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Unable to Read Data from Toyopuc PLC After Losing Connection

Last Update: 11/12/2018

The TCP window size can affect how the Toyopuc PLC (receiver) behaves. For example, some machines have a default TCP window size of 65535, and these machines may be unable to or have a hard time communicating with the Toyopuc device. One machine that was tested had a default window size of 16384 and could communicate with the Toyopuc device. These window sizes were determined through Wireshark, and no explicit registry settings could be found linking to them.

From this research, the TCP window size and the TCP buffer size are usually a multiple of the MSS. Toyopuc's MSS, as determined through Wireshark, is 1024 TCP buffer space: 31744/1024 = 31; TCP window size: 4096/1024 = 4. If the PC's TCP window size is greater than Toyopuc's TCP window size, there a potential for the buffers to run over. The communications problems encountered could be the Toyopuc's way of handling this potential.

Users can change the TCP window size by adding a registry entry as follows: "TcpWindowSize". Registry DWORD is not defined by default. Users must create it manually in one of two registry locations:

Users should set the TCP window size to 16384 and see if that resolves the problem. If it does not, try 8192 and then 4096 as a last resort.