. The second thing you notice is that you may have bought one of those brick outbuildings you hear so much about. Overbuilt has nothing on this computer....

The Xerox 820 was marketed as an office/business machine, and it was my first 'real' computer system. After all the time spent with the Ataris, and Commodores, and Color Computers, I finally had a computer that could do some real business processing! While I had plenty of exposure to them at work, now I had one on my desk at home. It was my first computer with a Hard Disk. An 8 Megabyte, belt driven monster. A noisy monster. And, the first time you fire it up, you find out something else about this machine. It makes Noise. Lots of noise. Noise that would most likely be lost in an office environment, but at home, it was like a freight train running through the living room. Fortunately the TV had good speakers, and a volume control that went to 11.

Xerox 820-II Computer system

The Xerox 820 runs CP/M ver 2.2, Gray Kildalls' contribution to the microcomputer world. Fortunately we had CP/M computers at work, so I was familiar with the commands such as 'pip'. And, this machine had a hard drive. A HUGE hard drive! I was in heaven. All that space. I could store all the data I would ever need on that 8 MB disk. Until the space ran out. Another first. My first experience at the rule that your data expands to fill all the available room on ANY hard disk, plus 10 percent.

I was a manager for my Condos at the time. So, my first task was to organize the Condo records. The system came with WordStar, and suddenly, the Condo correspondence was on real letterheads, and printed on a Daisy Wheel printer, instead of a Selectric typewriter. I had Dbase II, and I eagerly ventured into databases. My first task was to 'computerize' all the Condo records. This started my love of Database programming. A love that would peak with Computer Associates' Clipper language, and that would eventually evolve into the SQL/Access/Oracle/Asp world of the internet. This was an important machine in my life. A pre-cursor to the lure of the IBM PC. It also marked a departure for me away from hardware oriented hacking, to software oriented working. This was the first computer I didn't immediately build an interface for to control some important part of my technical world. I would return to that paradigm with the PC, but, to this day, the Xerox has had no wires, interfaces, or modifications attached to it. It was for Data Processing. What follows is a look inside my Xerox 820 computer. It still works fine, it still makes noise, and firing it up for this article has actually prompted yet another trip down memory lane as I looked at my data from 25 years ago. We've come a long ways, baby.




The Xerox 820 used a Single board Z-80A Processor. With 64k of RAM and a clock speed of 4 mhz. It has two serial ports for connection to a printer and a modem (Defined as LPT and COM1 in cp/m.) An unusual feature was that the CPU was built into, and a part of, the monitor. The larger expansion box only contained the disk drives. There were a number of different optional expansion boxes with different configurations of floppy drives, both 5 1/4 & 8 inch, as well as this one that contains one 8" DSDD floppy and a Rigid (hard) Drive. The rigid drive was initially partitioned into four partitions (E,F,G & H). Each one can be loaded from separately via the bootstrap monitor menu. The first four physical drives were reserved for floppies whether they were actually there or not. CP/M mapped the drives depending on which one it was booted from. CP/M always used logical drive A. This gives rise to a small utility called 'whatsa.com' that would tell you what physical drive was mapped to what logical drive. The 10 meg ridgid drive could hold 8.192 megabytes of data after formatting. The 8 inch Double sided, Double Density floppies hold 482k formatted, the 5 1/4" floppies were good for 155k formatted. The expansion box plugs into the monitor/CPU via a large cable and 36 pin 'D' type connector. There is an additional daisy chain connector on the expansion box.


Interior of monitor/cpu unit Interior of monitor/cpu unit Interior of monitor/cpu unit
Above are three photos of the inside of the Monitor/CPU unit from the back. Click on one for a larger image. The CPU board is in the bottom of the unit. At the left is the power supply, and at the right (vertical) is the monitor board. You can see the DIP header connector for the parallel port at the very front/bottom of the CPU Board. The Z-80 CPU is visible just to the right of the CRT Neck. On the left side of the CPU board are two expansion slots. One contains the PIO chip and support circuitry. The other one is empty. Major chips include a Z80A CPU, Z80A-CTC Clock chip, Z80A-SIO Dual serial port and two Z80A PIO Parallel Port Chips.

