RS232 choking


I have a remote DTE device that is directly connected to the RS232 cable from a Fastrac Extend. The modem is programmed with an application that issues the DTE with a 20 byte command string every 15 minutes. The DTE replies with about 120 bytes of data, which is sent ( POST) to a web server. The 120bytes is sent at 9600 to the modem. Full handshaking is connected. AT+IFC? returns 2,2
I have several of these and they have been running fine for about a year, each sending 120bytes of data every 15 minutes.

I now want to increase the data pay load by 5000 bytes.
If I modify the cmd string to the DTE ( by appending a 5 byte command to the existing command string) the DTE now sends 900 more numbers. However, the modem is unsuccessful in sending it to the server.
I get an err msg from my app “upload timed out”. The connection is successful, but there is no ack from the server.

But the additional data is identical to the other 120bytes. If I replace the modem with a PC running hyperterminal, send the command string from a txt file, I can save the 5000 byte data response to a file and examine it with a Hex editor; it is perfectly formatted, no funny control characters and HT receives it perfectly. The only thing I noticed is that the command string to the DTE is about 20 characters long, and the DTE starts sending data to the modem, before the last command bytes have been sent. i.e full duplex.
The data is just a string of about 1000 numbers, which look like E 45.1 E43.1 E65.3 E87.1 E33.2 etc
I’ve spent 3 days on this so far and am getting nowhere fast. Any clues/suggestions gratefully received.


I think I have a) found the problem and b) solved it.
The issue turned out to be the old Supreme modem I was using. It could not handle an incoming RS232 byte stream larger than about 200bytes without choking. An intercharacter delay did sort of fix it. (Hand shaking was turned on.) I moved on to a new “Extend” modem, and that appears to have no issues with large txt files coming in on the serial port. Testing is still progressing.


It sounds more likely that there is a flaw in your application.

Are the handshaking lines correctly connected, and does the sending device correctly observe them?


Make sure boost mode is on. We found that in low-power mode RS-232 is too slow.