Resolving LOGO! 24RC and INSYS GSM 4.1 SMS Communication Failures

The Siemens LOGO! 24RC logic module is a compact controller widely deployed for small automation and remote-alarm applications. When paired with the INSYS Microcom GSM 4.1 modem family, the combination delivers a low-cost remote notification solution: digital inputs and outputs on the LOGO! drive logic, the modem sends an SMS to a defined recipient list. In the field, this pairing frequently produces a situation where AT commands issued by a terminal produce SMS successfully, yet an output transition (Q1) on the LOGO! never reaches the modem's SMS engine. This article documents the root causes, the required HSCom configuration, and the field-proven reset/workaround procedures that bring the link up reliably.

1. System Overview and Architecture

The alarm chain consists of three blocks:

1. LOGO! 24RC — Logic module that evaluates inputs (I1..I24 depending on expansion) and drives outputs (Q1..Q16, with Q1..Q4 on the base unit). The base RS-232 port on the LOGO! 24RC (the 0BA5 and 0BA6 generations most commonly paired with the INSYS GSM 4.1) is used for serial communication to external devices.
2. INSYS Microcom GSM 4.1 — Industrial GSM/GPRS modem with a DB-9 serial interface, SIM card holder, and the HSCom configuration tool. The modem exposes an SMS-C engine that can be triggered by serial AT commands, by DTMF tones, or by polled monitored values coming from the attached LOGO!.
3. HSCom — Windows configuration software shipped with the INSYS modem. The HSCom project file (*.HGP) stores the recipient phone book, the serial port profile, the connection profile, and — critically — which LOGO! outputs are polled and forwarded as SMS events.

The integration diagram is straightforward: a 1:1 serial cable routes LOGO! TX → modem RX, LOGO! RX → modem TX, plus signal ground. No hardware handshaking is required for the alarm path because the INSYS GSM 4.1 operates as a DCE in transparent AT mode and as a polled master in LOGO! mode.

2. Required Hardware and Cabling

Verify the cable with a continuity tester before powering up. A swapped TX/RX pair will silently break the link and the modem will not produce any error indication.

3. Matching Serial Port Parameters

The first rule of a stable LOGO! ↔ INSYS link is that the serial parameters on the LOGO! and the parameters encoded in the HSCom project and the parameters in the modem's live profile and the parameters in LOGO!Soft Comfort's connection setup must all four match. The most commonly cited cause of a no-SMS-on-Q1 condition is a parameter mismatch — typically 9600/8/N/1 on the LOGO! against 19200/8/N/1 still active in the modem from a previous configuration.

4. HSCom Configuration: The Monitored-Values Trap

HSCom organizes the LOGO! interface under a dedicated menu node labelled LOGO. The trap that causes 80% of the field failures documented online is the dual-checkbox requirement: an output must be marked both in the Values in Use column and in the Monitored Values column. Marking only one of the two does not enable the SMS engine to fire on a transition.

4.1 Step-by-step HSCom setup for Q1

1. Launch HSCom and open the existing *.HGP configuration (or create a new one with File → New).
2. Configure the basic settings: SIM PIN (empty if PIN prompt is disabled), SMS-C number, and operator APN if GPRS is used for the data path.
3. Open the LOGO page at the end of the menu tree.
4. Locate output Q1 in the list of available values.
5. Tick Values in Use for Q1 — this allocates a polling slot in the LOGO! protocol frame.
6. Tick Monitored Values for Q1 — this declares Q1 as a watched element that triggers the SMS engine on state change.
7. Configure the action: in the pop-up window, set the recipient phone number in international format (+<country code><number>) and enter the SMS text. Repeat the number in the format +1234567890 for every recipient.
8. Save the project as *.HGP and upload it to the modem via the Modem → Send Configuration menu.
9. Press the Reset button on the INSYS modem once after the upload completes.

4.2 Required HSCom flags

5. Recipient Phone Number Format

The INSYS GSM 4.1 firmware enforces a strict E.164-style format on the recipient list. A number stored as 0412345678 is silently rejected by the SMS engine in many firmware builds, producing a "queued but not sent" condition with no log entry. Always store numbers as

+<country code>&code> followed by the subscriber number with no leading zero, no spaces, and no dashes.

