Scor­pio News

Power requirements are:–

+12V dc 1.2 amps maximum during stepping
+  5V dc 0.42 amps maximum

After a visual check of the drives when they arrived, I read the “Preliminary OEM Manual” which came with them. I understand that this was all that was supplied to Gemini when the drives were purchased for evaluation purposes, and it leaves a number of questions unanswered. The signal interface is virtually identical to the Shugart interface used for the VFC, and I was able to plug the drives in directly. Similarly, the power supply is via a standard + 5v and + 12v socket. After connecting the power and signal connectors, and using a simple program to check the ability to step in and out, I found that one drive was not working and that it became very hot in the process. Fortunately, Scorpio Systems were able to supply another drive, but this too proved to be faulty (though not so dramatically). More of this later.

With one drive apparently working mechanically, I had to tackle the problem of the clock rates, so as to test its ability to read from the supplied (and pre-formatted) disks.

Board Modification

No modifications were required for the signal interface. In common with most 80-BUS products, the MAP80 VFC card is supplied with a circuit diagram, and this was invaluable in dealing with the next stage, switching from a 5-1/4 to an 8 inch data rate under software control.

The clock signal for the 2797 floppy disk controller (FDC) chip is derived by division from the system clock. A wire link option is provided to adjust the clock between 1MHz for 5-1/4 and 2MHz for 8 inch drives. This is not so useful as it sounds, because there is no link to set up pin 17 of the FDC chip, which adjusts the internal VCO frequency for the two data rates, the pin being held low to indicate 5-1/4 inch data rates.

Perusal of the circuit diagram revealed a spare bit in the latch that is used for drive and density selection. This bit can be controlled by software, and can therefore select the appropriate clock rates. The method is similar to that described by R Mohamed in his article “GM809 Fast Stepper” in Scorpio News Volume 1 Issue 2 pp 46-49. I used the overkill solution of a multiplexer (741S157), thinking that other signals might need to be switched by this latch, but I could as well have used the 74LS00.