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RS485/RS232 Communications Interface
System Recommendations
Note:
It is possible to make serial communications operate without adhering to the following
recommendations, however, the recommendations will promote greater reliability.
•
An RS485 two-wire system can only be used in a network in which all devices use their tri-
state capability. Data flow is restricted, i.e. transmit and receive cannot be simultaneous (half
duplex). The driver in an RS485 system has tri-state capability (i.e. its output can be
disabled) which allows multiple transmitters to be connected to the same bus. RS485 thus
supports “multi-drop” operation. In multi-drop systems there is always one device which is a
“Master” and which sends messages to or requests data from the “Slaves”. A Slave never
initiates a communication.
•
An RS232 three-wire system always has a “Master” which sends messages to or requests
data from the “Slave”. The Slave never initiates a communication. There is only one
"Master" and one "Slave" in the system.
PLC/SCADA Supervisor
If possible, avoid using a PLC/SCADA supervisor which take its transmitter to a high
impedance state (tri-state) when idling. If it is unavoidable, then it is essential to use properly
screened cable.
RS485 Cable Specification
Use cable which has twisted pairs and
one overall screen, as shown. The
characteristic impedance should be in
the range 100 to 165 Ohms.
Recommended Cable Specification
Characteristic Impedance
100-165
Ω
at 3-20MHz
Cable Capacitance
<30pF/m
Core Diameter
0.34mm² (22 AWG)
Cable Type
Twisted pair cable
Resistance
<110
Ω
/km
Shielding
Copper braid, or braid & foil
Note:
Belden 9841 cable meets the above specification, but there are others.
RS232 Cable Specification
There are no special requirements for RS232 cabling, but we do recommend a maximum length
of 3 metres between Master and Slave.
screen
insulation
outer sheath
conductors
twisted pair
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