View Full Version : Adding an External Disk Activity LED to an IBM PC with a Glitch Works XT-IDE Card

November 3rd, 2017, 01:52 AM
Here's a quick, easy, cheap and easily reversible way to add an external disk activity LED to an IBM 5150 PC fitted with a Glitch Works XT-IDE card. Like many, if not most IBM 5150 PCs, my PC has two standard Tandon TM-100-2A floppy drives. Now that I have the XT-IDE card installed, I use the A drive only occasionally but I never use the B drive. The Glitch Works XT-IDE has a disk activity LED on the board, but I wanted one that I could see when the PC's cover was in place, so I looked for a way to repurpose the LED on the face of the B drive as a disk activity light for the Glitch Works card. It turned out to be surprisingly easy.

After a bit of poking around, I found that the P9 connector that attaches to the rear of the circuit board on top of the floppy drive connects to the LED on the face of the drive. After more poking around, this time with a DVM, I found which of the two pins on the P9 connector was the anode and which was the cathode of the LED. At that point, it was simply a matter of connecting the P9 on the drive to the external LED pins next to the bracket at the rear of the XT-IDE card (observing polarity). I used break away header pins and Dupont jumper cables. Once connected, I found that the LED on the face of the floppy drive was a bit dimmer than I liked so I replaced the 470 ohm R2 resistor on the XT-IDE card with a 270 ohm resistor.

As mentioned above, this hack is easily reversible if the need arises. It doesn't affect the functionality of floppy drive B. I can still use it as a floppy if I ever need it.






November 3rd, 2017, 08:20 AM
I've found that unless there's a steady stream of disk I/O, the duty cycle of an LED isn't long enough to really catch the eye. Using the select signal to trigger a 555-based pulse stretcher to drag the blink out to, say, a half-second is probably the ticket, but I haven't built it yet.

November 3rd, 2017, 10:19 AM
Neat! You could probably diode-OR the signal with that of the B: drive so that the B: drive would also trigger the activity LED.

The 470R resistor is definitely too big for older LEDs. The ones that ship with the kit are HP LEDs of reasonable efficiency, but when I've built up boards using vintage LEDs I usually go to 220R.

November 3rd, 2017, 01:49 PM
I'm guessing you have two full height drives?

My Tandy 1200 had half height stacked 5.25's, so I had a big empty plate blocking the right hand bay that I drilled a hole for a round LED, right where the original MFM Hard Drives had theirs.

Then, I dug through my box of parts, found a Round HDD LED, installed, glued, connected, and turned it on...

And it was blue! Damnit. Can't win them all. Gonna have to find a red one later.