Zener MSC-3 Instruction Manual page 58

Quick Setup for Config 3.
The purpose of the terminator is to supress electrical reflections (echo) that may occur on a long
communications cable where the time for the signal to travel the length of the cable becomes significant
compared to the rise time of the signalling pulses. In more compact installations, this will not be an issue
and the system may benefit from the slightly higher signal levels that result from terminating resistors not
being fitted.
General
• Use twisted pair shielded communications cable.
• Connect EIA/RS-485 common terminals in addition to data conductors.
• A linear wiring scheme (daisy chain) is preferred over a star arrangement or one with stubs.
• The cable should have its shield connected to ground at one end only (earthing recommended at
the computer / controller end).
• Avoid laying communication cables adjacent to power cabling and wiring. If not possible utilise
the best separation of communication cabling and power cabling. Communication cables should
cross power cables at right angles..
• The EIA/RS-485 standard allows a total of 32 standard load units on a network segment. Each
MSC-3 interface is 0.5 standard load units, allowing a master device and at least 62 MSC-3
drives. Network loading for other equipment may vary – check with the equipment manufacturer.
Shortcuts
From time to time we are asked if all this is really necessary and perhaps pointing out some functional
installation installed in some lesser way. Will it work? Can it be made to work? The short answer
is basically yes, The downside is that compromise arrangements tend to be a source of frustration
with issues like setups that work during a bench test but troublesome in the field. There is also some
equipment that doesn't have an accessible common connection. For these compromise situations – ask
us – we can almost always find a creative solution!
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Typical EIA/RS-485 wiring arrangement
IM00120 IM00120