Woodward EGCP-2 - Chapter 6. Modbus Rtu Communications for the Egcp-2 Panel; Introduction; Modbus Configuration Items

Manual 26181 EGCP-2 Communications
Woodward 35
Chapter 6.
Modbus RTU Communications for the
EGCP-2 Panel
Introduction
This chapter describes the second of the three EGCP-2 communication protocol
options, Modbus RTU.
The Modbus communication used by the EGCP-2 panel operates at 9600 baud,
with 8 data bits, 1 stop bit, NO parity, and NO flow control. The protocol used is
Modbus RTU (Master/Slave).
Modbus Configuration Items
There are three configurable menu items associated with the Modbus
communications located in the Sequencing and Comms menu: Modbus ID,
Modbus Timeout, and Modbus Reset.
The Modbus ID configuration menu item is adjustable from 1 to 247. This
address identifies the Modbus Slave to the Modbus Master using this address.
The Modbus address chosen for any particular EGCP-2 panel should be different
from all other devices on the Modbus network.
NOTE
The Network Address of the EGCP-2 panel is not linked to the Modbus ID in
any way.
The Modbus Timeout setpoint is located in the Sequencing and Comms
configuration menu. This item is the time which the EGCP-2 panel will wait
before either receiving a valid message from the Modbus master or indicating a
Modbus failure. If a time-out occurs, the EGCP-2 will no longer update the data
to its Modbus variable table. It assumes that there is no master present and that
the port is not being used. If a Modbus master does send a request after a
timeout the EGCP-2 may send data that is no longer current on the first request.
Then it will update its Modbus list of values again every 200 msec, until the
Modbus timeout timer elapses again. In order to avoid accessing old data, the
Modbus timeout setpoint must be set longer than the poll time of the Modbus
master.
There are several items to note when determining the Timeout setting. The
EGCP-2 must use a baud rate of 9600 bps. At this speed a complete transfer of
all of the EGCP-2 Modbus addresses (approximately 270) would take about one
second. If the Modbus master were to only request 50 % of these addresses,
then this could be accomplished in less time, and the data could be transferred in
about 0.5 seconds. Another factor to consider is the quality of the
communications.