Instruction manual

PF2100
Modbus Card Instruction Manual v1.1
©2011 Profire Energy Inc
Jan 5, 2012
Page 10
Termination
The two devices at the far ends of the bus should both be terminated. All other devices on the bus must NOT be terminated. For Cat-5e cable, a 120 Ohm
resistor is used to terminate the devices. The Modbus card has a built-in, jumper selectable 120 Ohm termination resistor. If another cable type is used, an
external termination resistor of a different value may be required instead.
Baud Rate
It is not required that all devices on the bus communicate at the same baud rate but it is generally a good idea to do so. It is, however, required that any devices
that must communicate with each other must do so at the same baud rate. Furthermore, all devices on the bus must communicate at less than the maximum
baud rate.
To determine the maximum baud rate that you can safely run your system at, you need to know the total cable length (L in meters) from the first device on the
bus to the last device on the bus. Plug that number into the following equation to determine the maximum baud rate in bps. Note that this equation provides
an approximation assuming that Cat-5e cable is used with proper termination.
ܤܴ
ெ஺௑
=
100,000,000
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Ensure that the baud rate setting selected on the Modbus Card DIP switches is less than or equal to the calculated maximum baud rate. Also ensure that all
other devices on the bus are configured to communicate at less than or equal to this baud rate.
Number of Devices
The maximum number of devices on the bus is a function of the input impedance of each device’s receiver. The device with the lowest input impedance will
determine the maximum number of devices. Generally speaking, older RS-485 devices will have lower input impedances and therefore lower maximum device
counts. The following table can be used to determine the approximate maximum number of devices supported given the input impedance of the lowest
impedance device on the bus.
Input/Output
Impedance
Unit
Load Maximum Devices on the Bus
12 kOhm
1
32
24 kOhm
½
64
48 kOhm
¼
128
96 kOhm
1/8
256
(Modbus further limits this to 247)