Song junchi

Thanks for your answer, I have checked this for several times. Now I am trying to renew the RS232 cable，because I didn't use the original assembly line before. 
 

I'd like to do delta-mode measurements with the Keithley 6221 & 2182A devices.
I have the 6221 and 2182A connected through RS232 null modem and trigger link cable, and the 6221 is connected to PC through GPIB bus. When I  configure the mode（ 19.2k rate, Xon_Xoff flow control, CR terminal）,and take the button "DELTA" in the GPIB interface, 6221 reports error-241"hardware missing". I can't find the problem.
 

I also have a problem about this. After I connect 6221/2182A with a trigger link cable and a null modem RS-232, I choose the GPIB interface. But 6221 report error-241"hardware missing". Does this indicate that the device is damaged, or are there other issues that haven't been taken into account?