SCPI via RS232 - no data returned?

Question

I'm trying to use RS232 to work with an SCPI power supply - HP E3644A.

I can do everything I want to, except get data back.

I can use my port object to get data from other devices or other computers just fine.  Same cable, etc.

I've tried a full-handshaking cable and set my port object to xon/xoff and that works...to set the voltage, current, output state, clear errors, etc.

I've also tried a loopback-handshake style cable.  (DB9 7,8 tied...1,6,4 tied.)  That also works to set the voltage, etc.

When I send queries like "volt?", I expect the power supply to send some data back...but I don't get any.  If I send another command, I will get an error indicating that the PS has data in the output buffer that it expected to be sent out before getting another command.

I've read the manual and perused the internet, and I've found a "read?" command referenced.  But that doesn't seem to work for me.

What is the simple command/trick/setting I've missed?

Thanks for the help!