80-Bus News

On studying the 829 manual, it became apparent that it was impossible to configure the card for both 8″ with Ready line, and 5.25 inch without Ready line. If the 5.25 inch drives had been Micropolis there would not have been a problem. Therefore I consulted the designer of the 829, who kindly advised a simple modification to the card which enhances it so that it can support this combination of drives.

For Pertec interface 5.25 inch drives, link 3 is normally removed, and this disables the Ready line input. This link should now be reinserted. Link 1 is left as before, linking B to C only. At this point the system will not work, since the 1797 will not operate without a Ready signal, and when the 5.25 inch drives are selected, no signal is provided. Therefore two diodes are added so that when drive 0 or 1 is selected, a Ready signal is provided. When drive 2 is selected, the 8 inch drive Ready signal operates as normal. Insert a diode between pin 22 and pin 26, also pin 22 to pin 28 underneath the 8 inch connector on the card. A hole in the card running from pin 22 to link 1 (A) can conveniently be used for one end of the diodes. The marked end (the cathode) must be at the pin 26 and pin 28 ends. In addition it is essential to alter the value of R18 from 150 to 3K3 (or rather more if you prefer) as otherwise the diodes cannot pull down the Ready line enough to activate it.

Summary of Modification

Applies to GM829 with Pertec or Teac 5.25 inch drives at addresses 0 and 1, and 8 inch drive(s) at addresses 2 and optionally 3.

Link 1: B to C only.
Link 3: Inserted.
Diode from PL3 pin 22 to PL3 pin 26 (cathode end at pin 26).
Diode from PL3 pin 22 to PL3 pin 28 (cathode end at pin 28).
R18 changed from 150 to 3K3 or rather more.

Further Difficulty

One further problem was encountered. When PIPing files to the 8 inch drive, it would sometimes fail to give the drive ready signal. It was being used as a single sided drive with a single sided disk. The drive can tell whether a single or double sided disk has been inserted, because 8 inch disks have the index hole in different places to allow this to be distinguished. Although the software was attempting only to use the first side (side 0), since the Teac drives were double sided and the file was on the second side, the BIOS was first selecting the drive, then selecting the side and doing the I/O operation. Therefore the drive was selected with the second side active. The drive immediately signalled that it was not ready (showing its lack of intelligence). This could be corrected by modifying the BIOS to select the side before the disk, but an easier method was to disable this option on the drive itself. This modification is documented in the Pertec FD650 manual, and is easily carried out. Another advantage of making this modification is that single sided disks may be used as if they are double sided, without the drive objecting. This generally works perfectly, just as 5.25 inch single sided diskettes generally work perfectly when used as double sided.