Troubleshooting CM 1241 RS422/485 Communication with RS232

Problem Overview

The SIMATIC CM 1241 (RS422/485) communication module is designed to attach a S7-1200 CPU to a half-duplex RS485 bus or a full-duplex RS422 point-to-point link. When a third-party device exposes only an RS232 interface, integrators frequently insert a passive or active RS232-to-RS485 converter between the device and the CM 1241, expecting the RCV_PTP instruction to read the same payload it does on a CM 1241 (RS232) module. The symptom is a silent receive buffer, no characters decoded, and the load cell or indicator transmitting valid frames that the S7-1200 never acknowledges.

Two distinct root causes are observed in the field:

• RS232 cable polarity — straight cables used where crossover (null-modem) cables are required on both sides of the converter.
• RS232-to-RS485 converter drive — the converter cannot determine when to release the RS485 bus because the RS232 control lines (RTS, CTS, DTR, DSR) are not asserted, and no timing parameter is configured in TIA Portal.

Both issues produce the same observable symptom: RCV_PTP reports no error and the LEN output remains 0, or the CM 1241 raises diagnostic event 8200 ("Port is busy processing a transmit request") when an attempt is made to switch directions on the bus.

CM 1241 (RS422/485) Hardware Specification

The CM 1241 (RS422/485) is a 9-pin sub-D female DCE that can be parameterized in TIA Portal for RS422 four-wire full-duplex or RS485 two-wire half-duplex operation. Electrical and protocol limits per the S7-1200 Programmable Controller System Manual are summarized below.

The full specification page is published in the TIA Portal online documentation at CM 1241 RS422/485 Specifications. Refer to the S7-1200 Programmable Controller System Manual for the absolute maximum ratings, including ±25 V DC fault tolerance on signal pins when the module is unpowered.

Root Cause: Cable Polarity Between DTE and DCE

An RS232 port is a DTE (Data Terminal Equipment) interface, and a Siemens CM 1241 is also implemented as a DTE in RS232 terms. Two DTEs connected back-to-back must have TXD crossed to RXD, RTS crossed to CTS, and DTR/DSR crossed on the opposing side. A "straight-through" cable connects pin 2 to pin 2 and pin 3 to pin 3, which works between a DTE and a DCE (modem) but fails between two DTEs. The same rule applies when a converter is inserted:

The most common field error is using straight cables on both segments: load cell to converter, and converter to CM 1241 (RS422/485). On the RS422/485 module the RS232-style pin numbering for the converter side is the same: TXD on the converter must arrive at RXD of the module, and vice versa. If only one segment is crossed, the converter will see the load cell's TX but the CM 1241 will not see the converter's TX, and the user observes "only Tx flashing, nothing to receive." This was the root cause in the original incident: both cables were straight; converting them to crossover (null-modem) wiring resolved the issue.

Pinout Reference for the 9-Pin Sub-D

In RS485 two-wire mode, only pins 3 (Data+ / T/R+), 8 (Data- / T/R-) and 5 (ground) carry payload. Some legacy converters tie TX and RX to the same pair internally; if the converter exposes a "T+, T-, R+, R-" terminal block, the integrator must short T+ to R+ and T- to R- on the CM 1241 side to build a true two-wire bus.

RS232 to RS485 Converter Operating Modes

Two converter architectures dominate the market, and choosing the wrong one is the second most common field defect.

Auto-Sensing (Snooping) Converters

These converters monitor the RS232 TXD line for the start bit of a character. When the start bit is detected, the converter enables its RS485 transmitter and re-transmits the byte onto the bus. When the stop bit is observed, the converter disables its transmitter and switches the bus back to receive. No configuration is required on the S7-1200 side beyond enabling half-duplex and matching the baud rate.

This is the architecture that "just works" for a load cell that never expects to receive. The trade-off is that the converter introduces a one-character turn-around delay and is sensitive to noise on the RS232 TX line: false start bits or break conditions will be re-broadcast as bus errors. Slew-rate-limited RS485 drivers (16 ns or 24 ns typical) are required to avoid reflections; check the converter datasheet.

RTS-Controlled (Hardware Handshake) Converters

These converters ignore the RS232 data lines entirely. The RS485 transmitter is enabled only while an RS232 control line (typically RTS, sometimes DTR) is asserted. The S7-1200 application must raise RTS, transmit, and lower RTS in a strict sequence. On the CM 1241 (RS422/485), the equivalent functionality is exposed through the Automatic use of the RS232C signals check-box and the Time to RTS off / Data Output Waiting Time parameters, even though physically no RS232 signals exist on the bus — TIA Portal reuses the same parameter block for the RS485 mode.

TIA Portal Configuration for Half-Duplex RS485

Configure the CM 1241 (RS422/485) in the device view of TIA Portal V15 or later. The Port Configuration group contains both the physical interface and the protocol framing.

