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SPC Diskitjr Hard Drive


When the PCjr was designed hard drives were a very expensive option found only on business class computers. IBM first started selling a 10MB hard drive with the introduction of the PC XT in 1983. In 1984 the PC AT was introduced with an option for a 20MB hard drive. The PCjr, being designed for the home market, had no ability to add a hard drive and even the single floppy drive was an option.

Hard drives are useful though; shuffling floppy diskettes on a single drive system is an exercise in frustration. People found ways to add a second or third floppy drive to their system, improving the usability of the machine. But the speed and capacity of a hard drive is what we all lusted for … a 10MB unit would hold the equivalent of 30 floppy diskettes, while a 20MB unit was basically infinite storage. Hard drives were lightning fast and you spent much less time moving diskettes back and forth by hand.

After a few years hard drives became more affordable in the market but hard drives for the PCjr remained unusual. The PCjr had a few problems working against it:
  • IBM basically orphaned the machine in 1985. Even with many in use, it was a dead-end market.
  • The machine physically did not have room to house a hard drive or a controller; any hard drive attachment would have to be external or physically make the PCjr larger using an expansion chassis.
  • The machine was non-standard; you could not use standard ISA controller cards. Every controller had to be designed specifically for the PCjr.
  • Given the option to add a hard drive to a PCjr or just sell the machine and buy a low-end PC XT clone, many people chose to do the later as it was more cost effective and it got them out of the PCjr compatibility hell that IBM had created.
Even with these challenges hard drive options became available. The March 1987 edition of "The Junior Report" featured advertisements for 4 different types of hard drive systems offering 10MB and 20MB options:
  • Paul Rau consulting: 20MB external hard drive for $699
  • Computer Reset (still in business as of 2017):
    • Racore expansion chassis with 10MB drive and DMA for $875 or 20MB for $985
    • External drives with interface card: $650 for 10MB and $750 for 20MB
  • SPC Diskitjr: $595 for 10MB, $695 for 20MB, or $245 for a controller card
Imagine paying $600 for a 10MB hard drive and controller … in 1987. In 2018 that would be close to $1340.

The SPC Diskitjr

The only hard drives that I have run into on a PCjr were homebrew units adapted from a PC XT controller or the very nice SCSI sidecar attachments that came later on from PC Enterprises. Recently I was thrilled to adopt an SPC Diskitjr from the original owner - I had seen the advertisements but I have never seen or heard of one in the wild.

First, the advertisement from the March 1987 edition of "The Junior Report":

SPC Diskitjr advertisement

And here is what one looks like 30+ years later:

PCjr with SPC Diskitjr
Looking past the cluttered workbench, this is what you are looking at:
  • A PCjr with a 128KB sidecar (closest to the machine) and the SPC Diskitjr hard drive interface sidecar.
  • A Seagate ST-238R 30MB RLL hard drive attached via a ribbon cable to the SPC sidecar. (The drive is being used as a 20MB MFM drive.)
  • An external power supply to power the drive and an external fan to keep it cool.
  • My usual 20GB Maxtor drive tucked in back, disconnected and unused for this experiment.
Normally the drive would be in a more professional looking enclosure as shown in the original advertisement. If you were really lucky you had an expansion chassis like the Racore Drive II to house the hard drive.

Here is an image of the bare controller card:

SPC Diskitjr controller card

Technical notes

  • The controller card uses a Western Digital 2010 Winchester disk controller chip. ("Winchester" is the old term for a sealed disk unit consisting of heads and platters.)
  • The controller is capable of driving two MFM hard drives. (Note the two sets of data pins at the lower right corner; one vertical and one at a right angle.
  • The controller card does not have a BIOS extension. To use the hard drive you must boot from the floppy drive and load a device driver that installs a block device driver, similar to the way a RAM disk or Zip drive is installed.

Software notes

The setup program uses a text file as the source of hard drive parameter information. The parameters for a drive generally look like this:
Seagate ST225 : 614 : 300 : 615 : 3 : 0 : 1
-1 : 0 : 6
I don't have documentation for the parameters, but comparing the drive specifications to the parameters one can deduce the following:
  1. Maximum cylinder number (based on 615 cylinders for the drive numbered 0 to 614)
  2. Write Precompensation?
  3. Reduced Write Current?
  4. Maximum head number (based on 4 heads numbered 0 to 3)
  5. Unknown, but always seems to be 0
  6. Step Rate Code (usually set to 1, rarely set to 2.)
  7. Unknown
  8. Unknown
  9. Unknown
  10. Unknown, but always set to zero. (Possibly an end of record marker.)
There are other formats to describe drives too; they have additional lines of data. I am still trying to decode what the data fields control.

All of the files that I have for the software can be found in Diskitjr.zip. If you have other files or documentation please let me know and I'll update the file and this page.

Lastly …

A special thanks to Paul W. for making the Diskitjr available for me to look at. I thought I was going to be debugging a dead piece of hardware; it turned out to be a trivial problem. I have since adopted it and now it has joined the misfit PCjr family. :-)

Created June 10th, 2018
(C)opyright Michael B. Brutman, mbbrutman at gmail.com

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