Delta RMC151 - Manual (Page 560)

RMC70/150 and RMCTools User Manual
19,200 3750 4000
9,600 4000 4000
4,800 4000 4000
2,400 4000 4000
6.10.3.6. RS-485 Termination and Biasing
Termination and Biasing are concepts that only apply to differential wiring. As such, they only
apply to RS-485 and not RS-232.
Note:
Delta recommends you always use biasing on a RS-485 network. To determine whether you
need termination, read the Termination Concept section in this topic.
Selecting Termination and Biasing on the RMC70
The RMC7xS termination and biasing can be independently selected with jumpers on the
8-pin RS-485 connector. The locations where the jumpers should be installed are marked
on the label.
To select termination: Insert a jumper between pins +T/R and Trm Jpr.
To select biasing: Insert a jumper between pins SCmn and Bias Jumper.
The Termination Concept
Cable termination is a way of absorbing transmitted energy at the end of a network. This
prevents signal reflections from bouncing back towards the transmitter and potentially
upsetting signal quality and communications.
The termination resistor should match the characteristic impedance of the cable being
terminated. The effective impedance of the RMC7xS's termination resistor and biasing
resistors is 114Ω. Therefore, cabling with impedance of 100Ω to 120Ω is recommended.
Termination should be placed at the end of the network for each wire pair. For RS-485
(2-wire, point-to-point or multi-drop), terminate the wire pair at each end of the network.
The diagram in Serial Network Topologies shows the correct location of the termination.
Termination and Cable Length
Termination is not required on all differential networks, but it does typically extend the
maximum cable length. The following chart shows the maximum cable lengths at various
baud rates with and without termination:
NOTE:
The maximum cable length is the length of the entire network and not just the distance
between nodes on the network.
Termination vs. Cable Length:
Baud
Rate
Max
Unterminated
Cable Length (ft)
Termination
Requirements
Max Terminated
Cable Length
(ft)
115,200 475 Required beyond 475 ft 3250
57,600 950 Required beyond 950 ft 4000
38,400 1900 Required beyond 1900
ft
4000
19,200 3750 Required beyond 3750
ft
4000
540 Delta Computer Systems, Inc.