TCP Read blocks remote Port

Question

HI,
I want to read a NMEA string(update rate 1 Hz) from a server. The original NMEA(serial, 4800 Baud) is converted by a MOXA to TCP.
Here is my minimal program, I have normally a "wait until 1000ms" timer inside.
The problem is that the MOXA closes its port every other minute. I still can read but every one else is blocked.
Other terminal programs work. The network guy tells me that he sees a lot of traffic coming from my adress and he thinks that at some point the MOXA is confused and closes its ports.
Is there a possibility that TCP read sends a status or failure string?
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Answer 1

azavarsky wrote:
HI,
I want to read a NMEA string(update rate 1 Hz) from a server. The original NMEA(serial, 4800 Baud) is converted by a MOXA to TCP.
Here is my minimal program, I have normally a "wait until 1000ms" timer inside.
The problem is that the MOXA closes its port every other minute. I still can read but every one else is blocked.
Other terminal programs work. The network guy tells me that he sees a lot of traffic coming from my adress and he thinks that at some point the MOXA is confused and closes its ports.
Is there a possibility that TCP read sends a status or failure string?
How fast does this device send its strings? Does it simply spew data without any query command having to be send? If so your Moxa may have a problem too, as it may not know what to do with the data if there is no client connected to read it. Also your Moxa converter may have trouble to serve more than one TCP client on the same port anyhow. What should it do if several clients try to access the same port? Send the same message to each? Or send the incoming message randomly to one of them?
Do you get errors from your TCP Read once Moxa "closes" the port as you say? If so you should terminate your loop on any error except possibly a timeout error and reopen the port in a second outer loop until your program wants to exit. Your loop never stopping on errors will prevent the TCP Close to execute which will keep the socket open and prevent the Moxa from seeing the proper handshaking that the connection has been properly closed by the client, which will likely prevent the Moxa device from being able to accept new connections. Network communication can be tricky and one of the first things you have to learn is proper error handling. Just about any error anywhere in the network connection, except usually a timeout error on read, should be handled by closing the connection and trying to reconnect.
Rolf Kalbermatter
CIT Engineering Netherlands
a division of Test & Measurement Solutions