Honeywell HC900 - Manual (Page 10)

Introduction
Modbus/TCP Interface
2 HC900 Hybrid Controller Communications User Guide Revision 10
12/07
The controller’s Ethernet 10/100Base-T Host port is used for the Modbus/TCP connection. Ethernet TCP
allows multiple concurrent connections to hosts for data interchange. The HC900 (C30/C50) supports 5
concurrent host connections using Modbus/TCP protocol messaging via this port; HC900 (C70/C70R)
supports 10.
Interface Preparation
ATTENTION
To access the controller you must have a current Hybrid Control Designer configuration file available. Some
data is referenced relative to number, such as Signal Tags and Variables.
Other principal blocks, such as PID blocks, have offsets for parameter access dependent on the order in
which the blocks were placed on the Function Block Diagram.
It is strongly recommended that you upload the controller configuration using the Hybrid Control
Designer configuration tool to assure that you have a current configuration.
The Hybrid Control Designer tool provides a series of reports for use in Modbus Address identification.
The "Tag Information" report lists the variables and Signal Tags in numeric order along with their Modbus
Addresses.
A "Block Modbus Address" report lists the starting addresses for all principal blocks configured, identifying
the offset.
Modbus/TCP Protocol
Modbus/TCP protocol, developed by Groupe Schneider’s Modicon Division, is a popular, open standard for data
interchange over Ethernet TCP/IP networks using a Modbus RTU command structure.
It is simply an encapsulation of Modicon’s Modbus RTU protocol within a TCP/IP frame as shown below, which
includes header information and the Modbus frame.
Figure 1-1 Modbus RTU Protocol within a TCP/IP Frame
The Open Modbus/TCP Specification is followed with respect to the physical, data link, and network layers. The
message structure within the Modbus frame uses standard Modbus RTU function codes.
The Address part of the Modbus frame is not used (set to 00) since there is no sub-addressing intended or required.
The controller IP address is the identifying address, set independently at the controller.
The error checking is supported by TCP/IP network protocols and not part of the Modbus frame.
The Transaction Identifiers and Protocol Identifiers in the header are normally all 0’s (4 bytes total) while the
Length field identifies the number of bytes in the Modbus frame. The controller will transmit the correct number of
bytes for the remainder of the frame. However, the controller does not check this field for messages received.
The standard IEEE 32-bit floating point and 16-bit integer formats are used.