Part 1: S7-1200 Modbus RTU Communication

(I) Overview

Modbus has two serial transmission modes: ASCII and RTU. Modbus is a master-slave communication mode with a single master station. Only one master station can exist on the Modbus network, and the master station does not have an address on the Modbus network. Each slave station must have a unique address, with the address range of 0 - 247, where 0 is the broadcast address, and the actual address range of the slave station is 1-247. Modbus RTU communication performs data transmission in a master-slave manner. During transmission, the Modbus RTU master station is the initiator, that is, the master station sends data request messages to the slave station, and the Modbus RTU slave station returns response messages. The following modules of S7-1200 support Modbus RTU communication:

Note:

(1) When using the communication module CM 1241 RS232 as a Modbus RTU master, it can only communicate with one slave.
(2) When using the communication module CM 1241 RS485 as a Modbus RTU master, it is allowed to establish communication with up to 32 slaves.

(3) When using the communication board CB 1241 RS485, the CPU firmware must be V2.0 or higher, and the software used must be STEP 7 Basic V11 or STEP 7 Professional V11 or a later version.

(II) Modbus RTU Instruction Versions and Restrictions

With the continuous updates of the Siemens Portal software and S7-1200 CPU firmware, different versions of the S7-1200 Modbus RTU instructions have emerged. Users need to correctly select and use the appropriate S7-1200 Modbus RTU instructions according to the software and hardware they are using to implement Modbus RTU communication. The software and hardware requirements and instruction version descriptions are as follows.

(1) Modbus RTUinstructions in the Siemens Portal

The Siemens Portal provides two versions of the Modbus RTU instructions. As shown in Figure 1 below:

• The earlier version of the Modbus RTU instructions (MODBUS (V2.2) in Figure 1) can only perform Modbus RTU communication through the CM1241 communication module or CB1241 communication board.
• The new version of the Modbus RTU instructions (MODBUS(RTU) V3.0 in Figure 1) expands the functionality of Modbus RTU. In addition to supporting the CM1241 communication module and CB1241 communication board, this instruction also supports Modbus RTU communication through the PTP communication module on the PROFINET or PROFIBUS distributed I/O rack.

(2) Restrictions on the use of new version Modbus RTU commands
When conducting Modbus RTU communication through the CM1241 communication module or CB1241 communication board with the new version of Modbus RTU instructions, the following conditions must be met:
a. The firmware version of the S7-1200 CPU must not be lower than V4.1
b. The CM1241 communication module should be V2.1 or above, or CB1241

Figure 1. Two versions of Modbus RTU instructions

(III) Common Issues

(1) Does the S7-1200support Modbus ASCII communication mode?

Siemens does not provide ready-made instructions that support the above communication mode, and users need to program in free port mode.

(2) What is the difference between Modbus RTUinstruction versions V1 and V2?

Modbus RTU instruction version V2 adds the parameters "REQ" and "DONE" to the "MB_COMM_LOAD" instruction. Furthermore, the "MB_ADDR" parameter of the "MB_MASTER" and "MB_SLAVE" instructions now allows a UInt value for extended addressing.

(3) Can different versions of Modbus RTU instructions, V1 and V2, be used simultaneously in the same CPU program?

It is not possible to use both V1 (V1.x) and V2 (V2.y) instruction versions simultaneously in the same CPU program. The Modbus instructions in the user program must have the same major version number; individual instructions within the major version group can have different minor version numbers.

(4) Can the S7-1200 communication module CM1241 be used as a Modbus RTU master or as a Modbus RTU slave?

The S7-1200 communication module CM1241 can be configured and programmed to call the "MB_COMM_LOAD" instruction, which sets it to Modbus RTU communication mode. By programming and calling the "MB_MASTER" instruction, the S7-1200 communication module CM1241 can be used as a Modbus RTU master, or by calling the "MB_SLAVE" instruction, the S7-1200 communication module CM1241 can be used as a Modbus RTU slave.

Note:Regardless of whether the S7-1200 communication module CM1241 is used as a Modbus RTU master or slave, the "MB_COMM_LOAD" instruction needs to be called for programming.

