Hi,
I wasn't home, so here a more complete answer.
I used Wireshark to sniff the network traffic between my PC and the SSA3000X, when using EasySpectrum, which requires NI VISA.
The SCPI communication starts at port 111 and/or 897 on the PC side.
However, any reply from the SSA3000X goes to port 4xxx. This port is different on every new connection and is told to the PC through what I imagine is a lower level protocol.
The problem I have with this, is that I haven't found a way in VB .net to read that incoming port number. So I don't know which port I should listen to.
Using Wireshark, I can see the raw TCP/IP packets and yes, there is the incoming port number. But how do I access it? No idea!
NI VISA supports such protocol and I managed to test communication under VB .net using NI VISA (hint: you need to install VISA with all options - somehow I missed the newer .Net runtimes on the first install).
But it kind of sucks having to install 600MB of software to then run a 200kB VB .net executable!
So what I am asking Siglent: please make SCPI work with a single fixed port number! The standard port number would be great, which is 5025.
Make it work in both directions and to test it, one could then just use "telnet 192.168.1.2 5025" to access the SSA3000X (which has IP 192.168.1.2 in this example).
As it is right now, you cannot telnet, because while you can SEND SCPI commands on port 111 or 897, the telnet session will NOT receive any incoming data on those ports! You can still verify the truth of what I am saying, because doing a telnet to the SSA3000X on port 111 and typing "*IDN?" will cause the device to show "REMOTE" on the screen and lock all keys except the ESC key (used to close the remote connection).
You won't see and reply, though, as the SSA3000X will reply to a different port.
Using one single port is perfectly doable and Rigol does that (at least with the DS1054Z I own: I can telnet, type commands like "*IDN?" and do get a propper reply). However, Rigol chose to use port 5555.
I have another professional device with SCPI support, that behaves like the Siglent. Again, that is not a bug and is according to TCP/IP specification. But it does prevent people like me to use it without NI VISA.
Hope this clears all up!
Regards,
Vitor