BENSHAW RediStart MX3 Series, User Manual

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Remote Modbus Communications
7.15
Remote Modbus Communications
The MX
3
starter provides Modbus RTU to support remote communication.
The communication interface is RS-485, and allows up to 247 slaves to be connected to one master (with repeaters when the number of
drops exceeds 31). Please refer to Figures 62 and 61 for connection diagrams.
7.15.1 Supported Commands
The MX
3
supports the following Modbus commands:
•
Read Holding Registers (03 hex)
•
Read Input Registers (04 hex)
•
Preset Single Register (06 hex)
•
Preset Multiple Registers (10 hex)
Up to 64 registers may be read or written with a single command.
7.15.2 Modbus Register Addresses
The Modbus specification defines holding registers to begin at 40001 and input registers to begin at 30001. Holding registers may be
read and written. Input registers may only be read.
In the MX
3
, the register maps are identical for both the holding registers and the input registers. For example, the Motor FLA (QST 01)
parameter is available both in holding register 40101 and in input register 30101. This is why the register addresses in the Modbus
Register Map are listed with both numbers (e.g. 30101/40101).
7.15.3 Cable Specifications
Good quality twisted, shielded communications cable should be used when connecting to the Modbus port on the MX
3
. The cable
should contain two twisted pairs and have an overall shield. Use one pair of conductors for the A(-) and B(+) signals. Use the other pair
of conductors for the Common signal. The cable should adhere to the following specifications.
•
Conductors: 2 twisted pair
•
Impedance: 100 Ohm to 120 Ohm
•
Capacitance: 16 pF/ft or less
•
Shield: Overall shield or individual pair shields
Examples of cables that meet these specifications are Belden part number 9842 and Alpha Wire part number 6412.
7.15.4 Terminating Resistors
The MX
3
does not have a terminating resistor for the end of the trunk line. If a terminating resistor is required, the resistor must be wired
to the terminal block.
The purpose of terminating resistors is to eliminate signal reflections that can occur at the end of a network trunk line. In general,
terminating resistors are not needed unless the bit rate is very high, or the network is very long. In fact, terminating resistors place a
large load on the network and may reduce the number of drops that may be placed on the network.
The maximum baudrate of 19,200 supported by the MX
3
is not high enough to warrant a terminating resistor unless the network is
extremely long (3,000 feet or more). A terminating resistor should only be installed on the MX
3
if signal reflection is known to be a
problem and only if the MX
3
is at the end of the network. Terminating resistors should never be installed on nodes that are not at the end
of the network.
7.15.5 Grounding
RS-485 buses with isolated nodes are most immune to noise when the bus is not connected to earth ground at any point. If electrical
codes require that the bus be connected to earth ground, then the Common signal should be connected to earth ground at one point and
one point only. If the Common signal is connected to earth ground at more than one point, then significant currents can flow through the
Common signal when earth ground potentials are different at those points. This can cause damage to devices attached to the bus.
7.15.6 Shielding
The shield should be continuous from one end of the trunk to the other. The shield must be tied to the RS-485 Common signal at one
point and one point only. If the shield is not tied to Common at any point or is tied to Common at more than one point, then its
effectiveness at eliminating noise is greatly reduced.
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7 - THEORY OF OPERATION