Floppy Interface

The Floppy Interface in Waveterm A und B

The original Waveterm A used 8" diskettes. The Eurocom computer has a 50-pin connector following the Shugart 50-pin standard for that. When the Waveterms were remodeled for 5.25" drives later on, there were 2 concurrent connection types available:

	the Shugart 34-pin connection that is highly compatible to the original Shugart 50-pin style of the older 8" floppy drives and
	the IBM PC 34-pin connector... that unfortunately isn't.

Of course, I can only speculate about the reasons that caused PPG to choose the Shugart 34-pin connection; I'd guess that the following ones were the most important:

	a move to the IBM variant would have meant a lot of redesign
	the IBM AT with the HD drives wasn't yet really prominent; the IBM-compatible HD floppy drives weren't yet available in big quantities.

Outcome: PPG designed a small adapter board that allows it with just a minimum of work to use 5.25" floppy drives following the Shugart standard.

Adapter Board Pin Layout

The following table is oriented to the result - the 34-pin floppy connector. A short description of the signals follows. The signal names have been taken from the Floppy User Guide.

from	Shugart 34	Shugart 50	Comment
	1,3,5...31,33 Gnd	1,3,5...47,49 Gnd
Controller	2 DENSEL	(2 DENSEL)	not connected; PPG uses only HD.
Controller	4 INU	18 LSD	assures that the heads only land on the diskette when that's necessary.
Controller	6 DS4	32 DS4
Laufwerk	8 IDX	20 IDX
Controller	10 DS1	26 DS1
Controller	12 DS2	28 DS2
Controller	14 DS3	30 DS3
Controller	16 MOT	Gnd	Motors rotate constantly!
Controller	18 DIR	34 DIR
Controller	20 STP	36 STP
Controller	22 WD	38 WD
Controller	24 WG	40 WG
Laufwerk	26 TK0	42 TK0
Laufwerk	28 WP	44 WP
Laufwerk	30 RD	46 RD
Controller	32 SS	14 SS
Laufwerk	34 RDY	22 RDY

Signal Description

Abkürzung	Beschreibung
DENSEL	selects between DD/HD and normal/reduced current; these two settings are coupled
INU	"In Use"; tells the drive that it's currently used. With the PPG drives, this signal is used to lower the heads onto the floppy disk.
DS1..DS4	"Drive Select 1..4"; selects one of the 4 possible drives. The Waveterm A software only uses 2 drives, but the underlying FLEX operating system could in theory use all 4; the Waveterm B operating system knows only 2.
IDX	is set by the drive when the index hole becomes visible
MOT	Motor Enable. In the Waveterm, this signal is always set, so the motors are continuously turning. This is the normal behaviour for 8" drives and was simply kept this way.
DIR	Direction the heads are moved to (outwards or inwards)
STP	Causes the heads to move a step into the direction given by DIR
WD	data written to the diskette
WG	determines whether data are read or written
TK0	reports whether the heads are over track 0
WP	reports whether the diskette is write-protected
RD	data read from the diskette
SS	sets the current head
DC	Disk Change; set as soon as no diskette is in the drive
RDY	Drive Ready signal
DC / RDY	IBM Mixture; the drive still signals "Not Ready" after the diskette is changed and a new one has been inserted. To find out whether this state is the result of a completed disk change, the controller has to cause the drive to change the current track; this resets the Disk Change flag and the "real" Drive Ready signal is reported.

A more detailed description can be found here.

Implications

Usage of the Shugart 34 Standard - and in a form that is not 100% standard - has some unpleasant consequences, from today's point of view. Because of that, you can only use some very special drives in the Waveterm that support this standard - and because of the success of the IBM AT and the following PC generations, the IBM standard has become the dominating standard. Drives that can be configured in a way that the Waveterm accepts are very hard to find by now.

It would be great if the Waveterm could be modified to accept IBM-compatible 5.25" (or, even better, 3.5") drives, too. Unfortunately, there are some obstacles:

the PPG behaviour to continuously turn the drive motors and only lower the heads when necessary can't be realised with the IBM standard, where the heads are always lowered when the motor is active, but the motor is only activated when necessary.
In theory, it's possible to change that to the IBM method, but this changes the drive behaviour, too - starting up the motor takes much longer than just lowering the heads, so it takes much longer before the drive is ready.
Andreas Voigt has modified his Waveterm A to use 3.5" disks - but to overcome this, he had to change a capacitor on the Eurocom motherboard (hardware timeout!), since the different timing lead to lots of runtime errors.

there's no RDY signal on IBM drives; instead, the DC/RDY signal is reported. The Waveterm expects only RDY; the received DC/RDY would cause the Waveterm to think that the drive is not ready after a disk change. Since it does not run through the IBM-compatible check (track change), the drive stays in this state. Some drives allow to return only the RDY signal, but unfortunately not all.

Modification of the Waveterm operating systems might allow to correctly implement the IBM behaviour, but that would mean a lot of work.

Another possibility would be to simply connect pin 34 to ground, giving the Waveterm the pleasant illusion of a drive that's always ready. Big disadvantage: the operating system has no chance to find out about status changes (for example, if the diskette is changed), which can lead to grave read and write errors.

Just thinking loudly now... in theory, it should be possible to develop a little circuit that simulates the IBM behaviour. When necessary (as soon as a Drive Select is set and the drive reports "not ready"), it would have to move the head one track up/down. In my opinion (which hasn't been tested yet) it should work to do the following in such a case:

if Drive Select & Not Ready, start motor for the drive
if changes occur on the IDC pin (i.e., if there's a disk rotating in the drive), step up and down one track

Last update: 04/03/04