Because the maximum transmitter output level is -7 dB, and the maximum receiver input level is -10 dB, the receiver may be overdriven on a short link and may require a 3 dB attenuator. Connector allowances for the transmitter and receiver are included in the above power declaration.
In all cases the System 57 operates as a MODBUS Remote Terminal
Unit (RTU), acting as a slave device node and transmitting data onto
the communication line only in response to a request from a master
device. The host computer system, typically a PLC, DCS or SCADA
graphics package, acts as the master device controlling the
communication system operation.
The subset of MODBUS functions supported by the System 57 RTU
are 02, 03, 04, 06 and 16. The MODBUS broadcast commands are not
supported and will be ignored, however, global commands are available
to reset all cards in the rack simultaneously. For a detailed description
of the available commands and data formats refer to the System 57
MODBUS Function Reference given in Section 7.
2.2 MODBUS INTERFACE MODULE - RS485/422
2.2.1 General
The RS485/422 Interface Module has two differential 5V transceivers
designated channel 1 and channel 2. Each transceiver channel can be
resistively terminated by setting LK1 and LK2 respectively (see Figure
1 and Section 3.4.3). The interface signals are isolated from the
System 57 power supply 0V and Ground to protect the host computer
against damage due to earth loops.
2.2.2 RS485 Operation
An outline RS485 connection diagram is shown in Figure 4.
When configured for the RS485 mode, transceiver Channel 1 is used
in dual highway systems for the primary highway or the only highway
in single highway systems. Transceiver Channel 2 is used only in dual
highway systems where it functions as the secondary highway.
Depending upon the type of installation, operation is possible over
distances up to 1.2km (3900ft). In the multi-drop mode up to 32 nodes,
including the host, can be connected. The terminal connections are:
CH1-A, CH1-B Differential Transceiver Channel 1 (Primary).
CH2-A, CH2-B Differential Transceiver Channel 2 (Secondary).
DGND Isolated Data Ground.
In a dual highway system the MODBUS uses, at any one time, only
one of the two available highways, either the primary or the secondary.
The interface automatically switches between highways when a
communication failure is detected.
