This situation is the simplest for writing ladder diagram programs. All the address information of the slave stations to be read and written needs to be established in the same Modbus master table. In the entire ladder diagram program, there is only one Modbus master communication instruction for the COM port, and users can use a certain enable terminal to control it. Users do not need to handle the conversion between multiple slave stations, nor do they need to understand how to implement it, because the PLC will automatically switch it internally, and the switching will be automatically performed in the order of the slave station serial number WR-ID in the Modbus table;

The advantage of this method is that ladder diagram processing is simple, requiring only one Modbus instruction and creating a Modbus table, making it easier for users to write ladder diagram programs;

The disadvantage of this method is that when there are many mounted slave stations or a large number of slave station information sequences to be read or written, switching between each slave station information in a polling manner results in longer time consumption;

Therefore, this method is suitable for situations where the master station must always maintain communication with all slave stations, or where there are few commands from slave stations and read/write slave stations;  

In some large-scale usage scenarios, it is more common for users to selectively communicate with certain slave stations, while unselected slave stations do not require read-write communication operations;

And this method can achieve any form of communication, including (1). A simple example will be given below to illustrate,

The advantage of this method is that it selectively communicates with certain mounted slave stations, resulting in faster communication speed compared to polling all slave stations in (1);

The disadvantage of this method is that the ladder diagram writing is relatively complex and requires the use of certain serial communication flags such as M8182/M8183 for users to manually control the switching of different Modbus tables and Modbus commands;

As shown in the table below, create a Modbus table named "All Slave Stations", where the master station mounts three slave stations with station numbers 1, 2, and 3. Each slave station has three different read and write sequences,

Table: All Slave Stations

Slave station number	Function code	Slave station register	Slave station length
1	0x03 (Read Double Word)	60200	1
1	0x01 (read bit)	30000	10
1	0x03 (Reading)	40000	10
2	0x03 (Read Double Word)	60200	1
2	0x01 (read bit)	30000	10
2	0x03 (Reading)	40000	10
3	0x03 (Read Double Word)	60200	1
3	0x01 (read bit)	30000	10
3	0x03 (Reading)	40000	10

After creating the table, select the Modbus master communication protocol configuration, check the slave serial port information and station number settings, and download the master station ladder diagram as shown in the following figure to the PLC. Monitor and observe whether it is running normally, and the WR-ID is constantly switching quickly and automatically between the slave station sequence 0~8.

command table:

NETWORK 000

LD M6000

MBUS K1 to all slave D5000 D5001

图3 MBUS2

Next, we will provide a method that can also implement Example 1. This method may be slightly more complex, but understanding its principles will lead to a deeper understanding of Modbus master communication instructions;

Firstly, separate the three slave stations in Example 1 into different Modbus tables, as shown in the following figure, and create three separate slave station tables;

Table：station 1

Slave station number	Function code	Slave station register	Slave station length
1	0x03 (Read Double Word)	60200	1
1	0x01 (read bit)	30000	10
1	0x03 (Reading)	40000	10

Table：Station 2

Slave station number	Function code	Slave station register	Slave station length
2	0x03 (Read Double Word)	60200	1
2	0x01 (read bit)	30000	10
2	0x03 (Reading)	40000	10

Table：Station 3

Slave station number	Function code	Slave station register	Slave station length
3	0x03 (Read Double Word)	60200	1
3	0x01 (read bit)	30000	10
3	0x03 (Reading)	40000	10

Write a ladder diagram as shown below, where D1234 is used to switch and control different Modbus tables, enabling different Modbus instructions. For example, when (D1234==1), it represents controlling slave 1, and when (D1234==3), it represents controlling slave 3;

M8183 is the completion flag for 485 serial communication reception. This flag is automatically set by the PLC after receiving data and needs to be manually reset by the user in the ladder diagram. This special register is a key flag used for manual switching;

The meaning of comparing D5001, D5003, D5005, and K2 in the second network of the ladder diagram: K2 represents the slave information sequence of each Modbus table. In this example, since there are a total of three sequences in the three tables, and the sequence numbers in each table are 0, 1, and 2 respectively, comparing D5001, D5003, D5005, and K2 here indicates that the corresponding Modbus instructions run to the end of the sequence in the table. In practical engineering, the total number of sequence numbers in different tables can be different, but the end sequence number in each table must be the total number of sequences n \ -1 in that table, that is, the WRIDs are 0, 1, 2, etc N \ -2, n \ -1;

Downloading the program to the PLC allows for monitoring and debugging by setting M6000 and D1234 to 1. Afterwards, it can be observed that the PLC will automatically switch quickly between three Modbus commands, and the read-write communication with multiple slave stations is also normal.

Command table:

NETWORK 000

LDP M8183

MPS

AW= D1234 K1

AW= D5001 K2

MOV K2 D1234

MOV K0 D5003

MRD

AW= D1234 K2

AW= D5003 K2

MOV K3 D1234

MOV K0 D5005

MRD

AW= D1234 K3

AW= D5005 K2

MOV K1 D1234

MOV K0 D5001

MPP

RST M8183 K1

POP

LD M6000

MPS

AW= D1234 K1

MBUS K1 station 1 D5000 D5001

MRD

AW= D1234 K2

MBUS K1 station 2 D5002 D5003

MPP

AW= D1234 K3

MBUS K1 station 3 D5004 D5005

POP

图4 MBUS3

图5 MBUS4

图6 MBUS5

If one can understand the program in Example 2, then it would be very simple and clear to achieve selective manual switching of control and communication with different slave stations;

Imagine a scenario where when the HMI enters screen 1, the user needs to communicate with slave 1 and not with any other slave, and when entering screen 2, the user needs to communicate with slave 2 and not with any other slave When entering screen n, it is necessary to communicate with slave station n and not with other slave stations; The different screens in this usage scenario are equivalent to D1234 in Example 2 above. Multiple different slave stations have multiple different Modbus tables and corresponding Modbus instructions. Example 2 implements automatic loop switching, while this usage scenario is manually switched by the user assigning different values to the D1234 register. The basic idea is the same, as long as the M8182/M8183 serial port reception completion flag is flexibly used and combined with the WR-ID in the Modbus instruction, any communication method can be achieved** Another key point to note is that the ladder diagram allows multiple Modbus commands to be included for the same COM port. However, within the same scanning cycle, only one Modbus command can be simultaneously conducted for the same COM port. If multiple Modbus commands are simultaneously conducted, data communication abnormalities may occur.