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Appendix A MODBUS Protocol
The Modbus Protocol is an option that can be enabled at the factory or can be enabled in the
field via a software key using the Web Browser Option Upgrade screen.
The following Modbus functions may be used.
Modbus Function
03
16
A function value of 03 (Read Holding Registers) is set to read alarm data from the system (in
slave configuration). Single and multiple register writes are supported. Exception codes for
unrecognized received messages are also included.
The system has the capability of operating with two distinct data formats: Status Mode (On/Off
status) and Event Mode (Alarm Status with Time Stamp). These formats are embedded within
Modbus data registers for transmission and reception.
Modbus Operation
The DMS-3K Modbus operates as a Master or Slave in both Modbus RTU and Modbus TCP/IP
modes. The DMS-3K Modbus port will transmit the ON/OFF Status (called 'Alarms') and the
timestamp recorded for every alarm and return to normal occurrence (called 'Events').
Modbus Alarms
Alarm ON/OFF Status is provided by reading 16-bit holding registers (Modbus Function 03) with
a single bit designated per input (or group of inputs). A 48 input system could indicate the status
of all alarms using three 16-bit holding registers. The Modbus implementation also allows the
transmitting or receiving of the Control Pushbutton functions for Control Group 1.
(Acknowledge, Test, reset, etc)
Modbus Events
Event Data is provided by reading six 16-bit holding registers (Modbus Function 03) that contain
the time, date, input number and alarm status for each event. An event could be an input going
into alarm or returning to normal. Event data is provided in blocks of ten events for a total of
sixty 16-bit holding registers (Six 16 bit registers per event x 10 events). If there are no new
events since the last Modbus poll, it will return all FF's. If there are more than 10 events in the
buffer, then it will continue to submit 10 events every new poll until there are no new events.
166
Description
Read Holding Registers
Preset Multiple Registers
Data Units
Mode
Words
Master or Slave
Words
Master or Slave
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