Solar Pump Inverter Chapter 7 Communication protocol
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4. Description of the protocol
CT112&CT112A series inverter communication protocol is an asynchronous serial
master-slave Modbus communication protocol and only one device (master) in the
network can establish a protocol (called "query/command"). Other devices (slaves) can
only provide data to respond to the master's "query/command" or make the
corresponding actuation according to the master's "query/command". The master refers
to a personal computer (PC), an industrial control device or a programmable logic
controller (PLC), etc. The slave is inverter. The master can communicate with a single
slave as well as send a broadcast message to all slaves. For the master’s single
"query/command", the slave has to return a message (called a response), for the
master’s broadcast message, the slaves do not need to respond to the master.
5. Communication data format
The Modbus protocol communication data format of CT112 series inverters is as
follows:
In RTU mode, the minimum interval time should be at least 3.5 bytes for message
transmission, which is the easiest way to achieve a variety of character time at the baud
rate. The first transmitted field is the device address. The transmitted characters can be
hexadecimal 0...9 and A...F. The network device constantly detects the network bus,
even during the interval time. When the first field (address field) is received, the
corresponding device decodes next transmitting character. After the last transmitting
character, the interval time of at least 3.5 bytes marks the end of the message. A new
message can start after this pause.
The whole message frame is a continuous transmitting flow. If there is an interval time of
more than 1.5 bytes before the frame is completed, the receiving device will renew the
uncompleted message and assume that the next byte is the address field of a new
message. As such, if the new message follows the previous message within the interval
time of 3.5 bytes, the receiving device will deal with it as the same with the previous
message. If these two phenomena all happen during the transmission, the CRC will
generate a fault message to respond to the sending devices.
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