(5) Does the S7-1200CM1241/CB1241 Modbus RTU communication support two stop bits?

Yes, it does.

Note:The stop bits can be set in the properties of the S7-1200 CM1241/CB1241, but when this module is used for Modbus communication, the set stop bits are invalid. It is necessary to modify the STOP_BITS stop bit value to 2 in the background DB of the Modbus_Commload instruction. As shown in Figure 2-3 below:

Figure 2. CM1241 configure stop bit parameters

Modify the STOP_BITS stop bit value to 2 (default value=1) in the background DB of the Modbus_Commload instruction

Figure 3. Modify the Modbus RTU communication stop bits

PART 02

Part 2: V3 Version Command Functions

(I) Overview of Modbus RTU Commands

The software version of B&R V13 SP1 provides two versions of Modbus RTU commands:

Figure 1. Two Versions of Modbus RTU Commands

(1) The early version of Modbus RTU command (MODBUS (V2.2) in Figure 1) can only perform Modbus RTU communication through the CM1241 communication module or CB1241 communication board.
(2) The new version of Modbus RTU command (MODBUS(RTU) V3.0 in Figure 1) extends the functionality of Modbus RTU. In addition to supporting the CM1241 communication module and CB1241 communication board, this command also supports Modbus RTU communication through the PTP communication module on the PROFINET or PROFIBUS distributed I/O rack.

(3) The PTP modules supported by the new version of Modbus RTU command are shown in Figure 2 below:

Figure 2. PTP Modules Supported by the New Version of Modbus RTU Command

(II) Introduction to Modbus RTU Command Instance Environment

The new version of Modbus RTU command includes both Modbus RTU master and slave commands. This article takes the CPU1217C+CM1241 RS422/485+ET200SP CM PTP module as an example to introduce the programming steps for master-slave communication using the new version of Modbus RTU command. In this configuration, the CPU rack CM1241 RS422/485 serves as the Modbus RTU slave, and the CMPTP module in the distributed rack ET200SP serves as the Modbus RTU master.

1. The network structure diagram is as follows:

Figure 3. Modbus RTU Network Communication Structure Diagram

2. The hardware and software used in this project are as follows:

(1) Hardware:
① CPU1217C (order number: 6ES7 217-1AG40-0XB0), firmware version V4.1.3
② CM1241 RS422/485 module (order number: 6ES7 241-1CH32-0XB0), firmware version V2.1
③ 24V power supply PS307 (order number: 6ES7307-1KA02-0AA0)
④ ET200 SP IM155-6PN HF (order number: 6ES7155-6AU00-0CN0)
⑤ CM PTP module (order number: 6ES7137-6AA00-0BA0)

(2) Software:
① Simatic PLC Simatic SP1 UP 9

(III) Device Configuration Example

1. Configure the CM1241 RS422/485 module

(1) Open the device view, add the S7-1200 CPU, and navigate to "Communication Modules" → "Point-to-Point" → "CM1241 (RS422/485)" in the hardware directory. Drag this module to the left side of the CPU, as shown in Figure 4 below:

Figure 4. Add CM 1241 RS422/485 Module

Note:The CM 1241 RS422/485 module with firmware version >= V2.1supports the new version of Modbus RTU commands.

(2) Next, select the CM1241 (RS422/485) module in the "Device View" with the mouse, and configure the hardware interface parameters of this module in "Properties" → "Port Configuration". In this example, the transmission rate is set to 9.6Kbps, parity is set to "No Parity", data bits are set to "8-bit characters", and stop bits are set to "1". The CM 1241 port configuration is shown in Figure 5 below:

Figure 5. CM1241 RS422/485 Module Port Configuration

(3) Finally, confirm that the hardware identifier is 269 (this parameter will be used in program programming) in the "Hardware Identifier", as shown in Figure 6 below:

Figure 6 Hardware Identifier

(4) In addition, the S7-1200 also provides system and clock memory functions. To facilitate subsequent instructions, it is recommended to enable this function. Enable the system and clock memory functions in the CPU "Properties" → "General" → "System and Clock Memory", as shown in Figure 7.