1. In the project tree, open Devices & Networks and select the CM 1241 (RS422/485).
2. Open Properties > Port Configuration.
3. Set Transmission mode to Half duplex (RS485), two-wire operation.
4. Match Baud rate, Parity, Data bits, and Stop bits to the load cell (typically 9 600 / N / 8 / 1 for weight indicators).
5. Enable Automatic use of the RS232C signals if the converter is RTS-controlled.
6. Set Time to RTS off to a value greater than the converter's worst-case bus release time. A safe starting point for 9 600 bit/s is 50 ms; raise to 200 ms if the converter datasheet is silent.
7. Set Data Output Waiting Time to the inter-frame delay expected by the load cell protocol (for continuous-stream load cells this can remain 0).
8. If the bus is physically long (> 10 m) or runs near VFD cabling, enable the integrated termination resistor via the hardware switch on the back of the module, or place a 120 Ω resistor across the data pair at the far end of the segment.
9. Compile the hardware configuration and download to the CPU.

The configuration interface, including the receive buffer section that became available with module firmware V2, is documented at Configuring the Communication Module in the SIMATIC S7-1200 Manual Collection.

Receive Buffer and RCV_PTP Diagnostics

The RCV_PTP instruction (FB / FC, library Point-to-Point Communication) reads characters from the receive buffer and assembles them into a message based on the configured start and end delimiters. Diagnostic visibility differs sharply between module firmware generations.

A typical RCV_PTP call uses the following inputs:

BUFFER := DB_LOADCELL.ReceiveBuffer

LEN := 0

ENQ_EN := FALSE

ENQ := 16#05

EOI := FALSE

TERM_CNT := 1

TERM_EN := TRUE

TERM_1 := 16#0D

TERM_2 := 16#0A

If the bus is silent, RCV_PTP.DONE never becomes TRUE and STATUS returns 16#0000. This is the signature of a wiring fault, not a protocol fault. The diagnostic step is to put an oscilloscope on the RS485 data pair and verify that the converter is actually releasing the bus. If the line is held continuously low (Mark) by the converter, the converter is failing to release its driver; the fix is RTS handshaking or a different converter.

Error Code 8200: Port Busy

Diagnostic event 8200 ("Port is busy processing a transmit request") is raised by the CM 1241 (RS422/485) when SEND_PTP or RCV_PTP is called while a previous request is still pending, or when the module cannot resolve the direction of the half-duplex bus within the configured turn-around window.

For a half-duplex link with a slow converter, 8200 is almost always a turn-around timing problem. Increase Time to RTS off until the error disappears, then back off by 20 % to leave margin for bus capacitance on long runs.

Step-by-Step Verification Procedure

1. Validate the load cell RS232 output with a laptop. Use a generic USB-to-RS232 adapter and a terminal program set to the documented baud rate. Confirm the load cell is transmitting a stable, repeating message terminated by the documented delimiter (typically CR/LF).
2. Test the CM 1241 (RS232) baseline. Connect the load cell directly to the CM 1241 (RS232) using a crossover cable. Verify that RCV_PTP returns DONE = TRUE and that the receive buffer contains the expected number of bytes. This proves the load cell and the S7-1200 application are functional.
3. Insert the converter between the load cell and the CM 1241 (RS422/485) module. Use a crossover (null-modem) cable on both sides of the converter. Confirm with a multimeter that pin 2 on each connector carries the receive signal of the device at the other end.
4. Power the converter. Verify the converter's power LED is on. If port-powered, ensure the upstream device asserts DTR/RTS. If externally powered, confirm the supply voltage is within the converter's specification (typically 5 V DC or 9–24 V DC).
5. Configure TIA Portal. Select half-duplex RS485, match baud rate, parity, data bits, stop bits. Enable the integrated termination if the bus is at the end of a segment. Enable Automatic use of the RS232C signals if the converter requires RTS to enable its driver.
6. Trigger RCV_PTP in a watchdog or cyclic OB. Observe the DONE, ERROR, and STATUS outputs. If DONE never asserts, place a scope on the RS485 pair and verify differential voltage swings of at least ±200 mV during the converter's active window.
7. Resolve 8200 errors. If STATUS returns 16#8200, increment Time to RTS off in 50 ms steps until the error clears. Confirm with a scope that the converter releases the bus (line goes to Mark) before the next request.
8. Long-term soak test. Run the link for at least 24 hours under full traffic. Capture all STATUS codes in a log DB and verify zero 8200, 80B0, or 80C0 events.

Troubleshooting Matrix

Best-Practice Cable Construction

When fabricating the RS232 cables by hand, the rule of thumb is that the cable entering and leaving the converter must both implement the null-modem crossover. A field-tested crossover for a 9-pin sub-D to 9-pin sub-D is:

2 <---> 3   (RXD <---> TXD)

3 <---> 2   (TXD <---> RXD)

4 <---> 6   (DTR <---> DSR)

5 <---> 5   (Signal ground)

7 <---> 8   (RTS <---> CTS)

8 <---> 7   (CTS <---> RTS)

For the RS422/485 side, follow the Siemens wiring diagram in the S7-1200 System Manual, Chapter "Communication interfaces":

Pin 3 (T/R+ on CM 1241) <---> Data+ on converter

Pin 8 (T/R- on CM 1241) <---> Data- on converter

Pin 5 (GND) <---> Ground on converter

120 Ω termination across pins 3 and 8 at the far end of the segment (last node only)

Frequently Asked Questions