The machine has a 6k rom that contained the bootstrap code, and a small resident monitor program. This is the menu you see when first powered up. It allows you to load the OS from one of the drives, go into a dedicated terminal mode, or use it as an electronic typewriter with the associated Xerox printer. The resident monitor had many additional commands for modifying and looking at memory, debugging, etc. The resident monitor also has many user accessible subroutines that could be called by your own programs to deal with disk drives, I/O, etc. These were well documented in the manual. The 6k ROM is bank switched out after the initial cp/m load, and replaced by RAM.


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Back of monitor/cpu unit

This is the back of the Monitor/CRT unit. The connectors From left to right are: the expansion box, keyboard, LPT and the Com1 port. The attached cables are for the expansion box, and keyboard.

The monitor is black and white, and displays 80 columns by 24 lines of White on Black text. The actual screen size is on the small side at 12 inches diagonal, but it is very readable.


Although there is a serial port on the back for serial printers, The Parallel Xerox printer plugged in to a 40 pin header connector inside the monitor cabinet via a ribbon cable. This port was driven by a Z80 PIO chip. These two 8-bit PIO ports can also be used for user hardware interfacing, and are well documented in the manual.

Please note that, although I would like to look at, and photograph, the motherboard, unlike most of what I document, I am not going to disassemble the Monitor/CPU at this time. It is 'well packed'. Since It is old, it still works, and the various bits of plastic may be brittle, I do not want to risk damage. If I sometime get a better camera that would make it worthwhile for very good photos, I might re-consider, but, for now....



The system comes with a very nice manual, bound in a Leather covered 3-ring binder. Very Business looking. It contains a complete description of CP/M straight from Digital Research. It also contained a lot of useful programming information. Disk formatting, I/O, cp/m memory usage, subroutine locations, pinouts, all were included. It is obvious that the user was expected to write their own software! The bundled software includes a Z-80 assembler (ASM), a Macro Assembler (Macro-80), a Dynamic Debugging tool (DDT), the famous ED text editor, and all the expected cp/m utilities (PIP, Copy, format). Additionally, it came with Mbasic (Basic-80 ver 5.21). Other software includes a telecom program - Modem7, dBase II, and Wordstar!!

Picture of Xerox 820 Instruction Manual
Above are four random photos from the manual. Click on them for a larger image. I tried to make them readable, but my camera is not quite good enough up close, and has autofocus only. Not a bad camera, but, not Jim Phelps' spy camera either...


The expansion box contains one Shugart model SA-851 8 inch Double Sided, Double density floppy drive, and a Shugart Rigid disk model SA-1004. It also contains an interface/controller board that connects to both drives, as well as a DC power supply. The spin motors on both drives run off of 115 vac.

Closeup of disk drive cabling
A Closeup of the Disk drive cabling. Click for a larger image.

Here is a closer picture of the cabling between the various drives. In the center is the controller card. The external cable (yellow wires) plugs into it. The large ribbon cable goes from the controller, left, to the Hard disk, and then to the far right to connect to the Floppy. From there it continues down to the 'daisy chain' connector on the back of the unit. There is an additional smaller ribbon cable from the controller to the Hard disk. There are 3 pin molex AC power cables to each drive (the colored wires). The large twisted white wires are from the DC power supply, and use 6 pin molexes to plug into the Hard Disk, the controller board, and the floppy drive. The power supply is an Astec model AA12070, and delivers +24 Vdc at 1.0 amp, Minus 5 Vdc at .5 amp, and Plus 5 Vdc at 8.6 amps.




Inside of 820 expansion box Inside of 820 expansion box Inside of 820 expansion box Inside of 820 expansion box
Above are four internal views of the Expansion Box/Drive assembly. Click on an image for a larger one.

SA-1004 Rigid disk drive SA-1004 Rigid disk drive

Here are a couple of pics of the SA1004 rigid drive logic board, and the top of the drive. You can see the white air filter, and the belt cover. The logic board is all discrete chips, and there really is nothing interesting on it!. (The blocks of wood are not part of the drive....)

