Caple C806C Bedienerhandbuch PDF herunterladen (Seite 14)

PF2100

Modbus Card Instruction Manual v1.1

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.

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100,000,000

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

24 kOhm

48 kOhm

128

96 kOhm

1/8

256

(Modbus further limits this to 247)

1 2 ... 9 10 11 12 13 14 15 16 17 18 19 ... 25 26

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