Need an Extra Drive or Two?

by Jim Wiegand
From newsletter of the Greater Akron 99er's.
Connecting a third disk drive to your TI 99/4a is relatively simple. A power "Y" connector and the proper Data ribbon cable will put you into operation. I used a Radio Shack "Tandy Color Computer" drive. It has a power supply and a single sided disk drive. I replaced it with a double sided, half height drive. These are plentiful and inexpensive.
Most disk drive Data cables will have a guide inserted into the controller connector between pins 3/4 and 5/6 (pin 1 always has a colored wire on the ribbon cable). The TI Disk Controller card has an external connector for a spare drive. This connector has a guide slot between pins 9/10 and 11/12. If this SPECIAL cable cannot be found, you can cut the guide out of the ribbon connector. Be careful to not damage the contacts. If you make this modification, you must remember to have the colored wire at the bottom when hooking it up. If you are technically inclined you can remove the disk controller card and cut a slot at the proper location. Don't cut too deeply as you do this. Watch for printed circuits in the line of cut.
If you prefer to use a 3.5 inch disk drive, these are also available and inexpensive. Adaptors to allow mounting these in the 5.25 inch drive bay are not as plentiful. I located some in a catalog from MCM ELECTRONICS CENTERVILLE, OH (1-800-543-4330). A call will probably get you a catalog. These kits contain the power adaptor and a Data connector adaptor.
A little info on disk drives may be helpful at this point. The TI disk controller uses connector pins 10, 12, and 14 as the Drive Select (DS) lines to select disk drives 1, 2 and 3 respectively. Most 3.5 inch drives are setup as drive B (2). Some of these drives will have DS jumper blocks allowing you to select its drive number. This block usually consists of a row or maybe two rows of five or six pins with a jumper across two of them. Look around the sides near the connector end. If no such selector can be found then the unit is probably hard wired as a drive B (2). A data cable modification will be required as follows:
From the colored wire on the ribbon cable (pin #1) count wires to 12 and 14. Cut these wires about an inch from the disk drive connector. Solder a wire between pin 14 from the disk controller and pin 12 from the disk drive. These connections should be insulated (tape, tubing). If you don't feel comfortable with this then ask for help. Most clubs have a "techy" who will be glad to assist you. The odd numbered pins are common (grounded) and are all on one side of the connector.
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