Configuring Siemens CM 1241 RS232 Module with S7-1200 CPU 1214C

Overview

The Siemens SIMATIC S7-1200 family supports serial point-to-point (PtP) communication through the CM 1241 communication module family. The CM 1241 RS232 (article number 6ES7241-1AH32-0XB0) plugs onto the left side of the CPU 1214C and provides an isolated RS-232C interface for free-port protocols, Modbus RTU master/slave, USS, and 3964(R). The module is detected and configured inside the TIA Portal Device View, not the Network View, because RS-232 is treated as a point-to-point channel rather than a routed subnet.

This article walks through the exact configuration path for a CPU 1214C plus a CM 1241 RS232 inside TIA Portal V13 SP1 (and later), identifies compatible firmware/tooling combinations, and documents the programming entry points used to forward a message received on the PROFINET port out through the RS-232 module.

Prerequisites

1. One SIMATIC S7-1200 CPU 1214C, any of the following order numbers:
   • 6ES7214-1AG31-0XB0 — AC/DC/Relay (1214C DC/DC/Rly, firmware V4.x)
   • 6ES7214-1AG40-0XB0 — AC/DC/Relay (F-suffix models and most V4.x production runs)
   • 6ES7214-1HG31-0XB0 — DC/DC/DC (1214C DC/DC/DC, firmware V3.x or V4.x)
   • 6ES7214-1HG40-0XB0 — DC/DC/DC (current production, firmware V4.x)
2. One CM 1241 RS232 (6ES7241-1AH32-0XB0) — backplane-mounted to the left of the CPU.
3. STEP 7 Professional V13 SP1 or later (V15, V15.1, V16, V17, V18 are also compatible with the same article number). Update 9 of V13 SP1 is the lowest recommended patch level for new deployments.
4. A 24 V DC power supply sized for the CPU plus the CM 1241 (the CM draws approximately 100 mA from the backplane, with no external 24 V required for the module itself; verify with the catalog data sheet).
5. RS-232 cable terminated to the device you are connecting to (DCE/DTE aware — see the wiring section below).

Hardware Identification

Two order-number variants of the CM 1241 RS232 exist:

The user's reported part number 6ES7241-1AH32-0XB0 is the current production article and is fully supported by the CPU 1214C. The -40- mid-section in the CPU part number is a firmware/revision marker, not a hard compatibility barrier: TIA Portal V13 SP1 Update 9 and later will see and configure all of the listed CPU part numbers. Earlier Update levels may flag the V4.x CPUs as "unknown module" and refuse to add the CM.

Step-by-Step Configuration in TIA Portal

Step 1 — Create the device

1. Open TIA Portal and create a new project.
2. From the project tree, choose Add new device > SIMATIC S7-1200 > CPU > CPU 1214C > <your article number>.
3. Confirm the firmware version that TIA Portal selects matches what is actually loaded in the physical CPU (visible in Online & diagnostics > Module information).

Step 2 — Open the Device View (not the Network View)

Selecting the CPU in the project tree and double-clicking Device View opens the hardware editor with the CPU in the centre. The left slot of the CPU is reserved for communication modules (CM) and the right slot for signal boards (SB). The CM 1241 RS232 must be dragged into the left slot of the CPU; if the slot is already occupied by a CSM 1277 or another CM, the new module is rejected and shown in red.

Step 3 — Insert the CM 1241 RS232

1. In the hardware catalog, expand Communication modules > CM 1241 > RS232.
2. Drag the 6ES7241-1AH32-0XB0 entry onto the highlighted slot to the left of the CPU.
3. Confirm the order number in the module's properties; TIA Portal will populate the diagnostic addresses automatically. The default input address is 256..257 and the output address is 256..257 (configurable under Properties > I/O addresses).
4. Compile the hardware configuration (Project tree > CPU > Compile > Hardware (rebuild all)) to surface any catalog mismatches.

Step 4 — Configure the port

Open Properties > Port configuration on the CM 1241 and set:

Step 5 — Download the project

Connect the PROFINET port of the CPU to the engineering PC, right-click the device, and choose Download to device > Hardware and software (only changes). After successful download, the CM 1241's diagnostic LED should transition from flashing green (no configuration) to solid green (configured and ready).

Programming Point-to-Point Communication

The CM 1241 is programmed through the Point-to-Point instruction set inside the CPU's program blocks. Open a new function block (FB or OB) and insert instructions from the Communication > Communication processor > Point-to-Point palette.

Receive

Use the RCV_PTP instruction (or the legacy RCV_CFG + RCV_REC pair on older firmware):

// SCL example: receive up to 200 bytes from the CM 1241 RS232
"rcvBuffer" : ARRAY[0..199] OF BYTE;

#rcvInst(REQ := TRUE,
         PORT := 269,            // CM 1241 hardware identifier from device configuration
         BUFFER := "rcvBuffer",
         LENGTH := 200,
         DONE => #rcvDone,
         ERROR => #rcvError,
         STATUS => #rcvStatus);

The PORT parameter is the CM 1241's hardware identifier, not the diagnostic I/O address. Locate it under Properties > System constants on the CM 1241. Typical value: 269 for the first CM 1241 in the left slot.

