Check the wiring!

Double-check the wiring!

In our experience, 80% of the reasons why SpecView cannot get through to the instruments is down to faulty wiring.

Things to check:

  • Is it wired up according to the Manufacturer’s installation instructions?
  • Are the terminals A/B or +/- the right way around? Try the other way, not every manufacturer has the same idea about A/B and +/-.  The correct relationship for RS485 is that A is - and B is +
  • If the convertor or instrument has 4 terminals, but you are using RS485 2-wire mode, then do the extra terminals need to be jumpered across or not? Check the instrument and convertor settings to make sure.
  • Is a termination resistor required at the end of the line (usually yes, and about 120 ohms)? Check the manufacturer’s guidelines for comms setup.
  • Do the A/B or +/- lines need to be biased?  Sometimes the comms lines need pullup and pulldown resistors to make then work properly. Sometimes the convertor does this, other times, the ‘termination’ settings on some instruments do it. Check which is which. Do not have too much termination because then the signals will struggle to get through at all.
  • Is there are Signal ground wire connected or needed? Often a convertor will have a third wire (RS485) or a fifth wire (RS422) that is designed to keep the signal grounds all connected. Is this needed or connected?
  • Shielding - if there is shielding, then it should be connected to Ground at only ONE point, not at every instrument.  If in doubt, consult the manufacturer’s wiring instructions.
  • Check connections - is there a loose screw on a terminal somewhere?
  • Check the cable routing, RS485/RS422 wiring should be daisy-chained, not ‘Star’ connected.  This means that the wires should run from the convertor to the back of the first instrument, then from that instrument to the next. See diagram on this page.
  • Does the cable run go through any connectors (such as to get inside or out of a cabinet)? if so, are these making contact properly?

Finally:

Triple check the wiring!

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(C) 2014 SpecView Corp & SpecView Ltd.  Page Updated: 11 April 2017

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