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I've been asked to test a MYDMX3.0 dongle as there is no ouput from it. It's installed correctly in device driver and hardware manager sees it and reports the serial number so I'm happy that side of things are working correctly but patching a light into the software produces no output. I've put together a simple two LED XLR tester but there's no LED activity either. I do have the DMX output LED flickering away on the dongle as well as the LCD showing Pc.

As it's well out of warranty I've removed the PCB from the case, disconnected the XLR board and checked the 5 pin output connector on the main PCB. What I am seeing is continuity between pin 7 of the MAX485 IC (DMX-) and ground and I suspect that that aint right. Before  I contemplate replacing a SM IC can someone confirm the DMX- line shouldn' t be grounded or is this perfectly normal and I need to look elsewhere?


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An update on this fault if it helps anyone else.

The circuit diagram shows Q1 an ESD device across both output lines and with a ground connection. I suspected it had done its job with the ultimate sacrifice so I lifted the ground pin from the SM device and I'm pleased to say we've now got good DMX output from the dongle.

Next step is to replace the duff diode with a PESD5V0L2BT, similar device and readily available then give it some soak tests.

One observation, the 'DMX indicator' LED between the XLR's is driven from the processor chip and bears no relation to whether there's good DMX at the output sockets.

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