ATI Q46 Modbus Communications Manual
5
O&M Manual
Rev-D (2/16)
1.2 Modbus Connection
Modbus wiring is done at a plug-in communication circuit board located in the outlined section of
Figure 12 above. A call-out in that figure shows the location of RS-485 connections. The earth
ground wire in the 485 cable should be terminated at “COM” on the Modbus card.
1.3 Registers and Coils
Modbus protocol was originally designed to transfer data to and from PLCs (Programmable Logic
Controllers), which organize data into groups of registers and coils. PLC registers containing i/o
information are called input registers and are numbered 30001 to 39999, while registers
containing data or the results of calculations are known as holding registers and are numbered
from 40001 to 49999. The term coils, on the other hand, refers to discrete (0 or 1) inputs and
outputs. Traditionally, these are inputs from such things as switch closures and outputs to the
coils of relays, which are under the control of the PLC.
All registers are 16 bit values, which may be read or written to individually, or in blocks by using
specific functions. Likewise for coils, which are one bit values. Since register functions transfer
16 bits and discrete (coil) functions transfer only one, it is usually more efficient to use register
functions, which reduces the number of messages required to transfer data. For this reason, the
Q46 Series transmitter organizes all of its data into input registers only, or more specifically, data
is organized into the holding registers starting at 30001.
The protocol specifies which registers to access by the value of the function code embedded into
the message. For example, to read one or more holding registers in a slave device, the master
must use function 3 – “Read Holding Register”. Similarly, the master must use function 4 – “Read
Input Register” to read one or more of the input registers. The Q46 only responds to request for
reading input registers (Function 4).
For more information on the protocol, please refer to the “Modicon Modbus Protocol Reference
Guide” at http://www.modicon.com/techpubs/toc7.html or, “Modbus Protocol Specification”,
available for download at http://www.modbus-ida.org/specs.php. Deviations from this guide are
noted in the appropriate section. More information regarding Modbus, in general, may be viewed
at: http://www.modbus-ida.org/
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