M86S

Received this item, which has all the terminals clearly labelled except the power input, which is 12-24 DC.  No "+" or "-" indications either on the board or in the manual.  Not much hope of a response from the (overseas) manufacturer.  It's a board I want to press into service, so returning it and buying another is undesirable, and there aren't many alternatives anyway (looks like most are the same).   

Removed it from its box and tried to see if I could work the polarity out from the circuit.  It turns out this device right next to the input connector  is a bridge rectifier.  Usually used to convert AC into (lumpy) DC, in this application it means that it doesn't matter which way round the power is connected!  I wish the instructions had just said that instead of omitting any mention of polarity!

Anyway, with this knowledge I got it working quickly enough!