ZENER 8000-V Installation Manual
IM00165
25
Terminating resistors
For best performance with long cable runs and high data rates a terminating resistor should be fitted at each
end of the cable run. Values of 100 or 120 Ohms are commonly used, connected between the A and B terminals
of the first and last devices on the cable run. For convenience, the ZENER 8000-V
EIA/RS-485 interface incorporates a 120 Ohm terminating resistor that is controlled by menu item G167
TERMINATOR. Refer to the
ZENER 8000-V Reference Manual IM00140
for details. A terminator should not be
fitted to, or selected at, intermediate devices on the communications cable.
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 ZENER
8000-V interface is 0.5 standard load units, allowing a master device and at least 62 ZENER 8000-V
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!
Master device
Typically PLC or
computer with
EIA/RS-485
adapter
ZENER 8000 Drive
(Slave device #1)
A
12
RS-485 / EIA-485
Serial Communications
B
13
C
14
A
12
RS-485 / EIA-485
Serial Communications
B
13
C
14
A
12
RS-485 / EIA-485
Serial Communications
B
13
C
14
A
RS-485 / EIA-485
Serial Communications
B
C
ZENER 8000 Drive
(Slave device #2)
ZENER 8000 Drive
(Slave device #3)
Communications
cable screen
grounded at
one end of run
only
Only 3 slave devices shown. More slave devices (ZENER 8000 and other) may be added up to the loading limit of 32 units.
Each ZENER 8000 interface is 0.5 load units.
Typical EIA/RS-485 wiring arrangement
