Coding system 8-bit binary
Number of data bits per
character
10 =
1 start bit
8 data bits
No parity bit
1 stop bit
Bit transfer rate 38400 bps
Duplex Half duplex
Error checking 2 byte CRC-16 polynomial
Bit transfer order LSB first
End of message Idle line for 3.5 or more characters
CB Falcon Modbus Communication
INTRODUCTION
Reference
This document describes the interface to the CB Falcon
boiler controller on either the MB1 or MB2 Modbus port.
These ports are RS-485 connectors that use the Modbus
communication protocol to allow configuration and status
data to be read from and written to the Falcon.
The CB Falcon functions as a Modbus slave (server) on this
interface. It responds to a single Modbus address to service
the requests of the Modbus master (client) on the RS-485
network.
This document does not describe the Modbus protocol. It only
describes how this protocol is used in this interface.
Definitions
The following definitions are used in this document:
Modbus Application layer communication protocol
standard adopted by the Modbus-IDA trade
association. Recognized as an industry standard
protocol for RS-485 serial communication.
PCB Parameter Control Block. Files that customize the
user interface with the CB Falcon. PCBs reside
in the non-volatile storage in the Falcon and are
uploaded from the controller into the user
interface.
PIM Plug-In Module. Plug that can be inserted into
the Falcon to enable Lead Lag and to backup &
restore parameter settings in the Falcon.
RTU Remote Terminal Unit serial transmission mode.
Mode used to encode data for Modbus where
each 8-bit byte is sent as two 4-bit hexadecimal
characters.
The following is used as a reference in this document:
MODBUS Application Protocol Specification V1.1a, June 4,
2004, http://www.Modbus-IDA.org
.
INTERFACE
Physical Layer
The Falcon Modbus port is a 3-pin connector that interfaces to
RS-485 signals as indicated in Table 1.
Table 1. RS-485 signals
Signal Terminal
Data + (a) 1
Data – (b) 2
Common (c) 3
The serial transmission mode on the Modbus network is the
RTU mode. Message format has the characteristics shown in
Table 2
Table 2. RS-485 message format
750-308
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