Mesa 7i77 MPG Encoder connection

Following the recent (for me!) revelation that an MPG hand wheel encoder can be (and is best) connected to a 7i77 on input pins 16 - 19, I've given it a go, but I'm having problems and I hope someone can help.

I've updated the firmware on the 7i77 and the 5i25 and set the 7i77 to mode 3 when it's loaded in the HAL file. Now I can see the hm2_5i25.0.7i77.0.0.enc0 & hm2_5i25.0.7i77.0.0.enc1 encoder pins, which is all good. However, when I connect my MPG (one of these ) to pins 16 & 17 and take a look at the output in Halmeter I can see it counting up in a rather random, jerky manner when I'm not touching the wheel at all. Moving the wheel doesn't seem to influence the count.

I've put an oscilloscope on the A & B outputs from the MPG and there's no discernible noise present. I've measured the output voltage and it's +-5v. This MPG worked pretty much perfectly when connected to the encoder input on a 7i76 in single ended mode.

When I check the other - not connected - encoder pin, there is no count, it stays at zero as it should. If I switch over the connections, the other encoder pin counts in the same manner.

Any ideas where I'm going wrong?

The most likely problem is that you do not have a common ground
between the MPG and the Field I/O (7I77 TB2 pin 8)

Ah yes, that sounds like it may well be the problem. Previously the MPG was supplied by 5v from the encoder interface, but the TB2 field power is 24v, so I took a 5v feed from a spare PC power supply cable, which also connects to TB1 - I never thought about the lack of connection between them.

I'll check it out tomorrow evening and see if that fixes it.

Yes, you were correct - connecting the 0v from the two supplies together fixed the problem.

I checked the resistance between them before making the connection and it was a couple of KOhms. The actual voltage difference between them when powered up was much less than a volt and the positive sides looked good relative to each other too, but it was clearly enough to give trouble.

It just goes to show that you can't really assume anything with electronics!

Many thanks

Ben