SINAUT MD720-3 Modem Not Responding: Cable & Baud Fix

Problem Definition

A Siemens SINAUT MD720-3 GSM/GPRS terminal modem is operated in AT-command (terminal) mode and connected to a PC COM port over an RS-232 cable. The modem's front-panel status LEDs indicate nominal operation, yet the modem returns no response to any AT command issued from Windows HyperTerminal. Selecting 115200 bit/s on the PC side instead of the documented 19200 bit/s produces a continuous stream of unreadable ASCII characters that the user reports as "NaN". Disconnecting, reconnecting, or power cycling the modem does not restore communication.

This pattern - silence at the documented baud rate and garbled output at a higher one - is the textbook signature of a UART parameter mismatch combined with an incomplete or mis-wired RS-232 cable. Both conditions are present in this report.

Symptoms observed in the field:

• Power and status LEDs indicate nominal operation (steady green ON, slow-blinking STAT).
• HyperTerminal shows no echo of typed AT commands.
• No OK, ERROR, or NO CARRIER reply to AT, ATZ, or AT&V.
• Setting the PC COM port to 115200 bit/s produces ASCII garbage visually resembling "NaN".
• No RING or unsolicited result codes appear after the modem is power-cycled.

Root Cause Analysis

The MD720-3 powers up with its serial service port fixed at 19200 bit/s, 8 data bits, no parity, 1 stop bit (19200 8N1) with hardware flow control on the RTS/CTS pair, as documented in the Siemens MODEM MD720 operating manual (PDF). Two independent failures must be cleared to bring the link up:

1. Serial parameter mismatch. The PC terminal is set to 115200 bit/s while the modem transmits at 19200 bit/s. The receiver oversamples each 19200 bit/s bit six times, producing groups of six identical ASCII characters. Where the underlying frame happens to be N (0x4E) a (0x61) N (0x4E), the terminal renders "NaN". The user is therefore not seeing a corruption bug - they are seeing correctly transmitted 19200 bit/s frames being mis-decoded at 115200 bit/s.
2. Incomplete RS-232 cable. The cable in the report is missing pin 1 (DCD) and pin 9 (RI). The MD720-3 firmware gates its service channel on the DTR/DSR/DCD handshake. A cable that does not carry those signals leaves the modem in a quiescent state where it neither echoes typed commands nor transmits unsolicited result codes.

UART Bit Timing - Why "NaN" Appears at 115200 bit/s

The bit period of the two rates is:

• t_bit(19200) = 1 / 19200 = 52.083 microseconds
• t_bit(115200) = 1 / 115200 = 8.681 microseconds

The ratio is exactly 6.000. A receiver sampling a 19200 bit/s line at 115200 bit/s reads the same logic level six times per transmitted bit, so each transmitted byte is decoded as a sequence of six replicated ASCII characters. The mapping that produces "NaN" from the modem's idle / banner frames is:

The three-byte sequence 0x4E 0x61 0x4E thus renders as the visible string "NaN" once each source byte is repeated six times by the over-sampling receiver. The same mechanism explains other reported garbled strings such as "aaa", "NNN", or any repeated block of identical characters.

Required Serial Parameters

The "Full duplex" setting named in the MODEM MD720 manual corresponds to enabling the hardware handshake in the terminal program. HyperTerminal does not expose an explicit "Duplex" field; selecting "Hardware" flow control enables the same RTS/CTS handshake the modem expects.

MD720-3 DB9 Connector Pinout (Male, Front View)

Inline Pinout Diagram (SVG)

Null-Modem (Crossover) Cable Wiring

The MD720-3 is a DTE device, electrically identical to the PC COM port in this respect. To connect two DTEs back-to-back, the cable must perform the standard null-modem crossover on the data and handshake pairs, plus DTR/DSR-DCD loopbacks that satisfy the modem's "terminal ready" detection. The pin map below is the wiring documented for service access to Siemens SINAUT modems:

Decision Flow: From "No Response" to "OK"

HyperTerminal Configuration Steps

1. Open HyperTerminal (Start -> Programs -> Accessories -> Communications -> HyperTerminal). Name the session (e.g. "MD720_Service").
2. Set "Connect using" to the COM port the RS-232 cable is plugged into. Verify the assignment in Windows Device Manager -> Ports (COM & LPT).
3. Set "Bits per second" to 19200.
4. Set "Data bits" to 8, "Parity" to None, "Stop bits" to 1.
5. Set "Flow control" to Hardware.
6. Open File -> Properties -> Settings -> ASCII Setup. Tick "Echo typed characters locally" and "Append line feeds to incoming line ends".
7. Click OK and connect.
8. Type AT followed by Enter. The modem must respond with OK within 1 second.

