Honeywell HC900 - Figure 23 - Modbus;Tcp Framing

38 HC900 Process & Safety Controller User and Installation Manual Revision 6
21 April 2017
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
Designer Software – 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. In general, 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 20.
Figure 20 – Modbus/TCP Framing
The HC900 Controller uses either Modbus/TCP or Modbus RTU, not ASCII. The “fixed” 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. Modbus Maps
In your controller’s configuration certain parameters are each automatically assigned a Modbus address.
These include certain types of function blocks (such as loops and set point programmers), signals and
variables, among other items. Through their addresses these parameters can be accessed or displayed
remotely, such as by a third-party operator interface. Collectively, these Modbus addresses and
parameters are known as the Modbus map.
Two Modbus Mapping options exist: Fixed map and Custom map
Using HC Designer, you can use a pre-defined fixed map where common parameters are mapped out
automatically at fixed addresses, or you can configure a custom map for configurations rev. 4.0 or
higher.
Function blocks added to the controller in release 4.0 and higher are not automatically included in the
Fixed Modbus Map. To access the registers of these function blocks, the Custom Modbus Map must
be used and the block data manually inserted into the custom Modbus map.
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.