Siemens CB 1241 vs CM 1241 RS485: S7-1200 Module Comparison

Overview: S7-1200 Communication Module Family

The SIMATIC S7-1200 PLC platform supports point-to-point (PtP) serial communication through a small but distinct family of plug-in modules. The two most frequently ordered RS-485 interfaces are the CB 1241 RS 485 (article number 6ES7241-1CH30-1XB0) and the CM 1241 RS 485 (article number 6ES7241-1CH30-0XB0). Although the MLFB suffixes differ by a single character (1XB0 vs 0XB0), the two parts represent different physical form factors, different installation locations on the CPU, and a subtle but field-relevant difference in how the bus termination is applied. They are not interchangeable hardware; however, they are software-identical from the perspective of the STEP 7 / TIA Portal instruction set.

This reference consolidates the official datasheet values from the Siemens Industry Online Support portal and the TIA Portal manual collection, adds the wiring detail for the CB variant, and clarifies the replacement path: the 0XB0 CM has been cancelled and superseded by 6ES7241-1CH32-0XB0. Engineers selecting a module for a 2-wire RS-485 link to a variable frequency drive, a third-party instrument, or a Modbus RTU multi-drop network will find the wiring, programming, termination, and substitution guidance in the sections that follow.

Product Identification and Nomenclature

Siemens uses a structured MLFB (Machine-Readable Product Designation) that encodes family, function, and revision. The two relevant codes break down as follows:

Decoding the MLFB segments:

• 6ES7 — SIMATIC S7 product group
• 241 — Communication function group (PtP / serial modules)
• 1CH30 — Function (CH = RS-485 half-duplex, 30 = base feature set)
• 1XB0 vs 0XB0 — Packaging/installation variant: 1 = Communication Board (CB, fits the CPU's onboard slot), 0 = Communication Module (CM, snaps to the left of the CPU or to the left of another CM/SM)

The CH32 successor uses the same RS-485 transceiver characteristics and the same TIA Portal block library as the CH30 parts. For greenfield designs the CH32 part should be quoted.

Form Factor and Mechanical Installation

The CB 1241 RS 485 is a low-profile board that inserts into the dedicated communication slot located on the front of the S7-1200 CPU, immediately below the wiring terminals and above the signal board slot. The CB occupies no additional DIN-rail width and is retained by the CPU's terminal cover. It is the only module that can populate that slot; you cannot install a CM 1241 in the CB location or a CB 1241 in the left-side signal module (SM) position.

The CM 1241 RS 485 is a standard-width module that mounts to the left of the CPU, in the same rail position used by signal modules (SM 12xx), battery boards, or additional CMs. Each CM 1241 extends the maximum I/O width budget and counts against the S7-1200's hardware configuration limit (typically six CMs/SMs per CPU, depending on the CPU's signal board/communication board support and the number of available module slots).

Engineers frequently choose the CB form factor when the cabinet layout is constrained horizontally and the application needs only one RS-485 port. The CM form factor is preferred when the design already requires an additional SM for I/O expansion and the rail width is not the limiting factor, or when more than one RS-485 network is needed.

Technical Specifications Comparison

The official datasheet for the CB 1241 RS 485 is published on the Siemens Industry Online Support portal and gives the values below. The CM 1241 RS 485 (CH30 and CH32) shares the same RS-485 transceiver and protocol capability, but draws its 5 V supply differently because it is backplane-fed through the left-side bus connector rather than the CPU's onboard slot.

The 50 mA 5 V backplane draw of the CB is the value that matters most for CPU power-budget calculation on the S7-1200. The CM variant's 220 mA draw is taken from the same 5 V rail but through the backplane connector on the left of the CPU. Both modules tolerate the 24 V sensor supply on the CPU's terminal block for the shield clamp and bias resistor network.

RS-485 Termination Resistor: The Key Functional Difference

RS-485 is a differential, multi-drop, half-duplex bus. The EIA-485 standard requires 120 Ω termination at both physical ends of the bus to suppress reflections on the twisted pair. The two S7-1200 modules implement this requirement differently, and the difference is the single most common source of field commissioning confusion.

CB 1241 RS 485 — Integrated Termination via Short Link

The CB 1241 exposes a 6-pin terminal block with pairs labelled A, B (RS-485 data), RTS (request-to-send, used as a transmit enable for RS-485 driver direction control in 4-wire mode), and TA / TB. TA and TB are the two pads of an onboard 120 Ω termination resistor. To engage the termination, the installer places a short link (a piece of wire or a jumper) between the TA and TB terminals at the CB's terminal block. The Siemens TIA Portal manual collection documents this directly:

"CB 1241 RS485 (6ES7241-1CH30-1XB0) — Connect TA and TB as shown to terminate the network. (Terminate only the end devices on the RS485 network.)"

This arrangement is convenient: the engineer can enable or disable termination by moving a single wire at the CB's terminals, no external resistor is required, and the part count is reduced.

CM 1241 RS 485 — External Termination Required

The CM 1241 RS 485 (both the CH30 and CH32 revisions) does not include onboard termination. The 120 Ω resistor must be installed externally across the A and B conductors at each physical end of the bus. In practice this is done either by soldering the resistor into the connector block, by using a D-sub backshell with a built-in termination switch, or by installing a Siemens termination connector (e.g., a 6ES7972-0DA00-0AA0-style terminator where compatible).

CB 1241 RS-485 Wiring Detail

The CB 1241 terminal assignment is the same on every CPU family that accepts the board. The wiring diagram in the TIA Portal manual collection illustrates the link between TA and TB as a discrete wire crossing the top of the terminal block, with the RTS line jumpered to the bus bias as required for 2-wire and 4-wire modes.

