The PLC communicates with the host computer, and BlandC is used to write the communication program between the host and the PLC, and the COM1 port must be initialized, the baud rate is 9600bps, and the parity check bit adopts even check, and the seven-digit valid data is used.
When the computer receives the ACK response character from the PLC, it can enter the data communication.
The computer can read, write and force ON/OFF operations on each software device in the PLC. Except for the PLC's timer and counter setpoint constants, as well as the data in the file registers, all switching inputs, outputs, and software devices of the FX2N series PLC are transparent to the computer.
In order to ensure the accuracy of the communication between the host and the PLC, the host computer must also be carried out and verified according to the communication protocol. If the received information is incorrect, it will be re-read. If it persists after 3 repetitions, an error message is displayed.
Using C language to achieve the above programming, the host can also write data to the PLC for single-point strong proximity positioning and reset.
Before using an external device to communicate with a programmable controller, it is necessary to specify some parameters such as data length, parity, stop bits, and baud rate, i.e., the communication format.
The communication format is set by a four-digit hexadecimal number, which is set to H6880, which means that the data length is 7 bits, the terminator is 7 bits, the parity is none, the control line is RS232 interface, the stop bit is 1 bit, and the check is automatically added, the baud rate is 9600BPS, the protocol is a dedicated protocol, the title is none, and the transmission control protocol is the protocol format 1.
The communication format is first set in the PLC. The communication format of the protocol format stipulates the control code, station number, PC number, command, message waiting, and check code in turn. In the FX series of programming controllers, the station number is set by means of a special register D8121. The setting range is from 00H to 0FH.
In the test bench, because the host computer only communicates with a PLC pneumatic test bench, the station number is 00H. THE PC NUMBER IS THE NUMBER USED BY THE A-SERIES MELSECNET(II) OR MELSECNET/B TO DETERMINE THE CPU OF A PROGRAMMABLE CONTROLLER. The PC number of the FX series is FFH, which is represented by two ASCII characters, i.e. "FF".
Commands are used to specify the required operations (e.g. read, write), and the main commands used in this control software to send to the PLC are:
Batch readout of BR commands for bit elements; Batch write BW instructions for bit components; Force PLC to run (RUN) RR commands; FORCE PLC TO STOP (STOP) RS COMMAND.
A message wait is a delay required by a computer to transition between the send and receive states. and the check code is used to determine that the data in the message has not been corrupted. It is calculated by adding and checking the hexadecimal values of ASCII characters in the region.
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