User Manual

4-2 Using Modbus RTU
20-COMM-H RS-485 HVAC Adapter User Manual
Publication 20COMM-UM009D-EN-P
Supported Modbus RTU Commands
The adapter supports the Modbus RTU commands listed in Table 4.B.
Data Addresses in Modbus Messages
All data addresses in Modbus messages are referenced to zero. That is, the
first occurrence of a data item is addressed as item number zero. Therefore,
when you create a message, you must address it to one less than the Modbus
address in the manual. The following are examples:
• Logic Command is Holding Register address 4x0001 in Table 4.F on
page 4-7, so you address it as register “0000” in the data address field of
the message.
• Feedback Hi is Input Register address 3x0003 in Table 4.H on page 4-8
,
so you address it as register “0002” in the data address field of the
message.
• Start is Coil address 0x0002 in Table 4.E on page 4-6
, so you address it
as coil “0001” in the data address field of the message.
• At Speed is Input address 1x0009 in Table 4.G on page 4-7
, so you
address it as input “0008” in the data address field of the message.
Adapter Modbus Register
Map
Table 4.C provides an overview of the Modbus register addresses and their
related functions.
Table 4.B Modbus RTU Commands Supported by RS-485 Adapter
Function Code Description
01 Read Coil Status
02 Read Input Status
03 Read Holding Registers
04 Read Input Registers
05 Force Single Coil
06 Write Single Register
08 Diagnostics
Subfunction 00 Only - Return Query Data (loop back)
16 Write Multiple Registers
23 Read/Write 4x Registers
Table 4.C Adapter Modbus Register Map
Modbus Register Function
Write Product Command Word Bits
0x0001 Stop
0x0002 Start
0x0003 Jog
0x0004 Clear Faults
0x0005 Direction
0x0006
0x0007 Local Control
0x0008 MOP Increment