View Full Version : Xerox 820-II Mixed size Floppy cable: New to these pages

February 24th, 2013, 08:13 PM
New to this forum...(wordy by nature...)
Plenty of experience in assembling, manufacturing, testing, servicing S-100 machines starting in 1981. Prior to that 2 years in Power Supply industry. Followed up with PC's Ltd/Dell for 15 years.

I recently jumped back into retro CP/M machines. I started out (unable to locate a Teletek SysteMaster SBC S-100) buying a CompuPro board set: CPU-Z, Ram-17, Interfacer II and Disk1 which came with a HUGE ParaDynamics cabinet and dual HH Qume 842 drives...and no diskettes...

I have an EPROM burner and a UV meat sterilizer bulb (works great...start up and leave room), so ready for that angle now.

Have Media Master software for IBMPC, so limited to 5.25" DSDD or SSDD diskette format and generation, just no CP/M boot capability...which is one reason why I want a mixed drive system.

Since then I have been driving forward from all different angles trying to get to the point that I can generate a bootable CP/M diskette (8").

My most recent purchase was for a Xerox 820-II with dual DSDD SA850 floppy drives. (Fixed an INDEX issue with the B: drive, ok now...and the system came with plenty of diskettes, Masters as well, at least 20 flavors.

How to get 8" and 5.25" floppies on the same cable, at the same time?
I understand in the 820-I days that the drive cabinets were provisioned to be daisychain-able, but my 8" cabinet for this 820-II does not have that feature (its all original twisted pair...uptown-like).

-I have the (2) 8" drives working.

-I created a 5.25" stand alone interface cable starting with a generic IBM floppy cable capable of interfacing to both edge card and 34-pin, 2x17 connectors for dual drives. I only connected 1 floppy, (prior to the IBM "twist") to my floppy edge card connector. Cut off the controller connector to be replaced with DB37 type.

-The cable manufacture is complex and Xerox documentation doesn't really come out and tell you how to generate it with ribbon cable...just that it is possible, so I used the Xerox 820-II Technical Manual pages 38 and similar page for 5.25", as well as drive jumper settings and interface tables of the Shugart SA850 and SA450 floppy drives that were used in the day.

-Once the 5.25" cable was made up, I would boot on the 8' a: drive first (after having generated a Xerox DSDD 5.25" diskette using MediaMaster with some CP/M files on it), bring up Sysgen on 8" ready to write, Then unplug the 8" cable and plug in the new 5.25" cable and have Sysgen target the C: drive. The 5.25" would complain of a bad sector, retry about 10 times, then if I ignored the error it would complete.....so maybe my generated diskette was bad somehow....I will try that one again later.

-The other task I was able to do was to get a directory of the C:drive....all the while switching cables back and forth between 8" and 5.25".

-So I may try making up a "Y" cable and splitting the J1 DB37 port with both cables connected (creating havoc with the termination folks). I may also install a switch on the J1 pin 2 to pin 20 connection that I think is telling the system the drive type, in hardware....if I just connect it all up at once I bet the system will see only 5.25" drives because of that J1 pin 2-20 connection and won't boot the 8" drive as a result.

-Here are the anomolies yet to be solved:
1. Checking floppy size on the 5.25" resulted in half the expected capacity. (Expected about 320K, got ~160K.

2. Despite the interface documentation of the SA450 showing MOTOR ON signal going to floppy pin 16, that signal does NOT show up in the Xerox documentation, and I am unable to read the floppy schematic block in the lower right hand corner of the "CTC" schematic page. I had to force the cable pin 16 to GND to get the motor of my IBM PCJr floppy (Qumetrak 142, which is working rather well on the Jr) to spin up the motor. Should anyone happen to have a readable schematic I would really appreciate a rundown on the J1 (DB-37 "DISK DRIVES") connector pinout, especially as relates to the MOTOR ON signal, or whatever is present on the J1 DB37, especially on pins 1,2,3,20 and 21. J1-2 shorts to J1-20 and does NOT go through the cable to the floppy drive...this connection does not exist on the 8" setup.

(Also, FYI, I found a misprint in the Shugart SA450 interface documentation that showed DIRECTION SELECT going to pin 19, pin 19 in fact is GND...DIRECTION SELECT goes to pin 18 on the Shugart SA450).

Hey, I promise to keep future posts short and sweet.

February 25th, 2013, 09:29 PM
It appears that the Xerox 820-II Disk Drive connector, (J1 pin2) must be at GND in order for a 5.25" DS/DD drive to be recognized. The Xerox Tech manual denotes this net as "8/N5" (on J1-2)

The Y cable I made up doesn't really help at all, just makes it easier to swap cables between 8" and 5.25" drives.

Another limitation is that there are only 2 drive select lines being passed through both the 8" and the 5.25" cables, namely SEL-1 (J1-5) and SEL-2 (J1-6).

I am able to pip files from an 8" to a 5.25" in the following manner:

Connect 8" drives to J1 connector
-Boot on the physical B: 8" drive (prompt shows A:)
-bring up PIP, (prompt is "*")
-switch drive cables to 5", remove 8", ensure J1-2 is at GND, jumper the 5.25" drive as SEL-1 (Drive 0)
-type in the pip command line: c:=a:sample.fil <CR>
The file copies, the process is now in slow motion....error messages appear when PIP wants to read or write...just switch the cables and Retry...etc till done.

I copied Sysgen.com and movcpm.com over to the 5.25" drives, but had the same issue not being able to write the system on the system tracks...quite possible that the MediaMaster program did not handle the system track format correctly.

Tried the same cable switch method with the format program, but the program saw the 5.25" drive as an 8" drive, trying to format 154 tracks...bouncing back and forth on the inner tracks a bundle of times (maybe I should hook up a 1.2Meg HD 5.25" and see if it formats out and works as a quasi 8" drive???).

Looking like the best route to take from here is to get the serial port file transfers working between the PC and the 820-II.

(I could have sworn the 820-II was setup to mix drives....)

March 13th, 2013, 02:41 PM
HI at all,
i bought recently a xerox 820-II motherboard. Only the motherboard. I discoveed that it not have any FDC aboard. Would you so kind to give me information on the name, codes of your interface FDC and the code of the FDC IC on it? Regards Enrico - PIsa - Italy www.vintagesbc.it