CM 1241 RS-485 Wiring Detail

The CM 1241 uses the same logical signal pair (A / B) but routes them through a removable 6-pin terminal block. Termination is achieved with an external 120 Ω 1/4 W resistor installed at the connector block of the end devices, or with a commercially available D-sub terminator. The 4-wire mode of the CM module (full-duplex) uses T(R) and T(T) for transmit and R(T) and R(R) for receive. For 2-wire Modbus RTU and the typical VFD link discussed in this reference, only the A and B terminals are used.

Programming and Instruction Set

From the controller's software perspective the CB 1241 RS 485 and the CM 1241 RS 485 expose the same logical port. The TIA Portal device configuration shows both as a PtP communication interface with the same set of baud rates, parity options, and protocol libraries. The hardware identifier (HW ID) is allocated automatically by the device configuration and is referenced by the user program as the input to the protocol blocks. There is no difference in the program code — the same MB_COMM_LOAD + MB_MASTER / MB_SLAVE pair, the same USS_PORT + USS_DRIVE pair, or the same Freeport SEND_PTP / RCV_PTP blocks work with either module.

A minimal Modbus RTU master configuration in TIA Portal V13 (or later) for either module is shown below.

// Hardware ID assigned automatically: 269 (example)
// RS-485: 19200 bit/s, 8E1, no flow control
// Slave address: 1, VFD start register 40001

MB_COMM_LOAD_DB(
    REQ    := TRUE,
    PORT   := 269,           // HW ID of the CB/CM
    BAUD   := 19200,
    PARITY := 2,             // 0=No, 1=Odd, 2=Even
    FLOW_CTRL := 0,          // 0=None
    TIMEOUT := 1000,         // ms
    DONE   => done_flag,
    ERROR  => err_flag,
    STATUS => status_word
);

MB_MASTER_DB(
    REQ    := done_flag,
    MB_ADDR:= 1,
    MODE   := 0,             // 0=Read, 1=Write, 2=Read/Write
    DATA_ADDR := 40001,      // 0-based Modbus offset → register 40001
    DATA_LEN := 10,
    DATA_PTR := P#DB100.DBX0.0 BYTE 20,
    DONE   => m_done,
    BUSY   => m_busy,
    ERROR  => m_err,
    STATUS => m_status
);

Changing the module from a CB to a CM (or vice versa) in the device configuration updates only the HW ID and the I/O address assignments in the background. The user program does not need to be re-written; the project can be recompiled and downloaded without source modification when the wiring side of the migration is correctly re-terminated.

Successor Module: 6ES7241-1CH32-0XB0

The CM 1241 RS 485 part number 6ES7241-1CH30-0XB0 was officially cancelled by Siemens and replaced by the 6ES7241-1CH32-0XB0 variant. From a field-engineering perspective the migration is drop-in: the form factor, terminal layout, mounting, and TIA Portal configuration screens are unchanged. The internal revision brings minor component updates (longer component lifecycle, RoHS/REACH compliance updates) and is functionally transparent to the user program. Stock of the CH30-0XB0 may still be available in distribution channels for some time, but new designs and spare-parts quotations should reference 6ES7241-1CH32-0XB0.

The CB 1241 RS 485 (6ES7241-1CH30-1XB0) has not been announced as cancelled at the time of writing and remains the active Communication Board for the S7-1200 platform.

VFD Communication Application Example (RS-485 to ABB Drive)

A typical field scenario uses one of these modules to control an ABB drives family member (such as the ACS150, ACS310, ACS355, ACS580) over the drive's integrated Modbus RTU interface. The drive's 2-wire RS-485 port is wired to the S7-1200's A and B terminals. Only two communication wires (plus shield) are required.

For this application the choice between CB and CM is governed only by cabinet space and the S7-1200 signal-board slot. If the CPU has its onboard slot free, the CB is the lower-cost and more compact option. If the onboard slot is occupied by a battery board (BB 1297) or a 4 AI/2 AQ signal board (SB 1232, etc.), the CM variant is the only path.

Selection Guidance

Use the following decision matrix when selecting between the two part numbers.

Diagnostics and Troubleshooting

The most common field issues with the CB/CM 1241 RS 485 family are wiring, termination, and address-conflict issues. The following matrix maps symptoms to the most probable cause and corrective action.

Configuration and Commissioning Steps

1. Open TIA Portal ≥ V13 SP1 and create or open the S7-1200 project.
2. In the device view of the CPU, insert the Communication Board > CB 1241 RS 485 (if using the 1XB0) or Communication Module > CM 1241 RS 485 (if using the 0XB0 / 32-0XB0).
3. Open the module's Properties dialog and confirm the protocol selection (Freeport, USS, or Modbus RTU master/slave) and the port configuration: baud, parity, data bits, stop bits, flow control.
4. Wire the field side: connect A to A and B to B on the slave(s). For the CB, install the short link between TA and TB if this CPU is the bus end. For the CM, install the external 120Ω resistor if the CM is the bus end.
5. Compile the project, download to the CPU, and go online.
6. Trigger the protocol block (MB_MASTER, USS_PORT, etc.) and observe the DONE, ERROR, and STATUS outputs. A non-zero STATUS with ERROR = TRUE points to the diagnostic matrix above.

Related Documentation

• Siemens Industry Online Support — Datasheet for 6ES7241-1CH30-1XB0 (CB 1241 RS 485)
• TIA Portal Manual Collection — CB 1241 RS485 Wiring Detail

Frequently Asked Questions