The Signature instruments can use both RS232 and RS422 protocol for communication. When the instrument is delivered to you it is by default set to communicate over RS232, so if you are going to communicate with it over RS422 you have to change the settings.
To change the communication protocol, connect to the instrument over ethernet and go to the terminal window in the Signature Deployment software. You can check which protocol is currently enabled by using the command GETINST,RS
To change the protocol from RS232 to RS422 follow these two steps:
1) Send the command SETINST,RS=422 (similarily, if you want to change back to 232 you write RS=232 instead).
2) Send the command SAVE,INST
The terminal window should now look like in the screenshot below, and you can disconnect your instrument and connect to it over RS422. You can find more information about the commands in the Integrators Guide – Signature.