©201
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Nelson Heat Tracing Systems
www.nelsonheaters.com
GA2497 Rev.
5
CM-2201/CM-2202
Communications
8.0
Communications
The Nelson Heat Trace CM-2201 supports a subset of the
Modbus
®
RTU protocol format that provides monitoring,
programming, and control functions using Read (03) and
Write (05-06) register commands.
8.1
General Information
Serial Port:
Baud Rate:
Select the serial portthat
corresponds to your RS-485
adapter. USB to Serial adapter
may be used for devices without
serial connections.
User Defined at 1200, 2400,
4800, 9600 or 19200
Data Bits:
8
Stop Bits:
Parity:
Device Address:
1
None
User Defined between 1 and 255
8.2
Modbus Registers
For all Modbus registers, see Appendix D.
Example of checking alarms via Modbus from a remote
terminal.
Controller SETTINGS and Status used on this example:
a. Modbus address 04
b. Baud rate 19200; non-parity; Stop bit 1.
c. High Temperature alarm active
1.
To check if any alarm is activated, send the
following instruction:
04 03 0046 0026 2590
04 = controller Modbus number
03 = Modbus function code 3
0046 = start reading from data address
70
0026 = the next 38 addresses
2590 = CRC
2.
The controller will answer with value 0x02 (High
Temperature Alarm) active Alarm
3.
Once the alarm condition has been resolved, the
alarm can be cleared by sending the following
instruction:
04 05 0045 FF00 9C4C
04 =controller Modbus number
05 = Modbus function code 5
0045 = writing to data address 165
FF00 = erase command
9C4C = CRC
At this point the controller enters the
alarm into the alarm log memory and
clears the active alarm. To check the
alarm log memory, see below.
4.
If the command to check if any alarm is
activated (as shown in (1) above) is now sent to
the controller, and assuming no other alarms
have since been activated, the controller will
answer with 0x0000 (no active alarms).
5.
To check the alarm log memory, issue the
following instructions:
04 03 006E 0026 A598
04 = controller Modbus number
03 = Modbus function code 3
006E = start reading from data address
110
0026 = the next 38 addresses
A598 = CRC
6.
The High Temperature Alarm will be listed here,
so the controller will answer 0x02. The Alarm
log can hold 20 alarms. After 20 alarms the
1
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