Figure 14 - Modbus/Tcp Framing - Honeywell HC900 Installation And User Manual

Components and Architecture - Ethernet Devices/Considerations
Note: PDE communications, discussed previously, do not use the PC host connection sockets. PDE
communications are separate from (and are transmitted concurrent with) PC host-to-controller
communications.
The PC host can include software that closely relates to and supports controller functioning and can also
include other software that is related remotely or not at all. Closely related software can include:
Either
Hybrid Control Designer - for generating and managing configuration files,
Or
HMI (Supervisory/Data Acquisition Software) or Operator Panel with Modbus/TCP driver
Or
Both configuration and HMI software (and or panel)
All communications between a controller and a PC host use Open Modbus/TCP protocol, whose
widespread use is making it an industry standard. Modbus/TCP is basically an adaptation of the Modbus
messaging structure that uses TCP/IP for a message carrier. Modbus messaging is available in two
versions: ASCII, in which each eight-bit byte is sent as 2 ASCII characters, and RTU, in which each byte
is sent as two four-bit hexadecimal characters. Each Modbus message frame is embedded into a TCP/IP
datagram as indicated in Figure 14.
The HC900 Controller uses either Modbus/TCP or Modbus RTU. The Modbus mapping structure for
the HC900 Controller is based on the mapping structure employed in Honeywell's UMC800 Controller,
and the function codes and methods for parameter access are also virtually identical.
From an implementation and installation aspect, the use of the Modbus protocol for HC900 Controller
configuration differs from the use for Controller-to-HMI communications. The Hybrid Control Designer
configuration package, which is supplied by Honeywell, is ready for use as soon as it is installed. This
package uses a subset of Modbus function codes that provide for very efficient and secure configuration
transactions. Communications details are transparent to the user; all communication drivers are included
with the package, and no mapping or other detailed setup is required.
HMI Supervisory/SCADA software is available from various suppliers, and functionality and setup
requirements vary with suppliers and with specific products. In all cases, the software selected must be
compatible with Open Modbus/TCP protocol.
The user can use the standard Modbus command set to generate a custom set of drivers for his specific
application, or may purchase additional software (for example, OPC with Modbus /TCP protocol) to
reduce or virtually eliminate development tasks.
16

Figure 14 - Modbus/TCP Framing

HC900 Hybrid Controller Installation and User Guide Revision 5
9/03