PuTTY Configuration Steps (Modern Replacement for HyperTerminal)

1. Open PuTTY, select "Connection type: Serial".
2. Set "Serial line" to the COM port (e.g. COM3) and "Speed" to 19200.
3. In Category -> Connection -> Serial, configure: Data bits = 8, Stop bits = 1, Parity = None, Flow control = RTS/CTS.
4. In Category -> Terminal, tick "Implicit CR in every LF" and "Local echo: Force on" for manual typing.
5. Click Open and issue AT.

AT Command Verification Sequence

Once the link is up, run the following commands to confirm modem health. Each line is terminated with Enter (\r). Expected responses are shown after the arrow.

AT          <-- OK
ATI         <-- Siemens MD720-3
AT+CGMI     <-- Siemens
AT+CGMM     <-- MD720-3
AT+CGMR     <-- firmware version, e.g. 04.10
AT+CSQ      <-- +CSQ: 12,99   (RSSI 0-31; 99 = unknown)
AT+CREG?    <-- +CREG: 0,1   (registered, home network)
AT+CPIN?    <-- +CPIN: READY  (SIM accepted)
AT&V        <-- active profile dump

If AT does not return OK, repeat the cable and baud rate checks. If AT returns ERROR, the SIM is missing or locked, the antenna is disconnected, or the modem is in firmware-update mode (STAT LED pattern will be a fast red blink).

LED Status Indicators

Step-by-Step Recovery Procedure

1. Power down the MD720-3 and the PC.
2. Inspect the RS-232 cable with a continuity tester. Confirm every one of the nine DB9 pins has continuity end-to-end and that the crossover pairs (2-3, 4-6+1, 7-8) match the table above.
3. Replace the cable if any pin is open. Recommended: a generic DB9 female-to-female null-modem cable rated for full handshake (e.g. L-com CSNULL9F-9F or equivalent).
4. Reconnect the cable. Power up the modem first; wait for the STAT LED to settle to a slow green blink (typically 10-20 s after power-up).
5. Launch HyperTerminal on the PC. Apply the 19200 8N1 settings with hardware flow control per the table above.
6. Issue AT. Confirm OK within 1 second.
7. Issue AT&F to reset to factory defaults, then AT&W to persist the change.
8. If the modem still does not respond, swap TX/RX pairs in case a straight-through cable was substituted for a null-modem cable.

Verification Checklist

• AT returns OK within 1 second.
• ATI returns "Siemens MD720-3".
• AT+CSQ reports a numeric RSSI (0-31), not 99.
• AT+CREG? returns +CREG: 0,1 or +CREG: 0,5 (registered, home or roaming).
• HyperTerminal echoes typed commands locally; incoming characters appear without manual carriage return.
• Hardware flow control is observed by toggling RTS at the PC; the modem's CTS line must follow.

Troubleshooting Matrix

Compatibility with Predecessor SINAUT MD720

The MD720-3 is functionally largely compatible with the predecessor SINAUT MD720 module, as stated in the MD720 operating manual. The serial service port defaults (19200 8N1, hardware flow control) and DB9 pinout are preserved. Existing MD720 service cables can be reused on the MD720-3 without rewiring, provided every pin (including pin 1 and pin 9) is wired straight through. Any deviation from the 9-pin wiring must be corrected per the null-modem map above before the service channel will respond.

Common AT Command Reference

SINAUT ST7 Integration Notes

When the MD720-3 is mounted on a SINAUT ST7 station, the service port is multiplexed with the central station (SINAUT ST7cc) link. Closing HyperTerminal without first sending +++ and waiting for OK may leave the modem in online data mode and break the SCADA connection. The recommended exit sequence is:

+++         <-- wait 1 s --> OK
ATH         <-- OK (on-hook)

Configuration changes that affect the GPRS APN, login, or central station IP must be written to the SINAUT project (TIM 4V module) and downloaded via STEP 7 rather than via the AT service port.

Safety and Field Notes

FAQ