CP 1242-7 V2 SIM Not Detected: LED Diagnostics and Firmware Fix

1. Problem Description

A S7-1200 station equipped with the CP 1242-7 V2 GPRS communication module (article number 6GK7242-7KX31-0XE0) shows a flashing red DIAG LED and a solid red NETWORK LED after the SIM card is inserted. All other status LEDs on the module remain off. According to the device manual, this LED pattern indicates that the CP cannot detect a SIM card, even though the card is functional, has no PIN, and was verified in a mobile handset.

The CP 1242-7 V2 is a GSM/GPRS (2G) communication processor that mounts on the left side of an S7-1200 CPU. It is widely used for telemetry, remote service, and SMS-based alarm distribution. When the SIM interface fails to register, the entire remote link goes down and the CPU cannot pass IP traffic through the WAN interface, so the symptom is reported from PLC programs, HMI panels, and external TIA Portal diagnostic views.

2. CP 1242-7 V2 LED Reference

The CP 1242-7 V2 has four front-panel LEDs. Read them in the order DIAG → STATUS → NETWORK, since each LED answers a different question. The values below summarise the operating manual; always cross-check with the current CP 1242-7 V2 operating instructions shipped with your firmware version.

Use this table as a triage matrix. A green STATUS LED confirms the CP is correctly mounted on the S7-1200 left bus, so the fault is downstream of the backplane. A dark STATUS LED means the CPU never sees the CP, and you are chasing a different problem (slot, firmware catalogue, defective backplane connector).

3. Root Cause Matrix

Field experience with the CP 1242-7 V2 narrows the "SIM missing" condition to a small set of recurring causes. Rank them in this order before you start swapping hardware:

4. Pre-Checks Before Touching Hardware

1. Confirm the CPU model and firmware. The CP 1242-7 V2 is supported on the S7-1200 CPUs 1211C, 1212C, 1214C, 1215C, and 1217C from firmware V4.2 upward. The "new version" 1212C DC/DC/RLY in the report is fine, but record the exact MLFB and firmware level before continuing.
2. Read the CP label. The 6GK7242-7KX31-0XE0 is the V2 variant. If the label shows 6GK7242-7KX30-... you have the V1 module and need a different configuration profile.
3. Open TIA Portal online diagnostics: Online > Online & Diagnostics > CP 1242-7 V2 > Diagnostics. The diagnostic buffer lists entries such as SIM card not inserted, SIM PIN required, GSM network search, and Login to APN failed. These text codes identify the failing layer more precisely than the LEDs.
4. Take a screenshot of the TIA Portal hardware configuration with the CP's article number and firmware version visible. This is the single most useful artifact when escalating to Siemens technical support.

5. SIM Card Verification Procedure

1. Power off the S7-1200 station before removing or inserting the SIM. Hot-plugging the card is not supported on the CP 1242-7 V2 and can corrupt the SIM or the holder.
2. Inspect the card holder on the underside of the CP. The CP 1242-7 V2 uses a standard mini-SIM (2FF) form factor. If you are inserting a micro-SIM (3FF) or nano-SIM (4FF) with a plastic adapter, the adapter can lift away from the contacts and produce the "SIM missing" LED pattern even though the card is mechanically present.
3. Wipe the gold contacts with a dry, lint-free cloth. Avoid solvents — the contact pads are bare nickel-gold and corrode quickly if contaminated with IPA residue.
4. Insert the SIM with the chamfered corner oriented as shown on the silk-screen beside the holder. The card should click and stay flush.
5. Power the station and wait 30 to 45 seconds. The CP needs roughly 20 seconds to initialise the cellular modem and another 10–20 seconds to register on the GSM network.
6. If the DIAG LED is still flashing red, move the SIM to a known-good mobile handset. Confirm the handset registers on 2G/GSM and that mobile data is functional. Many M2M SIMs ship with data disabled and require the carrier to enable the APN before any data session can attach.

6. Antenna and RF Link Checks

The CP 1242-7 V2 is shipped without an antenna. You must connect an external GSM antenna to the SMA female connector on top of the module. Without an antenna, the baseband cannot lock to a cell, and the module reports the failure as a network-side fault. In some firmware revisions the absence of an antenna is logged as SIM missing because the modem never completes initialisation.

• Use a quad-band GSM antenna covering 850/900/1800/1900 MHz. Siemens recommends the ANT794-4MR swivel antenna for indoor cabinets and the ANT896-4MA for outdoor use.
• Check antenna cable continuity. A shorted or open coaxial pigtail pulls the receiver noise floor up and prevents registration even with a strong signal.
• Verify the SIM is provisioned for a band that the antenna supports. In Europe and most of Asia the carrier is on GSM 900 and DCS 1800. In the Americas the carrier may be on GSM 850 / PCS 1900. A SIM from a band the modem cannot tune to is electrically invisible.
• Confirm RSSI. With TIA Portal online diagnostics, read the signal quality. A consistent value below -95 dBm produces intermittent registration and can be misread as a SIM fault when the SIM itself is healthy.

7. TIA Portal Configuration: Firmware and Catalogue Version

This is the single highest-probability fix for the symptoms in the source report. The CP 1242-7 V2 is a separate device in the TIA Portal hardware catalogue from the original CP 1242-7, and selecting the wrong entry causes a configuration/download conflict that the module reports as a SIM or firmware error.

