I have a Profibus connection as shown in the picture attached,

there is one CPU two remote modules and one HMI Connected on the same bus 

I checked resistance on connectors when ON and not connected to anything  is 220 Ohm and with both resistor ON CPU end and HMI end on the Bus it reads 110 Ohm - not connected on HMI or CPU only the profibus cable with middle two connectors OFF and connected on remote Modules 

But when the CPU end is connected on CPU and resistor is ON the HMI end with connector removed reads 198 ohm, just before it was connected it read 220 Ohm so the resistance drops when connected on CPU

Communication is working if i turn on the resistor on CPU and leave the HMI resistor OFF, If i turn on the HMI connector resistor the communication on HMI does not work .

BUT both should be On as the bus starts and ends on CPU and HMI respectively.

I am attaching pictures of the first and last connectors

first on CPU last on HMI

qwemx

I am attaching pictures of the first and last connectors

first on CPU last on HMI

 The first on CPU and the last on HMI are using A1 and B1 

The connection point's are as they should be. Have you tested another DP connector on the HMI side?

What is the PLC diagnostics buffer telling you whhen the HMI connection is not working?

Why don't you just use Profinet connection between the HMI and PLC?