VESDA ECO Detector by Xtralis
Product Guide
17898_05
Installation
15
RS485 Interface Connection
The Modbus RTU interface on the ECO detector is implemented using the EIA-485
standard for the physical interface. This is a two wire differential, half duplex
connection providing excellent noise immunity over cable lengths up to 1200m and
at high data transmission rates.
The ECO has been designed to meet the requirements of the “Modbus over Serial
Line, V1 2002” specification when using an RS485 interface.
Three connections are required to make the RS485 connection, A which is the
Modbus D0 connection, B which is the Modbus D1 connection and 0V to act as the
reference for A & B. If the RS485 interface device is being run from the same
power supply as the ECO device to which it is connecting, then there is no need to
include the 0V, but where there is any difference in the power supply arrangements
or where problems are experienced in long cable runs then the user should ensure
that there is a good connection between the ECO 0V terminal and the 0V reference
of the RS485 interface device.
Figure 7 Basic 2-wire Modbus Arrangement
An RS485 bus can accommodate up to 32 “Standard Loads” on the same data bus.
A standard load is an RS485 bus interface device, though some, like the ECO, are
specified as 1/8 loads, in which case up to 256 units can be include on the bus.
This is reduced to a practical limit of 247 units by the specification of the Modbus
interface. When using either the Xtralis VSC or VSM4 tools, the number of units on
a single bus is limited to 32.
If other, non-ECO, devices are being attached to the same Modbus bus, then the
installer must take account of the load definitions of all of these when determining
the maximum amount of units that can be connected to the Bus.
Each ECO device, and any other 3
rd
party devices, attached to the Modbus Bus
must have a unique Modbus address allocated and configured within them. All of
the units attached to the same bus cable must have the same protocol settings
configured.
