Carroll Waddell carrollwaddell at sc.rr.com
Tue Jun 20 15:56:40 CDT 2006

Since I started this diskette drive question and answer session, let me 
tell you what I have found out.
As I said, I bought a TRS-80 computer on EBAY to see if I could use the 
diskette drives in an H17. I could not get them to work.
Someone suggested that I try them in another computer. They worked fine 
on an IBM PC, proving that the drives themselves were good.

TRS-80 were TANDON TM100-1A. These are single sided drives, however they 
can write to both sides of the diskette with a double head. The circuit 
board can handle the Side Select signal from the controller. But with 
only one head, if you try to select HEAD 1, there is nothing there to read.

The Heathkit H8 Extended Configuration Option adds double headed 
operation, although Heath never implemented double sided operation. The 
Extended Configuration Option circuit has the circuitry to do head 
select. It is wired (Heath recommended) to pin 18 of the H8 bus. The H17 
controller picks up the signal from pin 18 and sends it out on pin 32 of 
the diskette drive cable.

Normally, this signal is held high (Logic 1). To select side 1 of the 
diskette drive, the signal is pulled low. Head 1 writes on the top side 
of the diskette, where head 0, and/or single sided operation writes on 
the bottom side of the diskette.

Now, on the H8 CPU card, pin 18 is normally jumpered to ground because 
pin 18 was not used in the original H8 configuration. So, the CPU had 
pin 18 held low, which had head 1 selected at all times. This is why 
diskettes written on an IBM drive could not be read on the Siemens 
drives, and was also why the Tandon drives would not work. Now, they all 
work the same.

The short of it is this: if you need diskette drives for the H17, you 
can probably find some old TRS-80 computers on ebay. Get the ones that 
are the full height drives. They will work in the H17.

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