Technical information

8 – THEORY OF OPERATION
Using Modbus Communication
137
8.11 Using Modbus
Supported Commands
The MX 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.
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, the register maps are identical for both the holding registers and the input registers. For
example, the Motor FLA parameter is available both in holding register 40050 and in input register
30050. This is why the register addresses in the Appendix D – Modbus Register Map, are listed with
both numbers (e.g. 30050/40050).
Cable Specifications Good quality twisted, shielded communications cable should be used when connecting to the Modbus
port on the MX. 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.
Terminating Resistors The MX contains a jumper site (JP2) located next to the Modbus connection terminals for installing a
120 Ohm impedance matching terminating resistor. Installing a jumper on JP2 connects a 120 Ohm
resistor between the RS-485 A(-) and B(+) lines.
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 baud rate of 19,200 supported by the MX is not high enough to warrant a terminating
resistor unless the network is extremely long (3,000 feet or more). A jumper should only be installed
on the MX if signal reflection is known to be a problem and only if the MX is at the end of the
network. Terminating resistors should never be installed on nodes that are not at the end of the
network.
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.
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.