February 15, 2020

ST506 hard drives

ST506 is a specific interface to an MFM hard drive. There were 2 cables.
A 34 pin cable had control signals and could connect to 2 drives.
A 20 pin cable had "data" signals and one went to each drive. These drives date from 1980 to 1990. My Micropolis 1320 series manual is dated 1985. This was the defacto standard for 5.25 inch hard drive until some time in the 1990's.

The DREM is affordable (about 2x the price of David's emulator), but will not read a disk, which David's will do. They call it the only option, but that is only true I guess if you want an emulator that emulates both HD and floppies. This claim is somewhat ingenuous.

Control signals

All these signals are active low. In general these are "open collector" signals and expected a 220/330 terminator to be installed on the last drive.

All odd numbered pins on this cable are grounds.

H - indicates the signal comes from the host (the controller)
d - indicates the signal comes from the drive 

2  - H - Head select 3 (or reduced write current)
4  - H - Head select 2
6  - H - write gate
8  - d - seek complete
10 - d - track 0
12 - d - write fault
14 - H - Head select 0
16 - - - reserved (key)
18 - H - Head select 1
20 - d - Index
22 - d - ready
24 - d - step
26 - H - drive select 0
28 - H - drive select 1
30 - H - drive select 2
32 - H - drive select 3
34 - H - direction in
A pulse on the "step" signal would move the heads in the direction given by the "direction in" line. Seek complete would go false on receipt of the first step pulse.

The four drive select signals allowed selection of one of four possible drives on a cable. Most systems I have seen only allowed 2 drives, and some only one. Jumpers on the drive would configure it to respond to one of these four signals.

Write gate would select either writing or reading.

Head select would select one of 8 heads (or of 16 heads on some later drives).

Write fault would indicate any condition that would prevent writing. The drive may not be up to speed, seek may have failed, heads may be shorted or open.

Track 0 indicates the heads are positioned on the outermost data track.

Index is true only briefly at one rotational position of the drive. It is true for 200 usec every 16.67 msec on a Micropolis drive. This is 60 Hz, so these drives must be spinning at 3600 rpm.

The drive recalibrates on power up. Seek complete and Ready will be false during this time. Once the drive is up to speed and heads on track 0, these signals will transition to true.

Data signals

Here most of the even numbered pins are grounds, but not all.
Once again, the drive selected signal is active low. 
1  - d - Drive selected
3  - - - reserved
5  - - - reserved
7  - - - reserved
8  - - - key
9  - - - reserved
11 - - - ground
13 - H - Write Data +
14 - H - Write Data -
15 - - - ground
17 - d - Read Data +
18 - d - Read Data -
19 - - - ground
So here we really only have 3 signals. We have the "drive selected" signal from the drive, and we have data coming and going as differential pairs.

The drive selected signal goes true when the drive select commanded on the control cable matches the drive jumpers.

Feedback? Questions? Drop me a line!

Tom's Computer Info / tom@mmto.org