Modem Troubleshooting

27/03/2025

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-NEP-LTEM (GL) Low Power Global LTE Cat M Modem with GPIO

ZETA-NLP-LTE1 (EU) Ultra Low Power European LTE Cat 1 Modem

ZETA-NLP-LTEM (GL) Ultra Low Power Global LTE Cat M 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 Once a Siretta ZETA modem is powered on, a combination of its 3 LEDs light up, showing different stages of modem connection to a network (default state).

Also, assuming the network connection is achieved, the modem can send a receive AT commands.

If any of the above fails to materialise, a troubleshooting procedure will help to investigate and solve basic issues.

Solution This application note covers the following:

No LED indication

Modem keeps rebooting

Failure to connect to a modem serial port

No response from the modem when using AT commands

No Carrier when dialling Voice/ data calls

Demonstration of Solution LEDs Not Lit

Check the modem has been connected to the correct power supply

Check the power connector is inserted correctly

Check the power has been switched on at the mains

Check the LED functionality has been set with either AT#SLED or AT#GPIO AT commands

Modem Reboot If your Siretta modem keeps rebooting, please check the following procedures:

Check the SIM card has been inserted correctly

Check the power supply is correctly specified or you are using an approved Siretta PSU.

No Serial Connection If your Siretta modem has no serial connection, please check the following procedures:

Check the serial cable has been inserted correctly

Check the serial cable has the correct pin alignment (see below)

Check the modem emulator program has the proper settings (Factory settings = 115200, 8, n, 1)

Check there isn't another program interfering with the communication program

Check there isn't a conflict with communication port access

RS232 Serial Port Interface This connector provides serial RS232 communication between ZETA-xxP modem and the connected equipment. The modem can be configured via the RS232 connection using AT commands as specified in the AT command manual.

Table 15. Pins Usage

Pin Name Usage Status Direction

1 DCD Output from UART that indicates the carries is present Connected OUT

2 RXD Output transmit line of UART Connected OUT

3 TXD Input receive line of UART Connected IN

4 DTR Input to UART and controls DTE ready condition

Connected IN

5 GND Ground

Connected IN

6 DSR Output from UART that indicates the module is ready

Connected OUT

7 RTS Request to Send - Input line of UART that controls hardware flow control

Connected IN

8 CTS Clear to Send - Output line of UART that controls hardware flow control

Connected OUT

9 RI Ring Indicator- Output line of UART that indicates the incoming call condition

Connected OUT

Supported baudrates

>> 2400 >> 38400

>> 4800 >>57600

>> 9600 >>115200

>> 19200 >230400

No Carrier Message If your Siretta modem returns a No Carrier' message upon an attempted call (voice or data), please consider the following possible causes:

If the modem returns Then ask? Action

No Carrier' Is the received signal strong enough? Use AT+CSQ' to check RSSI.

Check signal strength indication. *

Is the antenna connected correctly? Ensure antenna is firmly screwed into place.

No Carrier'

(When trying to issue voice communication) Is the semicolon ( ; ) entered immediately after the phone number in the AT command? Ensure that the semicolon ( ; ) is entered after the phone number in the AT command. (e.g. ATD123456:)

No Carrier'

(When trying to issue data communication) Is the SIM card configured for data/fax call? Configure the SIM card for data/fax calls. (Ask your network provider if necessary)

Is the selected bearer type supported by the called partly? Ensure the selected bearer type is supported by the call party.

Is the selected bearer type support by the network? Ensure the selected bearer type is supported by the network.

If no success, try bearer select type by AT COMMAND: AT+CBST=0,0,3'

Value of RSSI Interpretation

0 to 12 Insufficient or weak

13 to 19 Average

20-31 Good

99 No Signal

Additional Reading Description Author

ZETA Hardware user's manual Siretta

AT command manual Telit

Quick Start Guide Siretta

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