6. Root-Cause Matrix

7. Step-by-Step Resolution Procedure

1. Confirm the base alarm chain works. Open a terminal on the PC, connect to the modem over its DB-9, type AT+CMGS="+1234567890" and send a test SMS. If this fails, the problem is the modem, the SIM, or the antenna — not the LOGO!.
2. Confirm the LOGO! outputs are alive. Run a minimal program in LOGO!Soft Comfort that ties I1 to Q1 and download it to the controller. Toggle I1 and listen for the relay click on Q1. If Q1 does not click, the problem is the program or the wiring — not the modem.
3. Match the serial parameters on all four locations listed in section 3.
4. Re-author the HSCom project following section 4.1, paying special attention to the Values in Use + Monitored Values dual checkbox.
5. Enter recipient numbers in E.164 format with the leading + and country code.
6. Upload the project to the modem and press the Reset button on the modem once the upload completes.
7. Force a first contact by sending the SMS STATUS? to the SIM card. The modem will respond with its current state and, in doing so, will synchronize the polling handshake with the LOGO!.
8. Trigger Q1 by asserting I1. The RX/TX LEDs on the INSYS should flash within 1–3 seconds, and the recipient should receive an SMS within 5–10 seconds, depending on the GSM network.

8. Verification Checklist

9. Field-Proven Tips and Edge Cases

• Reset is a one-button bring-up tool. After every HSCom upload, press the Reset button on the INSYS front panel. The modem does not always restart the LOGO! polling thread on a soft reconfiguration; a hardware reset guarantees a clean handshake.
• The STATUS? trick. Sending STATUS? to the modem's SIM forces a poll cycle and kicks the LOGO! protocol into a live state. This is a reliable way to verify the link during commissioning when you cannot yet toggle Q1.
• Watch for relay wetting current on Q1. The LOGO! 24RC outputs are electromechanical relays rated 8 A / 240 V AC. If you wire a low-current modem input across Q1, place a bleeder resistor (10 kΩ across the load) so the relay contacts do not develop a high-resistance film.
• Use a level transition, not a steady level, to trigger SMS. Configure the LOGO! to drive Q1 with an edge-triggered pulse (e.g., a 2 s pulse from a monostable). A level-driven Q1 that stays high will fire the SMS engine on every poll cycle in some firmware versions.
• Disable the SIM PIN. The INSYS firmware expects a PIN-less SIM by default. If the SIM is PIN-protected, either enter the PIN in HSCom or remove the PIN from the SIM with a phone first.
• Supply the modem with a clean 24 V rail. The LOGO! 24RC has limited auxiliary current. Power the INSYS from a separate 24 V DC source rated at least 1 A to avoid brown-outs during GSM transmit bursts (peaks at 2 A).
• Check the LOGO! generation. The 0BA5 and 0BA6 generations support the polled LOGO! protocol natively. Earlier 0BA4 hardware needs the optional communication module CM LOGO! 0BA4, and the integration with INSYS is documented separately in the Siemens LOGO! product portal.
• Save the .HGP file as binary-readable. If you need to share the configuration with a support engineer, attach the *.HGP file — not screenshots of HSCom. The binary contains the recipient list, the monitored-value table, and the serial profile.

10. Diagnostic State Machine

The state machine above maps the commissioning flow: power-on and AT command test prove the modem path; the Q1 toggle test proves the LOGO! ↔ INSYS protocol is live; a failure on Q1 forces a hardware reset and a STATUS? trigger to re-initialize the polling handshake.

11. Specifications Reference

12. Related Siemens Documentation

Engineers deploying this integration should also review:

• Siemens LOGO! product portal — current portfolio, latest base modules, and the 4.3" color touchscreen generation.
• The LOGO! 0BA6 system manual for the polling protocol block and the RJ12 pinout.
• The INSYS Microcom GSM 4.1 hardware reference for the DB-9 pinout and the Reset button location.
• The HSCom online help bundled with the configuration tool for the full monitored-value table.