Circuit Switched Data (CSD) Setting Application Note
This application note explains how to configure a Siretta modem to communicate with an analogue modem using a Circuit Switched Data (CSD) connection. This allows serial data to be transmitted between a remote system connected to the Siretta modem and another system connected through an analogue modem.
Applicable Products
| Part number | Description |
| ZETA-GEP-LTE4 (EU) | Low Power LTE Cat 4 European Modem with GPIO and GNSS |
| ZETA-G-GPRS | Entry Level GPRS Modem with GNSS (GL) |
| ZETA-N2-GPRS | Entry Level GPRS Modem (GL) |
| ZETA-NEP-LTE4 (EU) | Low Power LTE Cat 4 European Modem with GPIO |
| ZETA-NLP-LTE1 (EU) | Ultra Low Power European LTE Cat 1 Modem |
| ZETA-N-LTE (EU) | High Performance LTE Cat 4 Modem with GPIO |
| ZETA-NSP-LTE1 (EU) | Low Power European LTE Cat 1 Modem |
Objective
Circuit Switched Data is a dial-up style data connection used to transfer information between two systems. Instead of using packet-switched internet connectivity, the modem establishes a direct data call.
In this example, an analogue modem dials the CSD data number assigned to the SIM card fitted inside the Siretta modem. Once the call is answered, data entered at either terminal is passed to the other side.
Requirements
| Item | Description |
| CSD enabled SIM card | The SIM card must support Circuit Switched Data and must have a separate dialable data number. |
| Terminal emulator | Tera Term, HyperTerminal or PuTTY can be used to configure the modems using AT commands. |
| Siretta modem | The modem must support CSD operation. |
| Analogue modem | A standard analogue modem connected to a working telephone line. |
| RS232 cable | RS232 to USB or RS232 to RS232 cable, depending on the PC connection. |
| Two PCs | One PC connected to the Siretta modem and one PC connected to the analogue modem. |
Important: The SIM card must be provisioned for CSD by the network provider. A standard voice or data SIM may not support this service unless CSD has specifically been enabled.
System Setup
| Siretta modem side | Connection | Analogue modem side |
| Siretta modem connected to PC or remote equipment using RS232. | CSD data call over the mobile network using the SIM data number. | Analogue modem connected to PC and working telephone line. |
Step 1 – Connect the Siretta Modem
1. Insert the CSD enabled SIM card into the Siretta modem.
2. Connect the antenna to the Siretta modem.
3. Connect the Siretta modem to the PC using the RS232 to USB cable.
4. Apply power to the Siretta modem.
5. Open Tera Term, HyperTerminal, PuTTY or another terminal emulator.
Step 2 – Configure the COM Port
Configure the COM port connected to the Siretta modem using the following settings.
| Setting | Value |
| Bits per second | 115200 |
| Data bits | 8 |
| Parity | None |
| Stop bits | 1 |
| Flow control | Hardware |
Step 3 – Configure the Siretta Modem
Enter the following AT commands into the terminal window connected to the Siretta modem.
| Command | Description |
| AT#DIALMODE=2 | Configures the modem for data dial mode. |
| AT+FCLASS=0 | Sets the modem to data mode. |
| AT+CRC=1 | Enables extended incoming call reporting. |
| AT&K0 | Configures flow control behaviour. |
| ATS0=1 | Sets the modem to answer automatically after one ring. |
Siretta Modem Terminal Window
| COM3 – Tera Term VT |
File Edit Setup Control Window Help |
| AT#DIALMODE=2 OK AT+FCLASS=0 OK AT+CRC=1 OK AT&K0 OK ATS0=1 OK |
The Siretta modem is now configured to answer an incoming CSD data call.
Step 4 – Connect and Configure the Analogue Modem
1. Connect the analogue modem to the second PC.
2. Connect the analogue modem to a working telephone line.
3. Open a terminal emulator on the PC connected to the analogue modem.
4. Configure the analogue modem COM port using the same serial settings.
| Setting | Value |
| Bits per second | 115200 |
| Data bits | 8 |
| Parity | None |
| Stop bits | 1 |
| Flow control | Hardware |
Step 5 – Dial the Siretta Modem
From the analogue modem terminal window, dial the CSD data number assigned to the SIM card inside the Siretta modem.
