#Modbus server windows update
Now, I can update information in my global DB from my code and read it from a Modbus TCP master, or the master can write data to my PLC. We should now have a single rung of logic to control our Modbus TCP server. Finally, I’ve added a Real, which is a DW and will use Registers 4004. The next Word will be the data for Register 40002, in this case, an Int. Looking at the DB above, you can see I’ve created an array of Bools that make up the first Word (this will be Register 40001). To wrap things up, I’m going to create a global DB for my hold registers and include the following: Unless you’ve modified the start position, the hold registers will begin at 40001 and continue for the length defined in MB_HOLD_REG. Getting back to the hold registers, these are accessed with function codes 3, 6, and 16. Check the help file for the other options. Function code 02 will allow read access to the input process image, mapping registers 10001-18192 to I0.0 to I1023.7. Likewise, function code 5 (Write Bits) allows write access to the same registers/bits. To sum it up, function codes 1, 2, 4, 5, and 15 will immediately support direct read and write access to the process image of the CPU.įor example, function code 01 (Read Bits) allows read access to the output process image, with registers 1-8192 corresponding to Q0.0-Q1023.7. There’s a handy table in the help file explaining the codes and data sources automatically made available. Now, ignoring the hold registers for a moment, let’s look at what this MB_SERVER instruction gives us out of the box. There are some status bits you can use to debug connection issues and a few static tags that can be accessed, but in most case you shouldn’t need them. You can set this pointer using the ANY format (“P#bit address” “data type” “length”):
These holding registers are used for Modbus functions 3 (read Word), 6 (write Word), and 16 (write multiple Words). This pointer can be any global data block or a memory area (M). MB_HOLD_REGISTER: This defined the location (start and size) for the available Holding Registers (40001 to max defined register).If you would like to use an alternative port, this is where you set it. IP_PORT: by default, the IP_PORT will be set to 502, which is the standard port used for Modbus TCP communications.This must be unique for each instance of MB_SERVER or MB_CLIENT. CONNECT_ID: this defines a connection number on the PLC.DISCONNECT: the default for this input is 'FALSE.' Setting the input to 'TRUE' will cause the server to sever any active connection and refuse any connection attempts.
You’ll see there are only a few inputs to this block: When you add the instruction, you’ll notice it will need its own instance DB, so go ahead and create one. I’m starting a project from scratch, so I’m going to add an instance of MB_SERVER to my OB1. Siemens provides two Instruction blocks for setting up Client/Server Modbus TCP connections – you can find them under Communication on the Instructions Window – MB_SERVER and MB_CLIENT.Īs an example, let’s set up a Modbus TCP server on our PLC. Have a need to make data available from your S7-1200 PLC to a Modbus TCP Master (or SCADA)? Luckily, this is easy to setup.
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