My CORE is powered through the MIDI OUT port - what to check?

[WickedBlade]

Hello,

I'm at a troubleshooting step in my project. So basically I fired it up and, as feared, nothing worth mentioning happened. However, I have checked that my CORE module with LCD is working. When I power up, I get the MIOS version message then the LCD prints "Ready."

I had previously tested the CORE, of course, and had been able to upload MIOS, as you can guess.

But I have made some changes lately (theoretically I was just wiring stuff to various J-ports, and I'm testing with the ports unused - nothing plugged), and now it seems that my CORE is powered through the MIDI out port: when I plugged the CORE to my PC via MIDI, I noticed the LCD lighting up. When my DOUT module is also connected, I notice that my common anode 7-segment LED digits lights up too. So it looks like there's some power coming through the MIDI out.

So I have 2 questions:

• is it normal that I measure about 5V between M+ and ground on my MIDI plug (on the PC), or is the PC MIDI output malfunctioning in some way?
  
• looking at the CORE schematics, I see that said M+ is connected to +5V through resistor R8, so it might be normal that MIDI out feeds the modules... Am I right and is this a normal behaviour, or is there indeed something wrong?
  

I hadn't noticed this behavior before, and I'd rather make certain before frying things.

Thanks.

[WickedBlade]

All right,

it looks like I mixed up the entries on the PC-side and actually plugged the PC MIDI Out to the MIDIBOX MIDI Out (instead of OUT with IN) -- my PC cable is a bit vague as to which connector is which.

I had a look at the MIDI specs for the PC cable (Sound Card Game Port to MIDI connector)  and it looks like this would effectively connect the PC +5V signal to the MIDIBOX +5V signal through 2 220 ohm resistors (one on each device). So it looks like it could result in some remote powering of the CORE... Unless I got mixed up regarding pin placement in the DIN plugs...

I'll try plugging my device in the correct way later, hoping I haven't fried anything...

I'd still appreciate some opinions on that, though.

[nILS]

If you fed the core with <=5V (polarity is correct so that's not an issue) you won't fry anything :-)

[WickedBlade]

Thanks for the help.

Well if I fire up my CORE module + LCD it does work correctly. Now that I plugged the MIDI in/outputs correctly, I do see the incoming SysEx message (F0 00 00 7E 40 00 01 F7, does this mean anything besides an upload request?) but I don't seem to be able to send data through the MIDI in port. I tried the LCD Debug feature in MIOS studio, it doesn't work. Likewise, Hex update doesn't work. (BTW I had already uploaded MIOS v1.9f some weeks ago when everything was working). Of course I tested my PC ports by using another MIDI device, it works.

So it looks like I have no MIDI in. I checked the wiring/soldering from the plug to the optocoupler, and from optocoupler to PIC, it looks to be OK. I'm wondering if I haven't fried the optocoupler somehow? Either that or I partially fried the PIC, but that is unlikely (I get SysEx 'welcome' and the LCD works fine), and that would be awful.

So what next? I guess I'll have to build the little optocoupler test circuit tomorrow...

[WickedBlade]

So I've built (awkwardly) the optocoupler test circuit as seen there: http://www.ucapps.de/howtodebug/mbhp_core_opto_test.gif

If I built it correctly, then I think I have a fried optocoupler on my hands. The thing is, I'm not sure of it. Yes, I've double-checked my circuit, but, you know, once you start doubting...

So here's the thing:

• I know for a fact that my MIDI cable and the MIDI port on the PC side is working: it works with other MIDI devices that I have, and it also worked with the very same CORE module weeks ago, I used it to upload MIOS to the CORE!
  
• the optocoupler has worked before (I used it to upload MIOS, and a few days later to impress my girlfriend with the LCD debug message from MIOS Studio)
  
• I may have fried it by plugging the MIDI plugs in the wrong order (IN with IN and OUT with OUT), though I fail to see how plugging 2 optocouplers together (one on each site of the MIDI connection) would lead to frying one (and apparently, not the other)
  
• the optocoupler testing circuit measures 5V when the switch is open... but I have 5V too when it's closed!
  

I'm a bit surprised at the latter value. I was expecting something maybe too big, at least bigger than the specified 0.10V... But 5V! That's what's making me doubt my circuit...

I should mention that as I didn't have any 1k or 1.2k resistor, I used a 1.1k that I had. I figured it should work the same. Was I wrong?

I'd very much like any opinion on that, please help, I'm getting nuts!

[nILS]

[ x ] I blame the opto.

They're easy to kill. Replace it.

[WickedBlade]

Well yes, that's what I'll try ASAP, but it will take a whole week for one to get to me, so I'd like to check that it's indeed the optocoupler's fault in the meantime, just in case, you know... I'd rather not wait for a new one to find out much later that it wasn't the culprit...

I measured stuff again, and I find one thing strange. When the switch is closed on the test circuit, I measure 4.98V between pins 6 and 5, as I said. But, stranger still, I measure 1.8V between pins 8 and 5. How is that possible? Shouldn't I at least be reading 5V there?!

I'm using the +5V and ground supplies from connector J9 on the CORE, by the way... How come the vreg doesn't keep giving me 5V... The PIC and LCD seem to be working fine at that point, though. I'm thoroughly dumbfounded!

EDIT: Well, I failed at building the circuit tester. Will try again and keep you posted.

[WickedBlade]

Alright, with the correct circuit I measured 44mV between pins 6 and 5.

Could this be too small a value?

[WickedBlade]

I did the loopback test as explained here:

http://www.ucapps.de/howtodebug/mbhp_core_extract_io_loopback.gif

And nothing happens, the signal my PC sends doesn't get back. So I guess it confirms there's something wrong with MIDI IN. I measured and checked the signal path on my CORE at various points around J13 and the optocoupler and was able to confirm that the signal path is OK and R5, R6, and R11 have the expected values. I confirmed that I had ground and +5V at the expected locations too.

So I guess it means that, indeed, my optocoupler died on me. What I don't quite understand is, if I had just stopped at the optocoupler test (mentioned earlier), I would have guessed that it worked correctly, what with reading 44mV when the switch is closed. Apparently, though, this is not a correct value.

Can a knowledgeable expert confirm that this is indeed too far from the expected value of 0.1V? This might help others... What is the acceptable range around that 0.1V value?

Thanks.

[WickedBlade]

Sorry for asking so many things but I'm not sure I've routed my devices correctly in MIOS Studio. If I didn't I may be sending things incorrectly, right?

This is my current routing, is it correct? The PC is connected through the MPU-401 interface.

[WickedBlade]

If anybody cares, I'll answer my own questions, in the hope that it may help someone:

• My settings on MIOS Studio were wrong. First, it's easier to hide the internal settings, second, a good routing would be thus:
  
• In the optocoupler test circuit, if you are too far from 0.1V when the switch is closed, this isn't good. I've experienced myself that 44mV is bad, and I've seen posts hinting that 70mV is bad too... No idea of a correct range
  
• The sysex message "F0 00 00 7E 40 00 01 F7" is the upload request, it should appear in the MIDI IN monitor of MIOS Studio when you fire up your MIDIBOX. Only once if you have a MIOS already uploaded, repeatedly if you only have the bootstrap loader.
  

FWIW I bought a new opto, it arrived yesterday, I popped it in and it worked. So yes, as everybody already knows, the optos are fragile things, it's better to order a few spares.