Command format: ATDxxxxxxxxxxx
Example: ATD07736393040
| Analogue Modem – Tera Term VT |
File Edit Setup Control Window Help |
| ATD07736393040 |
Replace the example number with the actual CSD data number supplied by the network provider.
Step 6 – Confirm the Incoming Data Call
While the analogue modem is dialling, the terminal window connected to the Siretta modem should show an incoming data call indication.
| Siretta Modem – Tera Term VT |
File Edit Setup Control Window Help |
| +CRING: REL ASYNC |
+CRING: REL ASYNC indicates that the Siretta modem has detected an incoming asynchronous data call.
Step 7 – Confirm the Connection
After the configured number of rings, the Siretta modem answers the call automatically. Both terminal windows should show that a connection has been established.
Analogue Modem Window
| Analogue Modem – Tera Term VT |
File Edit Setup Control Window Help |
| CONNECT 9600 |
Siretta Modem Window
| Siretta Modem – Tera Term VT |
File Edit Setup Control Window Help |
| CONNECT |
Step 8 – Change the Number of Rings Before Answering
The number of rings before the Siretta modem answers is controlled by the ATS0 command.
| Command | Result |
| ATS0=0 | Automatic answering disabled. |
| ATS0=1 | Answer after one ring. |
| ATS0=2 | Answer after two rings. |
| ATS0=3 | Answer after three rings. |
Step 9 – Test Data Transfer
Once the CSD connection is established, type text into either terminal window. The same text should appear in the opposite terminal window.
Example Data Sent from the Analogue Modem
| Analogue Modem – Tera Term VT |
File Edit Setup Control Window Help |
| Hello from analogue modem |
Data Received by the Siretta Modem
| Siretta Modem – Tera Term VT |
File Edit Setup Control Window Help |
| Hello from analogue modem |
If the text entered into one terminal appears in the other terminal, the CSD data connection is working correctly.
Step 10 – Final System Integration
After the CSD connection has been tested successfully, the PC connected to the Siretta modem can be replaced with the final remote system or equipment.
| During testing | Final installation |
| PC connected to the Siretta modem using a terminal emulator. | Remote equipment connected to the Siretta modem using RS232. |
| Manual test data entered through the terminal window. | Application data sent automatically by the connected equipment. |
| Analogue modem dials from a test PC. | Remote monitoring system dials the deployed Siretta modem. |
Troubleshooting
| Problem | Possible cause | Action |
| No response to AT commands | Incorrect COM port or serial cable issue. | Check the COM port number, cable connection and terminal emulator settings. |
| No incoming call shown | Incorrect number dialled or SIM not enabled for CSD. | Confirm the correct CSD data number with the network provider. |
| Call rings but does not answer | Automatic answering is not configured. | Use ATS0=1 or ATS0=2 on the Siretta modem. |
| Unreadable characters are displayed | Serial settings do not match. | Check baud rate, data bits, parity, stop bits and flow control on both sides. |
| No data is transferred | RS232 wiring or flow control issue. | Check TX, RX, GND and hardware flow control connections. |
Process Summary
| Step | Action |
| 1 | Connect the Siretta modem to PC 1. |
| 2 | Connect the analogue modem to PC 2 and to a working telephone line. |
| 3 | Configure both terminal emulator COM ports. |
| 4 | Enter the Siretta modem AT command configuration. |
| 5 | Dial the Siretta modem CSD data number from the analogue modem. |
| 6 | Confirm +CRING: REL ASYNC appears on the Siretta modem terminal. |
| 7 | Confirm both modems show CONNECT. |
| 8 | Send test data in both directions. |
| 9 | Replace the test PC with the final remote equipment. |
Additional Reading
| Description | Author |
| AT command manual | Telit |
| ZETA-xxP Quick Start Guide | Siretta |
For further assistance, please contact the Siretta support team.
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