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FAQ

Q.

What are the minimum I/O control signals required for the modem to work?

TD, RD and Gnd

Q.

Does the modem "Reset" on power up?

Yes, it is a soft reset.

Q.

Can Multi-Tech recommend manufacturers and part numbers of chip sockets, fuses, capacitors, and resistors to finish design work?

The developer’s guide has some recommendations.

Q.

Does Multi-Tech provide any SocketModems that go into a low power "sleep" mode?

Yes all models.

Q.

The Socket Modem is not responding to AT commands. What do I do?

a. First, check the supply voltage to the modem. It should be either 3.3V DC or 5V DC.
b. Next check for the presence of DTR (input to the modem). If it doesn't have this, it will not function.
c. Identify and re-evaluate Transmit/Receive and CTS/RTS to make sure they are connected properly. Often times, inputs and outputs can be swapped based on interpretation. The modem is considered a DCE device.
d. Lastly, if you've gone through all steps above, disable flow control.

Q.

What is the difference between line speed, serial speed and auto baud?

a. **Line speed** — is the modem negotiation rate (ie. 2400 baud)
b. **Serial speed** — is the speed between the DTE and the modem. This can be locked in or dynamically adjusted.
c. **Auto baud** — is the ability of the modem to connect at 300 baud to the maximum line speed available on the modem (ie. 56000 baud). This can automatically adjust during modem negotiation.

Q.

What is the polarity of Tip & Ring?

The SocketModem is not sensitive to Tip and Ring polarity.

Q.

How do I put the modem module into voice mode?

Use the AT+FCLASS commands. For details refer to the developer’s guide.

Q.

How do I design my LED driver circuit?

This can be handled in one of two ways:
a. All of the serial models (with the exception of the MT5600SMI and (MT3400SMI) have an LED driver designed into the module. You simply add the pull-up resister and LEDs to finish the design.
b. For all other models, you can add an external driver chip to connect to the serial I/O which in turn drives the LEDs.

Q.

I have a V.90 SocketModem solution and it negotiates tones but does not connect. What do I do?

a. If Originating a call: Disable the V.90 protocol on the SocketModem with the appropriate "AT" Command (Answering modem pool is NOT a digitally terminated V.90 modem pool). Next, we suggest that you disable "calling tones" with the appropriate "AT" Command.
b. If Answering a Call: Disable "V8 tones" with appropriate AT command.

Q.

Can I use 5v I/O circuit logic levels with 3.3v models?

Yes, however the modem inputs (TXD, DTR, RTS and Reset) need a 1K resistor in series when interfacing to 5V logic. Also, check the specs to make sure the modem outputs are high enough to drive the inputs of the logic family on the embedded system. Our minimum high output spec must exceed the minimum high input spec of the 5V logic circuit.

Q.

Do I need to design in the recommended capacitors and ferrite in the Tip/Ring circuit to prevent EMI leakage out the phone cable?

This is good design practice. You may have trouble gaining system approval for EMC without these components added to your design.

Q.

What is the minimum required pulse length on the "Reset" pin?

50 — 100ms

Q.

What is the minimum required pulse length for "DTR drop" to disconnect the modem?

100ms