Honeywell HC900 - Manual (Page 13)

Introduction
Modbus RTU RS232/RS485 Communication Ports
Revision 10 HC900 Hybrid Controller Communications User Guide 5
12/07
1.3 Modbus RTU RS232/RS485 Communication Ports
This implementation is designed to provide a popular data exchange format connecting the HC900 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.
These instruments DO NOT emulate any MODICON type device. The Modbus RTU specification is respected in
the physical and data link layers. The message structure of the Modbus RTU function codes is employed and
standard IEEE 32-bit floating point and integer formats are used. Data register mapping is unique to the HC900 and
other Honeywell instruments. Section 6 describes the parameter mapping for the HC900.
Modbus RTU Message Format
Table 1-1 Modbus RTU Message Formats
Coding system 8 bit binary
Number of data bits per
character 10, 11, or 12 Bits
start bits - 1
data bits - 8
parity bits – 0 or 1 selectable
stop bits – 1 or 2 selectable
Parity None, odd, even selectable
Bit transfer rate 1200, 2400, 4800, 9600, 19200, 38400, 57600 Selectable
Duplex Half duplex Transceiver or TX/RX
Error checking CRC (cyclic redundancy check)
Polynomial (CRC-16 10100000000001)
Bit transfer order LSB first
End of message Idle line for 3.5 or more characters (>1.82 msec for 19200).
Modbus RTU Link Layer
The link layer includes the following properties/behaviors:
• Slave address recognition,
• Start / End of Frame detection,
• CRC-16 generation / checking,
• Transmit / receive message time-out,
• Buffer overflow detection,
• Framing error detection,
• Idle line detection.
Errors detected by the physical layer in messages received by the slave are ignored and the physical layer
automatically restarts by initiating a new receive on the next idle line detection.