The two connectors and cables at the top right go to the heads. There are four identical cables (2 per connector), so I am assuming four heads, and maybe two platters. The connector on the bottom right goes to the head stepper motor - 12 wires. All of the external connections except the power supply are via the two edge connectors on the left. In the lower left you can just see a small 2 conductor connector that goes to something internal near the hub. I am thinking this is a tach sensor. The head stepper has an opto isolator on it with a tang to sense one rotation. I do not know if the stepper makes multiple rotations, or if this is the home sensor. I do know this thing makes a bunch of noise when it moves the heads. I have yet to find any detailed information on this drive. I did weigh it. 17 pounds!

Above are some assorted views of the SA1004 Rigid disk drive. Note the hefty spin motor, belt drive to the platter, and a good size stepper (under round yellow label) for the heads.
Picture of SA-851 Floppy Disk drive Picture of SA-851 Floppy Disk drive

Remember 8 inch floppies? These are pictures of the Shugart SA-851 Double sided, Double density floppy drive. In the picture of the top of the unit (right) you can see the spin motor in the lower right corner. The head stepper is the square motor directly above that. Above that motor, in the top right corner is the starting capacitor for the spin motor. At the top center is the head load solenoid (with the two red wire lugs attached). Directly below that is the head assembly. This is double sided, so there is a head on either side. The black plastic thing with the white sticker below the heads is a spring mechanism to open the door when it is unlatched. And, just below that, barely visible is another spring mechanism to pop the floppy back out. On the left side, in the middle are two optical sensors for the index hole. These have the green/yellow wires going to them. These are actually the LEDS, the sensors are on the other side directly behind them.. Even though the floppies are soft sectored, there is still an index hole in them to specify the start of the track. I don't know why there are two sensors at different positions? While they can not be seen in this photo, there are two interrupter type optos. One is for the head assembly home position, and the other senses the write protect notch. It is interesting to note there is only one mechanical sensor switch on this drive. That is the one that senses the door is closed.

ON the circuit board side (left), the flywheel pulley is in the center. To the right of that is a solenoid that locks the door release when the floppy is spinning, heads loaded. Above the flywheel is the switch that senses the door is closed. On the controller board, again discrete, no interesting chips. The Connector on the bottom right is for the heads, and in the top left is the connector for the head stepper motor. At top right is an edge connector for the rest of the wiring. The interface is connected to the edge connector in the left center of the board. The power is supplied by a 6 pin molex on the back side of this board - out of sight.



Picture of the 820 Disk controller card

Here is a picture of the expansion box disk controller board. This is a very dense board, and as such I did not have much luck photographing it. It was confusing my poor Mavica. Nonetheless, this board is interesting. It is a Data Technology Corporation (DTC) model 1403D controller. It has, in the upper left, header connectors for what appear to be four rigid drives. It also has a row of diagnostic leds, along the bottom left next to the power molex, that are buried, and hidden, when it is installed

in the 820. There are two 50 pin header connectors on the right side that are unused on the 820 (S-100 maybe?). Additionally, in the left center, right behind the edge connector is another 50 pin header connector. This connector is what the cable to the Monitor/CPU unit connects to.. I counted over 150 mostly 7400 ttl chips on this board. There is a NEC D765 floppy disk controller chip in the upper left, and an 8 position dipswitch (for the 820 switches 2,3 & 5 are on, all others off/open).


Finally, if you have or acquire one of these, here are a couple of items I had to re-learn after I set it up for the first time in 20 some years for this article. There is a Drive Lock bracket on the Hard Disk. It is a little metal 'L' bracket right near the motor pulley that locks the pulley with a metal tang. MAKE SURE THIS IS REMOVED BEFORE POWERING! The hard disk originally had four partitions labeled E,F,G & H. To load software from the hard-disk you have to type L and the drive letter. IE LE will load from the E Partition (C is a floppy). Note also that all the cases have interlocks. The Monitor/CPU interlock is on top of the CRT Frame, and the drive cabinet interlock is under the left screw that holds the top on. If you take a cover off, you will have to bypass those to fire it up.
Ridgid drive spindle lock picture of expansion box interlock Picture of Monitor/CPU interlock
Hard Disk Drive lock. Shown Engaged. Remove this bracket first. Turn it upside down & re-install Expansion Box AC Interlock. Pushed down by screw securing top. Monitor/CPU Interlock. Pushed down by top cover when secured.