Figure 7. System and Clock Memory Function

2. Configure ET200 SP CM PtP Module

(1) Insert an ET200SP distributed site, open the network view, drag in an ET200SP site, and assign it to the corresponding IO controller (in this example, CPU1217C is the IO controller), as shown in Figure 8:

Figure 8. Insert ET200SP Station

(2) Configure the ET200SP station. In the "Device View" environment of ET200SP, add signal, communication module, and server module to the ET200SP station. In this example, only the CM PTP module and server module are added. Select CM PTP in the ET200SP "Device View" with the mouse, and configure the hardware interface parameters of this module in "Properties" → "General" → "Interfaces" → "Operation Mode". In this example, set the "Specified Operating Mode" to "Half-Duplex (RS485) 2-Wire Operation" and "Initial State of Receiving Line" to "None". As shown in Figure 9 below:

Figure 9. CM PTP Operation Mode

Note: In the ET200SP station, the server module must be configured. The server module is purchased together with the interface module and does not need to be purchased separately.The ET200SP interface module requires an IP address and Device Name to be assigned. For detailed steps on configuring the ET200 SP distributed IO, please refer to the "Quick Start Guide for ET200 SP". This example does not describe the relevant settings and steps for Profinet IO communication. Download link for "Quick Start Guide for ET200 SP":

(3) Next, configure the port configuration parameters of this module in "Properties" → "General" → "Interfaces" → "Port Configuration". In this example, set the "Protocol" to "Freeport/Modbus"; "Port Parameters" settings: transmission rate = 9.6Kbps, parity = no parity, data bits = 8-bit characters, stop bits = 1 as an example. The port configuration settings are shown in Figure 10 below:

Figure 10. CM PTP Port Configuration

(4) Finally, confirm the hardware identifier of the CM PTP module in the "Hardware Identifier". This parameter will be used in program programming.

(4) Software programming of examples

1. Modbus RTU master station programming

Modbus RTU master station programming requires the use of the Modbus_Comm_Load and Modbus_Master instructions. The Modbus_Comm_Load instruction configures the communication module through the Modbus RTU protocol, while the Modbus_Master instruction enables communication as a Modbus master station through the port configured by the Modbus_Comm_Load instruction. The MB_DB parameter of the Modbus_Comm_Load instruction must be connected to the (static) MB_DB parameter of the Modbus_Master instruction. In this example, the CM PTP module in the distributed rack ET200SP serves as a Modbus RTU master station, and the relevant programming steps are as follows:

(1) Insert an FC function in OB1, and drag in the Modbus_Comm_Load instruction and Modbus_Master instruction within the function. As shown in Figure 11:

Figure 11. Dragging in the Modbus RTU Master Instruction

The meaning of each parameter of the Modbus_Comm_Load instruction is shown in Table 1 below:

Table 1: Meaning of MB_COMM_LOAD instruction parameters

The parameters of the Modbus_Master command are described in Table 2 below:

Table 2: Meaning of Modbus_Master instruction parameters

Note:
① The Modbus_Comm_Load instruction is not recommended to be called in the startup organization block OB100, and it is recommended to call it in OB1. When the Modbus_Comm_Load instruction is called in OB1, its input bit "REQ" needs to be triggered by a rising edge, and in this example, this input bit uses the "FirstScan" system memory bit.
② The static variable "MODE" in the background data block of the Modbus_Comm_Load instruction is used to describe the working mode of the PTP module, and the valid working modes include:

• 0 = Full Duplex (RS232)
• 1 = Full Duplex (RS422) Four-wire mode (Point-to-Point)
• 2 = Full Full-Duplex (RS 422) Four-wire mode (Multi-drop Master, CM PtP (ET 200SP))
• 3 = Full Full-Duplex (RS 422) Four-wire mode (Multi-drop Slave, CM PtP (ET 200SP))
• 4 = Half Duplex (RS485) Two-wire mode

The default value of this static variable "MODE" is 0 (RS232 Full Duplex mode), and it needs to be modified according to the actual configuration of the CM PTP module. In this example, the CM PTP module is operating in RS485 Half Duplex mode, and the value needs to be modified to 4, as shown in Figure 12:

