Specifications

MODBUS COMMUNICATION MANUAL
1-3
1.3.1.2 BMS Master To C-More Boiler Controller Slaves
Up to a total of 32 C-More Boiler Controllers can be connected to a BMS Master on the Modbus
Network. Multi-point drop network connections are made using shielded, twisted-pair wire. In
addition to the Modbus Network Boilers, up to 8 additional Legacy Boilers can be connected
using the BMS Pulse Width Modulation (PWM) wiring connection provided on connector J2.
1.3.1.3
EMS Master To C-More Boiler Controller Slaves
The number of C-More Boiler Controllers which can be connected to a Modbus Network which
utilizes a third party EMS Master will depend on the EMS’s limitations. Theoretically, the
maximum number of Slave devices is limited to 127. If the EMS contains a RS232 port, a
RS232-to-RS485 converter will be required to provide the necessary RS485 interfaces and
signal levels for the C-More Boiler Controllers. Multi-drop network connections are made using
shielded, twisted-pair wire.
1.3.2 Address Support
Address support is assigned as follows:
• BMS Address Support From EMS Master: 128 – 247 (80 – F7 hex)
• C-More Address Support From BMS or EMS Master: 1 – 127 (01 – 7F hex)
• Broadcast Messages: Address 0 is reserved for all Broadcast messages
1.3.3 Modbus Transmission Modes
Many Modbus Controllers can be set up to transmit using either the ASCII (American Standard
Code for Information Interchange) transmission mode, or the RTU (Remote Terminal Unit)
transmission mode. However, since RTU messages can be formatted using far fewer binary
bits than the corresponding ASCII message, it is far more efficient. Therefore, all Modbus
messages for the AERCO BMS and C-More Boiler Controllers use RTU transmission ONLY. If
a third-party EMS Master is being used in the Modbus network, ensure that it is set for RTU
transmission.
1.3.4 Timing Specifications
As Table 1-1 shows, Baud Rate and Heartbeat Timeout will vary depending on the
Configurations of the AERCO BMS and C-More Boiler Controllers being used in the Modbus
Network. Ensure that the Baud Rate used by the controlling Master (BMS or EMS) matches the
appropriate Baud Rate supported by the Network Slaves (BMS or C-More Controllers). Also,
ensure that the Modbus Master can refresh the control information to all C-More Slaves before
the Heartbeat Timeout period expires.
1.4 MODBUS FUNCTION SET SUPPORT
The complete Modbus protocol includes a total of 24 Function Codes. However, for AERCO
BMS and C-More Boiler Controllers, only the Codes listed in Table 1-2 are supported. The
supported Diagnostic Sub-Function Codes associated with Diagnostic Function Code 08 are
listed in Table 1-3.