1. In the project tree, open Devices & Networks, select the CP, and read the Article number and Firmware version shown in the device view.
2. Right-click the CP, choose Change device, and pick the entry that exactly matches the label: 6GK7242-7KX31-0XE0. Do not select the V1 entry 6GK7242-7KX30-... even if the icons look identical.
3. Confirm the firmware version in the catalogue matches the firmware flashed on the physical module. The CP 1242-7 V2 ships with firmware V2.1.x and can be updated via TIA Portal to V3.x for use with TIA V18 projects.
4. Compile the hardware configuration, download to the CPU, and power-cycle the station.

If the device catalogue does not list the V2 variant, your TIA Portal installation is too old. The CP 1242-7 V2 requires TIA Portal V14 SP1 with HSP 0296 or later, and is fully integrated from V15.1 onward. The recommended TIA Portal version for current firmware is V18 with all updates installed. Refer to the Siemens CP 1242-7 V2 product page for the latest HSP and firmware bundle.

8. APN and Carrier Provisioning

Once the SIM is detected and the antenna is connected, the module still needs a valid APN to attach to packet data. Configure the APN under CP 1242-7 V2 > Mobile network settings > APN.

• Use the APN string provided by the M2M provider. Common public APNs are "internet" (Vodafone, Telefonica in some regions), "web.vodafone.de" for Vodafone Germany, and carrier-specific M2M APNs for Aeris, Kore, Wireless Logic, and Soracom.
• Set the authentication type. Most M2M SIMs use PAP or CHAP. A mismatch between the APN credentials and the module's authentication setting is logged as a network-side fault and can look like a SIM problem from the LED perspective.
• Set the correct DNS. The CP 1242-7 V2 requires DNS to resolve Teleservice or OPC UA endpoints. Many APNs return DNS automatically; if you have to set DNS manually, use the carrier's recursive resolver.
• Verify with Online & Diagnostics > Mobile network > Status. A successful attach shows Attached with a PDP context IP address; a failure shows Detached with a cause code.

9. Hardware Module Verification

When the SIM is known-good, the antenna is correct, and the TIA Portal device matches the label, the CP itself is the suspect. Two hardware failure modes are common enough to warrant field test:

1. SIM holder wear: The push-push mini-SIM holder loses contact tension after several hundred insertions. On a desk, the SIM sits flat and the module reads it. In a cabinet with vibration, the SIM lifts a few tenths of a millimetre and the contacts open. The diagnostic buffer cycles between SIM inserted and SIM removed every few seconds.
2. Baseband IC failure: The cellular modem does not respond to AT commands. TIA Portal online diagnostics shows the CP as present on the backplane, but no mobile network status is reported. This is the failure mode reported in the source thread as "defective module, hardware claim to Siemens".

To differentiate, swap the suspect CP with a known-good unit on the same S7-1200. If the LED pattern changes to a normal green-on-gprs sequence, the original module is defective and must be RMA'd through your Siemens representative. If the symptom is identical, the problem is the SIM, antenna, or TIA Portal configuration.

10. Step-by-Step Diagnostic Flow

1. Verify the CPU is running and the CP STATUS LED is green (backplane OK). If not, fix the slot, CPU firmware, or backplane connector first.
2. Read the DIAG and NETWORK LED states. Match them to the table in Section 2.
3. Open TIA Portal online diagnostics and read the most recent diagnostic buffer entry.
4. If the entry is SIM card not inserted: power off, reseat SIM, test SIM in handset.
5. If the entry is Configuration error: verify the device catalogue entry matches the physical CP (V1 vs V2) and the firmware is compatible with the TIA Portal version.
6. If the entry is Login to APN failed: verify APN, authentication, and DNS settings; confirm with the carrier that GPRS service is active on the SIM.
7. If the entry is Hardware fault: swap the CP with a known-good unit and RMA the suspect.
8. Re-test after every change. Do not stack multiple changes in one download, because the diagnostic buffer can only show the most recent failure.

11. Verification Checklist

Mark the checklist in your commissioning sheet and store a screenshot of the green-LED state. This is the artefact your maintenance team will compare against the next time the LED goes red.

12. Common Pitfalls

• Carrier sunset of 2G. Several countries and operators have sunset 2G/GSM. The CP 1242-7 V2 is a 2G-only modem and will not register on LTE-M or NB-IoT. Confirm 2G coverage with the carrier before commissioning. For 4G/LTE migration use the successor CP 1243-1 or CP 1542SP-1.
• M2M SIM with data disabled. M2M SIMs often ship with voice and SMS enabled but data disabled. The handset will see signal bars but the APN attach will fail. Always confirm the APN is provisioned for the SIM.
• Wrong HSP installed. An older TIA Portal installation with the wrong Hardware Support Package silently substitutes the V1 catalogue entry. The download succeeds, but the firmware on the CP is incompatible and the module reports a configuration error.
• Antenna pigtail shorted to cabinet ground. A coaxial pigtail routed too close to a VFD cable picks up noise and can shift the receiver. Move the antenna cable at least 10 cm from any 400 V or VFD cable.
• Reading the LED too early. The CP needs up to 45 seconds to complete the SIM read, band scan, and PLMN selection. A quick glance during boot can read the red-flashing DIAG as a permanent fault.

13. Frequently Asked Questions