Detcon 880S-N4X - Manual (Page 34)

880-N4X
880-N4X Instruction Manual Rev. 1.0 Page 30 of 40
Slave Master MV Difference
ID 01 425 451 26
ID 02 425 430 5
ID 03 425 486 61
ID 04 425 410 -15
ID 05 425 420 -5
ID 06 425 432 7
ID 07 425 422 -3
ID 08 425 415 10
ID 09 425 418 -7
ID 0A 425 440 15
ID 0B 425 310 115
Results similar to those
shown here should be
expected. In this example
slave device 03 and 0B
would need to be changed.
Measuring at the master device between dc common and A, or dc common and B should give a reading of
around 2.5vdc. Remember this is with the power on, but with no communication taking place.
Another method of troubleshooting is to enable the communications and remove all the slaves. Proceed to
plug 1 slave in and see if communication is good. If so, plug in the next slave and check again.
Repeat this with each slave until all of the slaves are plugged in or one of the slaves takes the data bus down.
The following table consists of recommended cables and their specifications from Belden. These cables are
designed with RS-485 in mind and should be used for RS-485 data buses.
Belden# pairs AWG Impedance
9841 1 24 120ohm
9842 2 24 120ohm
8132 2 28 120ohm
The root cause of many RS-485 communication problems can be traced back to incorrect cabling, incorrect
cable and wire types, and the use of incorrect wiring schemes. It is easier to avoid these problems by
following a few simple guidelines:
9 Use good wiring schemes (refer to Appendix A).
9 Use recommended cables.
9 Properly connect data lines and shields.
9 Insure proper termination of data lines.
9 Use isolated repeaters to avoid grounding problems.