Transmit

Use the SEND_PTP instruction (or SEND_CFG + SEND for legacy firmware):

"sendBuffer" : ARRAY[0..99] OF BYTE;
#sendLen := 12;   // actual length of the frame to send

#sendInst(REQ := #triggerSend,
          PORT := 269,
          BUFFER := "sendBuffer",
          LENGTH := #sendLen,
          DONE => #sendDone,
          ERROR => #sendError,
          STATUS => #sendStatus);

Status / Error Word Interpretation

The STATUS output of the SEND/RCV instructions returns a 16-bit word. The most common values encountered on the CM 1241 RS232:

Architecture: Ethernet-to-RS232 Message Forwarding

The target application described in the original question — reader on PROFINET, PLC, downstream RS-232 device — maps cleanly to the S7-1200's two-port nature:

[Reader / Barcode scanner] ---PROFINET--> [S7-1200 CPU 1214C PROFINET port]
                                                       |
                                                       |  (user program:
                                                       |   Parse -> Modify -> SEND_PTP)
                                                       |
                                              [CM 1241 RS232 (6ES7241-1AH32-0XB0)]
                                                       |
                                              [RS-232 cable: TxD/RxD/GND]
                                                       |
                                                  [Downstream device]

Implementation pattern:

1. Receive the reader's TCP/UDP frame on the PROFINET port using TSEND_C/TRCV_C or an open user communication connection.
2. Parse and rewrite the message inside the user program.
3. Hand the resulting byte array to SEND_PTP on the CM 1241 RS232.

Wiring the CM 1241 RS232

The CM 1241 RS232 ships with a 9-pin sub-D male connector with the standard DTE pinout:

For a point-to-point link to a DCE (modem-style) device, use a straight-through cable. For a link to another DTE (e.g. another PLC, a PC's COM port), use a null-modem (crossover) cable that swaps TxD↔RxD and RTS↔CTS. Maximum recommended cable length is 15 m per the EIA/TIA-574 specification, though in practice 3 m is typical in panel wiring.

CM 1241 RS232 Specifications

For the full Siemens specification table see the official S7-1200 manual collection at the TIA Siemens Cloud S7-1200 CM 1241 RS232 specifications page.

Verification Procedure

1. Online diagnostics: Mark the CM 1241 in the project tree and choose Online & diagnostics > Diagnostics. The "Status of module" must read OK. If it reads Not available or Fault, the hardware identifier is mismatched or the module has not received its configuration since the last download.
2. LED check: DIAG LED solid green = configured and running. Slow flash green (0.5 Hz) = no configuration. Solid red or flashing red = fault; check STATUS word for cause.
3. Loop-back test: Disconnect the partner cable, install a wire jumper between pin 2 (RxD) and pin 3 (TxD) on the D-sub, and run a SEND/RCV back-to-back in the watch table. The CPU should echo the same bytes back to the buffer. If the echo fails, the CM 1241 is faulty; if the echo succeeds, the problem is the partner wiring.
4. Protocol-level test: Use a free port monitor on the partner PC (e.g. Docklight, AccessPort) to confirm the byte stream, frame delimiter, and inter-character delay match the partner device's expectations.

Troubleshooting Matrix

Edge Cases and Field Tips

• Mixing firmware generations: A CM 1241 RS232 ...AH30... (V1.0) is not interchangeable with ...AH32... (V2.0) in a project whose CPU is on V4.x firmware. TIA Portal will accept either at design time but the V1.0 module will fail to start on the bus.
• Slot chain length: The CPU has only one slot on the left for a CM. If you need two serial channels you must add a CB 1241 RS485 signal board (right side of the CPU) for the second channel — the slot to the left remains a single CM.
• Hot-swapping: The CM 1241 is not hot-swappable. Power down the CPU before insertion or removal; otherwise the CPU will detect a configuration change and stop.
• Firmware update: A CM 1241 ...AH32... can be firmware-updated online through TIA Portal Online & diagnostics > Firmware update. Use a firmware file at least as new as the one bundled with your TIA Portal version.
• Isolation budget: The 707 V DC isolation between the RS-232 signal and the CPU logic common is the type-test value, not a continuous working voltage. For permanent installations keep the field-side RS-232 cable within the same equipotential bonding zone as the S7-1200 rack.
• Freeport framing: If you are not using Modbus RTU, leave "End of receive criteria" at the default Message timeout with a value of 4 × character time. Avoid "Character delay time" with low values when the partner may insert flow-control pauses.

Reference Documentation

• S7-1200 Manual Collection — CM 1241 RS232 Specifications
• S7-1200 Manual Collection — Using the Serial Communication Interfaces

FAQ