CQM1 SMS Alarm: Configuring GSM Modem with CX-Supervisor

1. Overview

The Omron CQM1 compact PLC and CX-Supervisor SCADA package form a classic alarm-notification stack that is still common in retrofit and small-machine installations. The engineering pattern in this reference uses the CQM1's built-in RS-232C ports to communicate with CX-Supervisor on COM1, while a separate GSM modem occupies COM2 for SMS dispatch. When the ladder program raises an alarm bit, a CX-Supervisor script sends AT commands to the GSM terminal, and the modem transmits a short text message to a maintenance phone.

Three SMS delivery paths are available, listed here in increasing order of hardware footprint:

• Serial GSM terminal (e.g., Siemens TC35/TC35i) attached to a PC COM port.
• Legacy handset with a serial data cable (older Siemens, Ericsson, Nokia products).
• Conventional landline modem using a GSM-to-PSTN adapter or direct SMS-capable PSTN modem.
• HTTP SMS gateway (Twilio, Clickatell, BulkSMS, vendor SDK) reached over ordinary Internet.

The first three paths are local to the cabinet and need no Internet service; they only require a SIM card with a voice/SMS tariff from any mobile operator. The fourth path is preferred when the cabinet is moved frequently or the GSM reception is poor at the plant.

2. Prerequisites

3. Hardware Topology

The topology separates the deterministic data path (PLC <-> SCADA) from the asynchronous alert path (SCADA <-> GSM modem). Both paths terminate on the same PC, but they never contend because the PLC never talks to the modem and the modem never talks to the PLC.

+-----------+    COM1 (Host Link)    +-------+
| CQM1 CPU  | <--------------------> |  PC   |
+-----------+                        |  CX-  |
                                     | Sup.  |
                                     |       |
                                     |       |   COM2 (AT cmd)
                                     |       | <----------+
                                     +-------+            |
                                                          v
                                                  +----------------+
                                                  |  GSM Modem     |
                                                  | (TC35i + SIM)  |
                                                  +--------+-------+
                                                           |
                                                           | SMS
                                                           v
                                                   +---------------+
                                                   | Maintenance   |
                                                   | Phone         |
                                                   +---------------+

Pin assignment follows Omron's standard Host Link cable (XW2Z-200S-CV or equivalent). The modem leg uses a straight-through cable unless the TC35i documentation specifies otherwise; the module's DCE/DTE jumper is normally set for DCE.

4. CQM1 Port Configuration

Open CX-Programmer and inspect the PC Interface Settings dialog before downloading the project. The PC port here refers to the CX-Programmer download channel, not the runtime SCADA channel. The values below correspond to the CQM1's DIP-switch and DM-parameter settings used at commissioning.

If the application needs the PLC to originate SMS messages without the SCADA PC, configure Port 2 with the RS-232C free-data mode (DM6646 = 0x1006) and use TXD(48) instructions in the ladder to send ASCII frames directly to the modem. This bypasses CX-Supervisor entirely and is the recommended path when the PC is optional.

5. Ladder Logic for Alarm Generation

A minimal alarm-detection rung uses a comparator against a threshold and a SET instruction on a dedicated alarm word. CX-Supervisor polls this word cyclically through Host Link; the SCADA script reacts to the rising edge.

-- Rung 1: Threshold alarm
|--[ LD 200.00 ]--[ > D200 D201 ]--[ SET 100.01 ]--|
|   (process    | (analog value   | (alarm bit     |
|    running)   |  vs setpoint)   |  for SCADA)    |

-- Rung 2: Latch reset on operator acknowledge
|--[ 250.00 ]--[ RSET 100.01 ]--|
|   (ack from HMI)

-- DM201 holds the alarm setpoint in BCD

Word W100 can be assembled into a 16-bit alarm map so a single Host Link read retrieves all 16 alarm sources. CX-Supervisor maps each bit to a discrete point tag.

6. CX-Supervisor Script Configuration

CX-Supervisor supports Microsoft VBScript and, in newer builds, Java-based extensions. Either scripting engine can open the second COM port, write AT frames, and read the modem's response.

6.1 Create the Alarm Point

1. Open the CX-Supervisor project and select Project > Points > Add.
2. Define point ALM_HIGH_PRESSURE with PLC address 100.01 and update on Change.
3. Add an action in the point's Events tab: select the On True event, choose Run Script, and call SendSMS(ALM_HIGH_PRESSURE.Value).

