TRS-80 8 MB Hard Drive Three Wire Fix

Starting in 1981, Tandy Radio Shack released a number of different hard drives for their popular line of microcomputers. The first one to be released was the 8 MB Hard Disk Drive for the TRS-80 Model II. This unit contained an 8″ Shugart SA1000 hard drive bubble along with an 8×300 based controller board. With the subsequent industry introduction of the smaller 5 1/4″ MFM hard drive bubbles, the next hard drive unit released was the much smaller 5 MB Hard Disk for the TRS-80 Model I and III and a similar 12 MB Hard Disk for the TRS-80 Model II and 16. Future updates included the 15 MB, 35 MB and 70 MB units which were all sold for the Model II/12/16/16B/6000 line of business computers.

The TRS-80 8 MB Hard Disk Drive sold by Tandy Radio Shack in 1981.

All of these hard drive units were built by Radio Shack and consisted of several components: the hard drive bubble, the controller circuit board, power supply, cooling fan, keyed power switch, activity lamp, write protect switch and wiring. The power switch, activity light and write protect switches required special wiring that tapped into certain signals on the hard drive bubble to function properly.

This wiring becomes an issue when you want to replace the hard drive bubble with one that is different from those that were provided by Tandy. The common solution typically applied was written about by Roy T. Beck in a series of articles called “My Adventures with Hard Drives”. Ian Mavric elaborated about this fix in the June 2013 issue of TRS8BIT. Roy’s fix involves moving the three wires from the bubble to matching locations on the controller board. Once this fix is applied, you can easily swap in different MFM hard drive bubbles with ease.

The SA1000 8 MB hard drive bubble. Notice the 3 wires attached to various signal points.

While Roy’s article mentions the 8 MB Hard Disk as the original implementation of the Tandy 3 wire configuration, the fix he defines only applies to the later 5 1/4″ hard drive units. Since I am an enthusiast of the Model II line of computers, I’ve spent some time figuring out the appropriate fix for the earlier 8 MB hard drive unit. The fix is listed in the table below which I suggest to be added as an addendum to Roy’s article.

While the wires in the 8 MB unit tap the same signals as those referenced in Roy’s article, the wires are different in color.


Addendum to Table 1

Master 8 MB Hard Drive with 8X300 Controller (Note 7 & 8)

Wire Color Function Original SA1000 Connection Generic Connection
Orange Drive Select E2 Pin 1 HDC J6 Pin 26
Blue Write Protect J2 Pin 5 HDC J1 Pin 5
Brown Seek Complete C3 Pin 8 HDC J6 Pin 8


7. All of these wires should be soldered to the underside of the HD controller board.
8. This configuration is only for the primary 8 MB Hard Drive.


The 3 wires are now connected to points on the hard drive controller board. Note that I used yellow for brown and red for the orange wires as I had to extend the wires and did not have brown or orange wire on hand.


The assembled 8 MB Hard Drive after the 3 wire fix has been applied. The SA1000 bubble is on the left and the controller board on the right. Note again that we used yellow extension wire for the original brown wire and red extension wire for the original orange wire.