How does the "Read Status Byte" work in the VISA test panel?

Question

I'm currently trying to replace an HP-85 with a modern computer running VB.NET, which will communicate with an HP 3421A. Unfortunately the 3421A was created before SCPI was created, so I believe the commands I am able to send are dictated by what the 3421A lists in the manual (http://exodus.poly.edu/~kurt/manuals/manuals/HP%20Agilent/HP%203421A%20Operation,%20Programming%20%20&%20Configuration.pdf).
The commands I'm trying to use to troubleshoot are "DCV", which reads a voltage, and "SR", which returns 24 different registers, with register 1 being the status byte. When I write "DCV" and then read, it returns a correct voltage. When I write "DCV" and click "Read Status Byte" in the VISA panel, it lets me know that data is available to be read and when I read it returns a correct voltage. When I write "DCV", then "SR" and then read, it returns 24 different bytes, with the first byte being the status register, but it doesn't recognize the "DCV" command from before and the status byte is now 0, which leads me to believe that it's overwriting the "DCV" command from before.
How can I mimic the functionality of the "Read Status Byte" button in the VISA panel? How does this work on instruments created before SCPI was implemented? Sending "*STB?\n" doesn't work. I am new to instrumentation so if I left out some necessary information please let me know. 

Answer 1

Thank you for the suggestion. I found a solution by adding NationalInstruments.NI4882.dll and using the SerialPoll function.