MOD3CT
User ManualUser Manual
5A. TYPE OF CABLE TO USE
The cable to be used is a shielded twisted pair (telephone type). AWG18 cable may be
specified, but different types of cable with equivalent characteristics can be used.
The twin cable consists of two conductors that are twisted together.
This arrangement improves immunity to electromagnetic disturbances because the cable forms
a series of successive coils, each of which faces in the opposite direction to the next one.
In this manner any magnetic field in the environment traverses each pair of coils in opposite directions
and its effect is therefore reduced.
5B. MODBUS BASICS
A Modbus RS485 connects a Master device to one or more Slave devices,
which are measuring instruments with serial communication.
Each device has a communication port with two terminals, which can be
marked A and B. In both terminals the communication cable is connected
so that all the devices that form part of the communication are connected
in parallel.
All the ‘A’ terminals must be connected together and all the ‘B’ terminals must
be connected together, respectively.
Inverting the ‘A’ and ‘B’ connections of a device does not only prevent it from
communicating but may also stop the entire communication system from
working owing to incorrect direct (polarisation) voltage found on the terminals
of the incorrectly connected device.
In order to avoid errors when many devices are connected, cables of the
same colour should be used for all the connections to the terminals A and
cables of the same colour should be used for all the connections to the
terminals B of the various devices (e.g. white for A and blue for B).
5C. BAUD RATE & BIT RATES
Bit rate is a measurement of the number of data bits (that's 0's and 1's) transmitted in one second. A figure of 2400 bits per sec
means 2400 zeros or ones can be transmitted in one second, hence the abbreviation 'bps'. Baud rate by definition means the
number of times a signal in a communications channel changes state.
One of the more common baud rates, especially for simple communication where speed is not critical, is 9600 bps.
Other “standard” baud are 1200, 2400, 4800, 19200, 38400, 57600, and 115200. The higher a baud rate goes, the faster data
is sent/received, but there are limits as to how fast data can be transferred.
Common Settings for Modbus - 9600 or 19200. Parity (Error Checking) is EVEN or NONE.
ALL items MUST have the same settings otherwise they CANNOT communicate.
6. METER READING
The viewing angle Operator meter should be at 45°.
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