Silicon Laboratories SI2494/39 manual  At Ok?,  Inappropriate Commands,  Command Timing

AN93

AT OK?

The modem responds with an OK to the command AT<CR>. This indicates that the host processor/software is communicating with the modem controller, and problems are in one of the following areas:

Inappropriate Commands

Verify that all AT commands used are supported by the ISOmodem and comply with the proper format. Be sure each command and argument is correct. Do not mix upper- and lower-case alpha characters in an AT command. An AT command string must contain 48 or fewer characters followed by a carriage return. Command strings greater than 48 characters are ignored.

Command Timing

The execution time for an AT command is approximately 200 ms. Execution is complete when OK is received. Subsequent AT commands should wait for the OK message, which appears within 200 ms after the carriage return. The reset recovery time (the time between the rising edge of a hardware reset or the carriage return of an ATZ command and the time the next AT command can be executed) is approximately 300 ms. When a data connection is being established, do not try to escape to the command mode until after the protocol message.

Register Configurations

The ATS$ command lists the contents of all S registers, and the AT:R command lists the contents of all U registers.

Si3018/10 and/or Associated Components

If the modem goes off-hook and draws loop current as a result of giving the ATH1 command, go to "10.1.6. Si3018/10 Troubleshooting".

If the modem does not go off-hook and draw loop current as a result of giving the ATH1 command and receiving an OK message, begin troubleshooting with the isolation capacitor at the ISOmodem. First, check all solder joints on the isolation capacitors, Si3018/10, and associated external components. If no problems are found, proceed to "10.1.5. Isolation Capacitor Troubleshooting" to verify whether the problem is on the ISOmodem or the Si3018/10 side of the isolation capacitor. If the problem is found to be on the ISOmodem side, check C50, C51, C53, the corresponding PCB traces, and the ISOmodem pins. Correct any problem found. If no problems are found with the external components, replace the ISOmodem. If the problem is found to be on the Si3018/10 side of the isolation capacitor, go to "Si3018/10 Troubleshooting".

If the modem does not respond with an OK to the command AT<CR>, this indicates that the host processor/ software is not communicating with the modem controller, and the problem can be isolated as follows:

ISOmodem Clock is Oscillating

First, be sure the ISOmodem is properly reset and RESET, pin 12, is at 3.3 V. Next, check the DTE connection with the host system. If this does not isolate the problem, go to "10.1.4. Host Interface Troubleshooting".

ISOmodem Clock is Not Oscillating

Check the voltage on the ISOmodem, pins 5 and 21, to be sure the chip is powered. Also, check that pins 6 and 20 are grounded. Next, check the solder joints and connections (PCB traces) on C40, C41, Y1, and the ISOmodem Pin 1 and Pin 2. Measure C26 and C27 (or replace them with known good parts) to ensure that they are the correct value. If these steps do not isolate the problem, replace the ISOmodem.

10.1.4. Host Interface Troubleshooting

The methods described in this section are useful as a starting point for debugging a prototype system or as a continuation of the troubleshooting process described previously. The procedures presented in this section require a known good ISOModem evaluation board and data sheet. This section describes how to substitute the evaluation board for the entire modem circuitry in the prototype system. Substituting a known operational modem can help isolate problems quickly. The first step is to substitute the evaluation board for the complete modem solution in the prototype system. This immediately demonstrates whether any modem functionality problems are in the prototype modem circuitry or in the host processor, interface, or software.

Verify ISOModem Evaluation Board Functionality

Connect the evaluation board to a PC and a phone line or telephone line simulator. Using a program, such as HyperTerminal, make a data connection between the evaluation board and a remote modem. Remove power and the RS232 cable from the evaluation board and proceed to the next step.

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