Figure 12. Modify the static variable "MODE" in the Modbus_Comm_Load background data block to

③ The "DATA_PTR" parameter of the Modbus_Master instruction is used to point to the address of the data area where data is to be written or read. This data area supports optimized access data blocks or non-optimized (standard) data blocks, and it is recommended to use non-optimized access data blocks.
The data block used in this example is a non-optimized access data block. To modify the data block to a non-optimized access data block, simply cancel "Optimized Block Access" in the data block's properties (right-click on the data block, select "Properties", and cancel "Optimized Block Access"), as shown in Figure 13:

Figure 13. Setting data blocks to non-optimized access

When the "DATA_PTR" of the Modbus_Master instruction points to a data block that is not optimized for access, this input parameter needs to be filled in using a pointer method, such as P#DB3.DBX0.0 WORD 5.
④ When there are multiple Modbus RTU slaves in the Modbus RTU network or when a single Modbus RTU slave requires both read and write operations simultaneously, multiple Modbus_Master instructions need to be invoked, and these Modbus_Master instructions must be called using a polling method.
The following Figure 14 describes the method of polling and invoking two Modbus_Master instructions.

Figure 14. Modbus_Master Polling and Invoking Method

(2) Insert "Pull or plug of modules" interrupt OB83

In this example, the Modbus RTU master module is installed on a distributed IO site, so the program needs to consider faults such as distributed IO site failures and CM PTP module plugging and unplugging. When modules are plugged in or unplugged in the distributed IO site, the operating system will call OB83 once. The faulty module and type can be determined through the input variable "16#Event_Class" in the OB83 interface area: event type 16#39 indicates that the module has been unplugged, and event type 16#38 indicates that the module has been plugged in. When the CM PTP module is re-plugged in, the Modbus_Comm_Load instruction needs to be called in the interrupt OB83 to reconfigure the communication module, as shown in Figure 15.

Figure 15. Re-calling Modbus_Comm_Load instruction in OB83

Note:
① The background data block for calling the Modbus_Comm_Load instruction in OB83 needs to be the same as the background data block for calling the Modbus_Comm_Load instruction in OB1.
② The hardware identifier of the CM PTP module can also be queried in "PLC Variables" ---> "System Constants", as shown in Figure 16:

Figure 16. System Constants

(3) Insert "Rack or Station failure" interrupt OB86

During the failure and recovery of a distributed IO site, the operating system will call OB86 once. The faulty module and type are determined by the input variable "16#Event_Class" in the OB86 interface area: event type 16#39 indicates site failure, and event type 16#38 indicates site recovery. When the IO site where the CM PTP module is located recovers, it is necessary to call the Modbus_Comm_Load instruction in the interrupt OB86 to reconfigure the communication module, as shown in Figure 17:

Figure 17. Calling Modbus_Comm_Load Instruction in OB86

Note:
① The background data block for calling the Modbus_Comm_Load instruction in OB86 needs to be the same as the background data block for calling the Modbus_Comm_Load instruction in OB1.
② The hardware identifier of the distributed IO site can also be queried in "PLC Variables" ---> "System Constants".

2. Modbus RTU Slave Programming

Modbus RTU slave programming requires calling the Modbus_Comm_Load instruction and the Modbus_Slave instruction. The Modbus_Comm_Load instruction configures the communication module through the Modbus RTU protocol, and the Modbus_Slave instruction can communicate as a Modbus slave through the port configured by the Modbus_Comm_Load instruction. The MB_DB parameter of the Modbus_Comm_Load instruction must be connected to the (static) MB_DB parameter of the Modbus_Slave instruction. In this example, the CPU rack CM1241 RS422/485 serves as a Modbus RTU slave, and the relevant programming steps are as follows:

(1) Insert an FC function in OB1, and drag the Modbus_Comm_Load instruction and the Modbus_Slave instruction into the function.

