In my furious attempt to not do anything constructive until the Retro Challenge officially starts, I’ve completely disassembled the NorthStar Horizon to prepare everything for cleaning and testing.
During disassembly, I discovered that the floppy cable that runs from the floppy controller board goes to the two Tandon TM-100 floppy drives, then continues on and exits the back of the chassis and terminates in a dangling connector. I can only assume that this was for additional floppy drives, but the cable harness is certainly not the original configuration for this machine.
Neither of the internal floppy drives have a terminator resistor pack, so I will have to either purchase one, or make one up with 150 ohm resistors.
Next, I got in to checking the jumper settings for the floppy drives. The first drive was jumpered as drive 3. The second drive was different. It had a long wire soldered on to edge connector pin 34 and ran over to pin 9 of the jumper configuration socket. There were no jumpers in any of the drive select positions.
This puzzled me for a few days, until I happened to stumble upon a paragraph in the Horizon documentation:
To program a drive as drive number 4 perform the following instructions on the printed circuit board mounted on top of the disk drive. a. Solder a jumper wire between pin 34 of edge connector J1 and location 1F pin 6. Take great care to connect the wire to the very tip of pin 34 so that the ribbon cable connector can slip as far onto J1 as possible. b. Solder a jumper wire between 1F pin 9 and 1F pin 11. c. The uninarked strap location between MX and MH at location 1F is now the location for programming drive 4. It can be labeled DS4. This strap should remain connected and the straps labeled DS1, DS2, and DS3 should be disconnected. Be sure to disconnect this strap if the drive is ever programmed to be drive 1, 2, or 3.
For some reason, the NorthStar Horizon puts DS4 on pin 34 instead of the more normal pin 6. These drives were jumpered for drives 3 and 4. I will need to change this back to drive 1 and 2 for my configuration.