Rev 9/25/01

ADDING A FOURTH DRIVE

by Jim Wiegand
From newsletter of the Greater Akron 99er's.
(Note from Ted Peterson of PUNN. On the TI disk Controller the drive lines for DSK1, 2, 3 are line 10, 12, 14, in the cable line. There is no DSK4 on the TI Disk Controller)
This is a discussion of and work associated with the addition of a fourth drive to the TI 99/4a. I use 3 1/2" drives for backups. See the last paragraph of this article for 3 1/2' disk information.
This modification requires some wiring changes (with soldering) and drilling small holes. Materials needed are: a SPST miniature toggle switch, a dual drive data cable to match your drives (3 1/2" or 5 1/4") and a proper power "Y" adaptor. These items can usually be purchased at Radio Shack or a local Computer store.
As mentioned in a previous article, pin #14 of the TI Disk Controller is the Drive Select (DS) line for disk #3. This is the wire we will be working with. The task here is to install the toggle switch to allow selection of DSK 3 (A or B).
To accomplish this, you must count the wires in the Data cable (the colored wire is #1) to wire #14. Cut this wire between the disk controller connector and the disk drive connector, about one inch from the disk drive connector.
Now, find wire #14 between the two drive connectors and cut it near the center. Strip and tin these two ends plus the end selector switch and mount it. With a 3 1/2" drive installed in a 5 1/4" bay. There is ample room.
Cut a switchable length of three conductor wire ( a piece of ribbon cable works quite well) to connect the switch to the data cable. Solder these wires to the switch with the center wire connected to the center terminal. At the drive connector, solder the center wire to wire #14 from the disk controller. The other two wires must be soldered to the cut wires between the disk drive connectors. Insulate all soldered junctions and install the drives.
If all went well, the drives selector position (A or B) can be indentified. Type in, OLD DSK3.TEST and press ENTER. Watch the drive lights and label the switch position appropriately.
Remember that, if this is beyond your ability, ask for assistance. Someone will surely help.
If you are using a 3 1/2" drive and cannot find the double density disk, (with only one hole on the back edge), you can use the high density disk if you cover the second hole with a write protect tab. Do not cover the hole with the little slider on it. This is the write protect slot and may be needed later.
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