6221/2182A error-241“hardware missing”

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.
 

Maybe obvious, but this error means the 6221 does not receive the expected response from 2182A on the rs-232.

Double check the rs-232 settings are same in both instruments.
Then set LAN or GPIB the active interface on 6221.
Press delta button on 6221 and accept defaults.
Error?

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.