D03-3003-000 Honeywell

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.

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