Testing MIDI in/outs

[diydidi]

Hi.
Is there an easy way or a circuit I could construct to test MIDI in/outs of devices that I get for repair?
I often get keyboards, sound cards, drum modules etc, that need testing of the MIDI ports.
I dont feel like buying expensive software, interfaces.....

[Jazz P Bass]

Good question!

[eschertron]

Before I knew very much about electronics (as if I know anything now) I successfully built a simple MIDI interface from a schematic I found on the internet. A couple opto-isolators and some TTL logic if I remember correctly. I know you don't want to build a MIDI interface - not a standard kind of one, anyway - but I thought I'd share that it's pretty basic stuff, at least the 5-pin DIN variety is, as it's based on 70's and early 80's technology. Look around for schems and tutorials, I'd think you can make a breakout box with just a few parts and almost no cost.

Or even with no hardware besides a scope, a pinout of the MIDI connection should help you test some basic functions. JM2C

[Enzo]

I still have a couple of the old Ensoniq MIDI-bug testers. It was a rubber insect with LED eyes and a MIDI cord. You plug it into any MIDI OUT or MIDI THRU and its eyes blinked whenever a MIDI signal went by. It was a fast go/no-go test.

MATRIXSYNTH: Ensoniq MIDI Bug


The way MIDI works is you have a current source (a resistor to +5v) in the sending unit, and it connects that to one pin of the cable. In the receiving unit, that current source is fed through the LED in an optocoupler with a series resistor to limit current and back out the cable back to the sender. And finally back in the sending unit, the is a logic gate or a transistor driving the line with its own series resistor. All the resistors are like 220 ohm. SO in essence, the MIDI transmit is just a little circuit to control an LED that is at the other end of a cable.

To make a tester, try putting an LED and a 220 ohm resistor in series betwen the second and fourth pins of the MIDI plug.

I can never remember which pins are which, so if you get it backwards it stays dark. But just probe the pins, the one with 5v on it is the 5v pin. MIDI uses pins 4 and 5, but the pins are not numbered in order, so those are the second and fourth pins.

[Steve Conner]

It is pretty much as simple as that. I made my own from the head of a Darth Vader plastic toy, with red LEDs mounted in the eyes. "Darth Midi" was a permanent fixture sticking out of the MIDI patch bay in my home studio.

Of course this just shows that some sort of pulses are coming out, not that they are valid MIDI data. But usually the MIDI stuff is integrated in a chip somewhere and it either works fine or not at all.

[Enzo]

Right, it doesn't identify the pulses, but if you just press a key down, you see the note-on flash, then release the key and see the note off. If you have aftertouch, press a key lightly, wait a second, then push down harder. The flash will tell you aftertouch data was sent. MIDI doesn't usually junk up the pipeline when the instrument is sitting there, so it isn;t like a data line in a microprocessor circuit. SO if you only generate one event at a time, it is likely that is what the MIDI information relates to.

[diydidi]

Thanx guys. I have found a little circuit at instructables.com.