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Introduction
Modbus RTU RS232/RS485 Communication Ports
4 HC900 Hybrid Controller Communications User Guide Revision 4
9/03
Ethernet 10Base-T Network Connections
Ethernet 10Base-T networks operating at 10MB/sec. are supported. A typical network arrangement is as shown
below.
Figure 1-2 Ethernet 10Base-T Network Connections
Setting Up the Modbus/TCP Double Register Format
The HC900 predominantly uses an IEEE floating point format for communicating data to software applications
providing Modbus/TCP protocol communications drivers. A floating point value is sent as (2) consecutive 16-bit
registers, each register of which consists of two 8-bit bytes. Some software packages require the registers and bytes
to be sent in a certain order. The controller can be configured to deliver the data in four different byte orders.
The Hybrid Control Designer software tool allows this order to be selected as follows:
1. Using the Utilities Worksheet in the HC Designer software, access the Set Controller Network
Parameters button and make the selection to change the Modbus TCP Double Register Format (middle
radio button).
2. With the Port selected for downloading this order (using a COM port or Network port), select the
appropriate byte order format if the default (FP B) is not appropriate for the application. See Table
3-1, page 10.
3. Select Next and verify (by the response in the dialog box) that the change has been made in the
controller. This order can be changed in the RUN mode.
The Modbus TCP double register transmission format selection, FP LB “Little Endian Byte-Swapped”, would be
selected for interface to most third party software packages which use this format as standard. The default, FP B
“Big Endian” is used with SpecView32 or Honeywell’s PlantScape/Experion software and follows the “Honeywell”
default format of other control and recording products. It should be noted that most PC software packages offer a
register (word) swap selection in their driver package anyway, so there should never be an incompatibility.
1.3 Modbus RTU RS232/RS485 Communication Ports
This implementation is designed to provide a popular data exchange format connecting these instruments to both
Honeywell and foreign master devices via the RS232 and RS485 communication ports. The Modbus RTU allows
the instrument to be a citizen on a data link shared with other devices, which subscribe to the Modicon Modbus
Protocol Reference Guide PI-MBUS-300 Rev. G specification.