ABB module 3BSE038415R1 AO810V2 8-channel analog output module PLC

Description

Since the subject of AO810V2 did not mention higher level protocols, we also ignored them. But it needs to be clear that from the network layer to the top, the information unit sent between the layers is already a complete message. The OSI model also specifies the syntax structure of the message, which is ignored only for the length of the message.

It is worth noting that RS232/RS485/RS422 communication interfaces and their definitions are very clear. This includes the pin level, pin function definition, and the data flow timing relationship of the interface when sending and receiving information. These must be accurate and strict, otherwise information exchange cannot be performed.
When PLC AO810V2 exchanges information with some power instruments, and these power instruments conform to RS485/MODBUS-RTU communication specification. What are we going to do?
First, we wired according to the communication link requirements of the chrysanthemum petal structure, and connected the PLC communication interface with N power instrument interfaces. The end of the last electric instrument shall be equipped with a 100 ohm terminal resistor.
Second, for AO810V2, we determined the addresses of these N electrical instruments according to the principle of increasing addresses, such as 01H, 02H, 1FH, etc. Here, H means hexadecimal, and 1F means 16+15=31.
Thirdly, we set the communication rate specified by the power instrument in the PLC programming software.
Fourthly, we set the MODBUS communication code according to the data area address code of the electric instrument in the PLC programming software, as well as the cyclic relationship of each substation.
Note that the MODBUS communication code here meets the requirements of the IEC 61131-3 programming module of the PLC. General PLC ladder diagrams do not have this function. The ladder diagram meets the requirements of IEC 61131-1, but does not meet the requirements of IEC 61131-3.
Fifth, a special data area is opened in the memory of the PLC to store the information read from the electric instrument and processed, so that the higher level master station can read the information. This data area has a name, which is called data point table, sometimes also referred to as communication protocol.
Finally, of course, it is the boot test. There are many contents in it, which will not be introduced due to space limitations.
Let’s look at an example of using MODBUS-RTU to read data on the RS485 network, as follows:
The address of an electric instrument is 01H. Six data such as three-phase current and three-phase voltage are placed at the 2000th position in the memory of the power meter. Each data occupies two bytes, a total of 12 bytes.
The communication rate of this power meter is 9600bps. What do you mean? Bps represents a 0/1, that is, a bit, which means that 9600 bits can be sent on this bus every second. We already know that a byte has 8 data bits, 1 start bit, 1 parity bit, just 10 bits or 10 bits. Therefore, if the communication rate of the power meter is 9600bps, then one second can send 9600/10=960 bytes.
We also know that in the frame structure (downlink frame) of the read data of the master station, there are 1 byte address, 1 byte function code, 2 byte memory address, 2 byte data quantity, and 2 byte CRC check code. There are 8 bytes in total. Therefore, the time taken by the master station to send the read data MODBUS communication frame is: 8X10/9600=8.33 milliseconds.
For this example, we know that the command for MODBUS-RUT to read data is 0X03H, that is, the 03 command. Note the writing here: 0X is the prefix, 03 in the middle is the command, and the last H is hexadecimal.
The specific communication frame is: 01 03 07 D0 00 06 C5 45, where 0X01H is the address, 0X03H is the command, 0X07D0H is the memory address 2000, 0X0006H means to read six consecutive words, that is, the current and voltage parameters in the memory, and 0XC545H is the CRC check code of 01 03 07 D0 00 06.
Then the frame structure of the response frame (uplink frame) of the power instrument is: 1 byte address, 1 byte function code, 1 byte data area bytes, 12 bytes data, 2 bytes CRC check code, a total of 17 bytes, and the occupation time is 17X10/9600=17.7 milliseconds.
The specific instrument response communication frame is: 01 03 0C 00 64 0064 0064 00 DC 00 DC 00 DC D6 F5, in which 0X01H and 0X03H have the same meanings as before, 0X0CH indicates that there are 12 bytes in the upload data area, 0X0064H indicates that the A phase current is 100A, the following two groups are B phase and C phase currents, both of which are 100A, 0X00DCH indicates that the A phase voltage is 220V, the following two groups are B phase and C phase voltages, both of which are 220V, and the last 0XD6F5H is the CRC check code.
Initiate the downlink communication frame from the master station, wait for another 10ms for the slave station to respond, and then receive the uplink communication frame sent back from the slave station. The total duration is:
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If there are 31 identical instruments waiting for the master station to visit one by one, the total duration of the master station from the first instrument to the final response is:
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The 1.12 seconds here is the data reading cycle of the 31 instruments at a communication rate of 9600bps. The waiting time for the master station to send the downlink communication frame again is ignored, and the actual time will be slightly longer.
It is believed that after seeing this, we should have a more profound understanding of the communication frame under MODBUS-RTU.
Reminder: A word has two bytes. Generally, bytes can only be used to express 8 switching values. But for analog quantity, it should be expressed in words. For example, the current of 1250A is 04E2H in hexadecimal system, which requires 2 bytes to express completely. Therefore, in all kinds of electrical instruments, analog quantities are expressed in words.

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