IBM I/O Tester

This beautiful blinkenlight machine was donated to the channel, and we did not originally know what it was. But we quickly figured out it is an IBM I/O Tester, a customizable control panel that you can plug in many peripherals from the IBM 360 and 370 computer series to diagnose them. And, as it turns out, it plugs right into my IBM 2501 card reader, so we'll certainly need to us it in the future! It is in essence a portable logic analyzer. It features a slot in the front where you can slide an overlay, which will name everyone of the lights according to the peripheral you are testing. You can also slide a name insert for the switches at the bottom.

Photos below are by Ken Shirriff, who wrote a Twitter thread about us power up this machine:

Here is my IBM 2501 card reader that this instrument can plug into for debugging. This tester supported many more instruments, see full list in the documentation section.

The lights are smarter than you'd think, with comparators and amplifiers that will sample the signals for the correct logic level, and not just simple lights. See schematics in the documentation section.  The comparator / driver circuits are IBM SLTs, the format used for the IBM 360 construction. 

They are neatly arranged in little paddle cards, the same construction arrangement as in the IBM 360.

An internal power supply takes 24 VAC in borrowed from the IBM device you plug it in through two pins from connector A and generates +3V and +12V internally. Its electronics are in the IBM SMS card format, the older format used in the construction of our transistorized IBM 1401 computer from 1958. It has only 3 transistors on it.

The switches are just that, and you have direct access to their common, N.O. and N.C. contacts.  I repaired our broken ones by machining new parts, but I later found the exact part number. See the restoration tips below.

Finding a mating connector for the two main connectors was a challenge also. I eventually found the exact part and part catalog. These connectors are highly configurable and you have to get the pins, the housings, and the keying options separately, or mix and match from eBay connectors. Also see the restoration tips.

Here is one of the the matching connectors in my IBM 2501 card reader.

IBM I/O Tester Video

We restore the mysterious IBM I/O Tester, find out how it works, and use it for something new.

Restoration Tips


Lamp bulbs are 2158 type, T1-3/4 wire terminal 3V 15 mA, available here:

and here


Connectors are AMP/TE M-Series 104CF

Male AMP 201692-4

Female AMP 201532-4

Contacts, hood and receptcale are separate parts (see catalog in documentation section)

Corner guide pin male: 202173-8

Back Hood: 202169-4

Male Pin Solder: 164162-1 (gold) 164162-1 (Nickel)

[Male Pin Wrap Post: 66460-7 

Male Crimp Contact: 164163-2]


Toggle switches are branded C-H 3A 125V Uno. Lab. Insp.

The switch is IBM p/n 738826. Also used on 3741 Data Station, 1052 printer keyboard and IBM 029 punch.


This instrument is normally powered by the device it is plugged in. It gets 24V AC from pins 82 and 80 on connector A. 

But you also have to provide a jumper from pin 103 to pin 84 on connector A to complete the 3V circuit that powers the lamp.

An internal power supply generates 12V that powers the electronics and 3V that powers the lamps. 

It is possible to bypass the internal power supply and power it directly with 3V on pin 103 and 12V on pin 99, with the common ground on pin 98 (connector A). In this case, do not use the jumper. 

See details in my connector pinout reverse engineering notes.

I modified my instrument by adding a special plug on the side to power it up with 24V AC, and at the same time automatically close the pin 103 to 84 jumper, so I can use the instrument on its own. But if I disconnect this external supply, it reverts to its normal IBM configuration and can be plugged into the IBM peripheral and powered as originally intended.


A few overlays came with the machine, Al Kossow kindly donated more of them. They have all been scanned and are available in the documentation section.  One funny note about that, one of the original overlays that came with the machine had a bold handwritten inscription on the back of one of our overlays, that warns that you'd endure the wrath of Howard Porter if you removed the overlay from the lab. We show it at the end of the video. It turns out that Howard Potter was the manager of the new Diagnostic Support Center established in Poughkeepsie in 1974, to help support their field engineers, the famous IBM C.E. (Customer Engineers). Thanks to the IBM archives for that tidbit of information.

I had remade my own version of the switch overlay, which I printed on high quality photo paper on a color printer. The vector design files (in Affinity Designer format) and the .jpg are in the documentation below.