IntelliSAW IRM-48 Reader
Installation Manual
Page 18 of 28
IRM-48 Reader
910.00371.0001
segment of the bus should be shield-terminated to the protective earth in the cabinet from which
it originates.
4.2.1.3 Bus resistive termination
The RS485 bus needs to be terminated
at each end with 120 Ω resistors when long stretches of
cable are used. This ensures that the bus has the correct impedance. In general, RS485
adapters and bus masters provide the source impedance internally and should be located at
one end of the bus.
If the bus length is less than 2% of the maximum (20 meters at 9600 baud), the termination
resistor may be omitted, as long as the bus master has termination and failsafe resistors.
4.2.1.4 Bus data rate (baud rate) considerations
The RS485 bus data rate is dependent on the bus cable length and the number of readers on
the bus. In industrial environments, slower data communication rates are generally more
reliable; IntelliSAW recommends either 19200 baud or 9600 baud for the data rate.
Bus length
Bus cable length has an impact on the overall data rates which can be achieved. A
conservative rule for RS485 uses the equation: (baud rate * cable length (m)) < 10 x 10
6
. A
9600 baud network would require a bus less of (10 x 10
6
÷ 9600), or 1042 meters (about 3400
feet). This is perfectly adequate for most substation installations.
Number of Readers
As the number of readers increase on the RS485 bus, the baud rate must be adjusted to
accommodate the electrical characteristics of the communication protection circuits. The
following table is a guide for number of readers versus system baud rate:
Readers
Baud Rate
1
115,200
1
– 8
38,400
1
– 16
19,200
1
– 32
9,600
