BRUEL & KJAER VIBROCONTROL 8000 - 9.1.2.1 RS-422 (4-Wire) Daisy Chain; 9.1.2.2 Termination for RS-485 Communication; 9.1.2.3 Maximum Racks on an RS-485 Network; 9.1.2.4 Fail Safe Biasing Resistors

Page 56 of 188
█ © Brüel & Kjaer Vibro ● S1079330.002 / V04
Technical alterations reserved
EN
9.1.2.1 RS-422 (4-Wire) Daisy Chain
The VC-8000 rack does not support RS-422 daisy chain. If the DCS (Modbus Client) is an RS-422
port, we recommend two viable options.
• Use a Modbus Serial (RS-422) to Modbus TCP converter and configure the VC-8000 racks for
Modbus TCP.
• An RS-422 (4-Wire) client (the DCS) can connect to field devices configured for RS-485
(4-Wire).
9.1.2.2 Termination for RS-485 Communication
Normally termination is not needed on RS-485 cables. If the cable length is short and the baud rate
low, reflections are attenuated by the time the signal is read, so termination is not required. However,
if your cable length exceeds the length shown in Table 9-4, you will need to terminate your network.
Terminate at the farthest ends of the network (both ends). Keep connections, from the main trunk to
each rack, as short as possible. To terminate, place external 120-ohm termination resistors between
RX- and RX+ and between TX- and TX+.
Baud Rate (bps)
Maximum Cable Length Without Termination
Resistors
1200, 2400, 4800
Termination not required.
9600
850 m (2800 ft.)
19200
425 m (1400 ft.)
38400
213 m (700 ft.)
57600
143 m (470 ft.)
115200
71 m (234 ft.)
Table 9-4) RS-485 Cable Lengths
9.1.2.3 Maximum Racks on an RS-485 Network
RS-485 multi-drop networks are limited to 64 devices and 4000 ft. (120 m) of cable total.
9.1.2.4 Fail Safe Biasing Resistors
The VC-8000 Modbus serial port does not require biasing resistors. The fail-safe function is built in.
9.1.2.5 RS-485 (2-Wire) Half Duplex Communication
Externally connect the receive and transmit pairs together (RD+ to TD+, and RD- to TD-) for half
duplex RS-485 communication. The SAM settings (RS-485 2-wire, and RS-485 4-wire) are the same;
no communication electronics are changed between these two settings.
9.1.3 Redundant Modbus Communication
Redundant Modbus is accomplished using a primary SAM (in slot 2) and a secondary SAM (in slot 3).
Each SAM works independently. and must be configured independently. For example, if you are
using a custom Modbus map, the map must be configured in both modules.