Figure 18. Drag in Modbus RTU Slave Command

The meanings of the parameters for the Modbus_Slave command are shown in Table 3 below:

Table 3: Meaning of Modbus_Slave instruction parameters

Note:
① The Modbus_Comm_Load instruction is not recommended to be called in the startup block OB100, and it is recommended to call it in OB1. When the Modbus_Comm_Load instruction is called in OB1, its input bit "REQ" needs to be triggered by a rising edge, and in this example, this input bit uses the "FirstScan" system memory bit.
② The static variable "MODE" in the background data block of the Modbus_Comm_Load instruction is used to describe the working mode of the PTP module, and the valid working modes include:

• 0 = Full-duplex (RS232)
• 1 = Full-duplex (RS422) four-wire mode (point-to-point)
• 2 = Full-duplex (RS422) four-wire mode (multi-drop master, CM PtP (ET 200SP))
• 3 = Full-duplex (RS422) four-wire mode (multi-drop slave, CM PtP (ET 200SP))
• 4 = Half-duplex (RS485) two-wire mode

The default value of this static variable "MODE" is 0 (RS232 full-duplex mode), and it needs to be modified according to the actual configuration of the CM1241 RS422/485 module. In this example, where the CM1241 RS422/485 module operates in RS485 half-duplex mode, the value needs to be modified to 4. How to modify the "MODE" static variable is shown in Figure 12.

③ The "MB_HOLD_REG" of the Modbus_Slave instruction is used to point to the data area address of the Modbus hold register, which supports optimized access data blocks or non-optimized (standard) data blocks. It is recommended to use non-optimized access data blocks.

(V) Common Issues

1. Are there any restrictions on the use of the new version of Modbus RTU instructions?
When the new version of Modbus RTU instructions are used for Modbus RTU communication through the CM1241 communication module or CB1241 communication board, the following conditions must be met:

a. The firmware version of the S7-1200 CPU cannot be lower than V4.1;
b. The firmware of the CM1241 communication module or CB1241 communication board cannot be lower than V2.1.

The CPU with firmware version V4.0 of the S7-1200 can be updated to version V4.1 through firmware updates, and the CM1241 communication module with firmware version V2.0 can also be updated to version V2.1 through firmware updates. For the method of firmware updates, please refer to the S7-1200 firmware update instructions.

2. What is the role of the static variable "MODE" in the background data block of the Modbus_Comm_Load instruction? Why is it necessary to modify this variable in general projects?
The new version of the Modbus RTU instructions expands the functionality of Modbus RTU. In addition to supporting the CM1241 communication module and CB1241 communication board, this instruction also supports Modbus RTU communication through the PTP communication module on the PROFINET or PROFIBUS distributed I/O rack. The PTP communication module on the distributed I/O rack supports multiple working modes. Taking the ET200SP CM PtP module (order number: 6ES7137-6AA00-0BA0) as an example, it supports multiple working modes such as RS232, RS485, and RS422. The static variable "MODE" in the background data block of the Modbus_Comm_Load instruction is used to define the working mode of the PTP module. The default value of "MODE" is 0, representing the "full duplex (RS232)" working mode. In actual project configurations, this variable needs to be modified according to the actual working mode.

3. Can the Modbus_Comm_Load instruction be called in the startup organization block OB100?
The Modbus_Comm_Load instruction is not recommended to be called in the startup block OB100. The new version of the Modbus RTU instruction extends the functionality of Modbus RTU, and this instruction supports PtP communication modules on PROFINET or PROFIBUS distributed I/O racks to implement Modbus RTU communication. The operating system needs to call instructions such as read data record and write data record to implement Modbus RTU communication with PtP modules on the distributed I/O racks. The read data record and write data record instructions are asynchronous read and write instructions, and the execution of these instructions requires N scanning cycles. Therefore, it is not recommended to call the Modbus_Comm_Load instruction in the startup block OB100.

4. How to query the error code when there is a Modbus RTU communication error?
Taking the Modbus_Master instruction as an example, when a communication error occurs, the "ERROR" output bit of the Modbus_Master instruction will become TRUE but only remain TRUE for one scanning cycle. Therefore, when monitoring through the TIA software program, the error cannot be queried. The error code in the "STATUS" parameter of the Modbus_Master instruction is only valid during the scanning cycle when "ERROR" = TRUE. To obtain the error code for Modbus RTU communication errors, we can use the method shown in Figure 19 below.

Figure 19. Obtaining STATUS