6.2 Open the GSM Port

The following VBScript snippet, placed in the project script section, opens COM2 at 9600 8N1 and verifies that the modem responds to a basic AT command before any alarm is allowed to fire.

' Project_Scripts.bas
Dim g_modem
Sub Project_Start()
    Set g_modem = CreateObject("MSCOMMLib.MSComm")
    g_modem.CommPort = 2            ' COM2
    g_modem.Settings = "9600,N,8,1"
    g_modem.InputLen = 0
    g_modem.RThreshold = 1
    g_modem.PortOpen = True
    AT_Query "AT"                   ' expect "OK"
    AT_Query "AT+CMGF=1"            ' switch to text mode
    AT_Query "AT+CSMP=17,167,0,0"   ' set SMSC and validity
    g_modem.PortOpen = False
End Sub

Sub AT_Query(cmd)
    g_modem.Output = cmd & vbCr
    Wait 0.2
    Debug.Print g_modem.Input        ' echoes back to CX-Supervisor log
End Sub

Sub SendSMS(reason)
    Dim number, body
    number = "+15551234567"         ' operator on-call
    body   = "Plant alarm: " & reason & " at " & Now
    g_modem.PortOpen = True
    g_modem.Output = "AT+CMGS="" & number & """ & vbCr
    Wait 0.2
    g_modem.Output = body & Chr(26) ' Ctrl+Z terminates SMS
    Wait 1.0                        ' SMS submission
    g_modem.PortOpen = False
End Sub

Use Project_Start() as the project's Startup macro so the modem hand-shake occurs once at SCADA boot. Avoid opening COM2 until after CX-Supervisor has finished initialising its Host Link session on COM1; otherwise the OS may allocate both COMs to a single process and collide with the PLC driver.

7. AT-Command Reference for SMS

All SMS-related AT commands follow the ETSI GSM 07.05 specification. The subset below is sufficient for alarm text messages.

PDU mode (AT+CMGF=0) is required for binary content, concatenated messages above 160 characters, or Unicode. For a one-line alarm text, PDU mode is unnecessary.

8. SMS Gateway Alternative

When a SIM card and a GSM modem are inconvenient (no cellular coverage, frequent PC moves, or large message volume), an HTTP-based gateway delivers the same outcome over the existing Internet connection. The CX-Supervisor script is rewritten to call the gateway's REST endpoint rather than open a serial port.

' Gateway path - example using BulkSMS-compatible API
Sub SendSMS_Gateway(reason)
    Dim http, url, body
    Set http = CreateObject("MSXML2.XMLHTTP")
    url = "https://api.example.com/sendsms?u=USER&p=TOKEN&to=" & _
          "+15551234567&text=" & Server.URLEncode("Plant alarm: " & reason)
    http.open "GET", url, False
    http.send
    Debug.Print http.status, http.responseText
End Sub

This approach demands outbound HTTPS on TCP 443 and a stable Internet connection. It removes the SIM management burden and the AT-command round-trip and is the recommended path when the SCADA PC is already networked. A hybrid is also valid: keep the local GSM path for primary alerts and the gateway as a redundant secondary channel.

9. Verification Procedure

1. Download the ladder project and the CX-Supervisor project to their respective targets.
2. Force-set W100.01 from CX-Programmer's watch window.
3. Confirm the CX-Supervisor log records the alarm going True and that Project_Start() executed without error.
4. Within five seconds the operator phone should receive a text beginning with the configured prefix.
5. Use AT+CMGL="ALL" on the modem to confirm the message was sent and acknowledged.
6. Reset the alarm bit from the HMI and confirm ALM_HIGH_PRESSURE returns to 0; check that no further SMS is dispatched until the next rising edge.

10. Troubleshooting Matrix

11. Field-Proven Caveats

• Set the TC35i to a fixed baud rate with AT+IPR=9600 and store it with AT&W; autobaud can hang on cold start.
• Always pre-pend the international + and country code; a national-format number silently fails in some operator networks.
• Long alarm bursts can saturate the SIM. Use a downstream queueing service if the application generates more than one alarm per minute.
• CX-Supervisor's VBScript host is single-threaded; do not block on a slow modem inside a frequently-called point event. Push the alarm into a queue and drain it from a periodic timer.
• Disable PIN on the SIM before deployment; if the PIN must remain, store it in the modem with AT+CPIN="1234" and not in the SCADA script.