GDS GasVac LG-1, Руководство по эксплуатации

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10
MODBUS SET UP AND INDICATORS
Modbus is a means of obtaining data from the gas detection system via a MODBUS master
controller. The system is a SLAVE to the commands received from the master via two RS485 twisted
pair communication cables. When a command is received on one of these cables, the reply is sent
on the same cable thus giving rise to half duplex communication. For maintenance purposes, two
LED’s situated on the main processor PCB close to the connectors show when a reply is being sent
on that particular cable.
In a MODBUS system that makes use of both these cable, both these LED’s flash periodically
to indicate successful communication. Once the communication has started, the system will
monitor how long since the last command was received and if this is greater than 50 seconds a
communications failure (COMMS FAIL) alarm will occur.
Each system has an internal address, which is used both for Modbus address and CAN bus address
(see repeaters). This address and other parameters can be set and will be remembered at power up
using
ENGINEER’S menu 27 Setup Network. The speed of communication (baud rate expressed in
bits per second) is set using menu 34.
All communicating devices in a Modbus system should be set to the same baud rate and the system
is capable of supporting 19200, 9600 and 4800 baud. 9600 is set by default.
MODBUS BAUD RATE
Using ENGINEER’S menu 34, the baud rate used by Modbus can be changed. Pressing UP (1)
and DOWN (5) will alter the baud rate. Pressing SELECT (3) returns you to the ENGINEER’S menu.
The Modbus protocol for the data packets received from a master is usually a request for specific
data to be sent back to the master. This protocol is covered in a separate document known as
MODBUS RTU protocol. In its simple form, the master will ask for the data kept in any of the 250
registers located in the system and will reply with the requested register data.
These registers are 16 bit numbers representing 0 to 65535 in decimal. A view of these registers
can be seen by using
ENGINEER’S menu 1. UP (1) and DOWN (5) will display different register
numbers with the data. It is useful when first setting up the system.
Additionally,
ENGINEER’S menu 4 allows you to view the Modbus data packets received and
transmitted. Again, this is used when first setting up the system and can diagnose requests and
responses.
See Modbus register definitions in the appendix. See C1148 for configuration.