A LabVIEW serial read that returns the same AT text that was transmitted has captured command echo, not necessarily the modem information being requested. Preserve the raw receive buffer, separate the echoed command from any following lines, and avoid interpreting the echo as the complete response.
Distinguish command echo from modem data
The confirmed symptom is that LabVIEW writes an AT command and reads back identical text. Treat echo as the leading hypothesis, but verify it from the complete raw byte stream because the available evidence does not identify the modem model, its command set, serial settings, message terminator, or response-completion rule.
| Observed receive data | Interpretation | Decision |
|---|---|---|
| Only the transmitted command | Possible echo, incomplete read, or missing response | Capture a longer complete transaction before parsing |
| Transmitted command followed by additional lines | Echo plus response is likely | Remove only the verified echo and parse the remaining lines |
| Data differs from the transmitted command | A response or communication fault may be present | Compare the raw text with the modem's documented command syntax |
Capture the complete LabVIEW transaction
Use the existing basic serial write and read VIs, but inspect the unmodified receive string before attempting conversion. The modem-specific documentation must define the required command terminator, supported information command, response format, and completion condition; none of those details are present in the evidence.
- Record the exact bytes written by LabVIEW, including any terminator.
- Read and retain the complete raw modem transaction rather than stopping after the first returned characters.
- Compare the beginning of the receive buffer byte-for-byte with the transmitted command.
- If the prefix matches, classify that portion as a verified echo and inspect all remaining data separately.
- Send a documented modem-information command and confirm that the remaining response matches its documented format.
Parse without inventing a response format
Do not convert the echoed AT text into modem information. First split the complete receive buffer into the transmitted echo, response content, and any status text according to the modem documentation. Keep the raw buffer available for diagnostics so a read-boundary problem cannot be mistaken for an empty response.
write_buffer := documented_command + documented_terminator
raw_receive := read_until_documented_completion
if raw_receive starts with write_buffer:
response := raw_receive after write_buffer
else:
response := raw_receive
parse response using the modem's documented response formatIf no data remains after removing the verified echo, do not assume the modem lacks information. Verify that the chosen command requests information, that the command terminator is correct, and that LabVIEW waits for the documented response-completion condition.
FAQ
Why does my RS-232 modem return the same AT command?
The returned text may be command echo. Capture the complete raw transaction and check whether additional response lines follow the echoed command.
Should I convert the echoed AT text into modem information?
No. Remove only a byte-for-byte verified echo, then parse the remaining response according to the modem's documented command set and response format.
What should I check if LabVIEW reads only the AT command?
Verify the exact transmitted bytes, the documented command terminator, the selected information command, and the response-completion condition. Retain the raw receive buffer while diagnosing the transaction.