Key Features
Note: The CB 1241 does not have a diagnostic LED like the CM 1241 modules; only TX (transmit) and RX (receive) LEDs are present .
Technical Specifications
General
Communication Interface
Integrated Protocols
Modbus RTU Specifications
Electrical
Physical & Environmental
Certifications
Pinout & Wiring
The CB 1241 RS485 uses a 9-pin sub-D connector. For RS485 2-wire mode, the key connections are :
Important Wiring Notes:
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Use shielded twisted pair cable and connect the shield to ground
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Terminate only the two ends of the RS485 network (127 Ω termination)
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A/B polarity convention varies between manufacturers. Siemens labels pin 8 as A (non-inverting) and pin 3 as B (inverting), but many third-party devices use the opposite convention. If communication fails despite correct parameters, try swapping the A/B lines—the transceiver is protected against this
Applications
The CB 1241 RS485 is ideal for applications requiring serial communication between S7-1200 PLCs and third-party devices :
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Variable Frequency Drives: Modbus RTU or USS control of VFDs from manufacturers like SEW, Danfoss, and ABB
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Modbus RTU Networks: Integrating with Modbus-enabled instruments, sensors, and power meters
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Legacy Equipment: Communicating with existing RS485-based controllers and devices
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Barcode Scanners & Scales: Reading data from serial-output industrial equipment
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Building Automation: Connecting to HVAC controllers and energy meters
Key Benefits
Important Notes
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CPU Firmware Requirement: The S7-1200 CPU must have firmware V2.0 or higher to use this CB .
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Modbus Library Compatibility: When using Modbus RTU with TIA Portal V13 SP1, the library version must match the CPU firmware. Modbus library V3.x requires CPU firmware V4.1.1 or higher. Older firmware (V4.0, V4.1) only supports library V2.x .
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Hardware Identifier: In TIA Portal, the RS485 port hardware identifier is 271, not 269 (269 is the board's diagnostic interface). Using the wrong ID will result in error 16#7000 .
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No Flow Control: RS485 does not support hardware or software flow control .
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No Diagnostic LED: The CB 1241 has only TX/RX LEDs; there is no diagnostic LED like the CM 1241 modules .
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Termination: Only terminate the two end devices on the RS485 network. Intermediate devices should not be terminated or biased .
FAQ
Q1: What is the primary function of the 6ES7241-1CH30-1XB0?
A: It is a CB 1241 RS485 communication board that adds a single RS485 serial interface to SIMATIC S7-1200 PLCs, enabling point-to-point communication with third-party devices such as VFDs, sensors, and Modbus RTU equipment .
Q2: What protocols does this module support?
A: The module supports Freeport, ASCII (library function), Modbus RTU Master, Modbus RTU Device (slave), and USS protocols . Note that 3964(R) is not supported on this RS485 variant .
Q3: What is the maximum communication distance?
A: The maximum cable length for RS485 communication is 1,000 meters using shielded twisted pair cable .
Q4: Does this board require an external power supply?
A: No. The CB 1241 is powered directly from the CPU backplane bus, drawing approximately 50 mA at 5V DC .
Q5: What CPU firmware version is required?
A: The S7-1200 CPU must have firmware V2.0 or higher to use this CB . For Modbus RTU library V3.x, CPU firmware V4.1.1 or higher is required .
Q6: What is the difference between CB 1241 and CM 1241?
A: The CB 1241 is a compact signal board that plugs directly onto the CPU front, providing one RS485 port. The CM 1241 is a larger communication module that mounts on the S7-1200 rail and is available in RS232, RS422/485, and other variants .
Q7: What is the hardware identifier for the RS485 port in TIA Portal?
A: The RS485 port hardware identifier is 271. Using identifier 269 (the diagnostic interface) will result in error 16#7000 on MB_COMM_LOAD .
Q8: Can I use this board with Modbus RTU devices from other manufacturers?
A: Yes. The CB 1241 supports Modbus RTU Master and Device modes, making it compatible with standard Modbus RTU devices. However, note that A/B polarity conventions vary between manufacturers—if communication fails, try swapping the A/B lines .
Q9: What is the maximum number of Modbus devices supported?
A: The module supports up to 247 slave devices (addresses 1-247), with a maximum of 32 devices per network segment without a repeater .
Q10: Does this board support USS protocol for controlling Siemens drives?
A: Yes. The CB 1241 supports USS protocol as a library function, enabling communication with Siemens drives that support USS .
