Is it possible to install a ctrlogix rack with a ethernet comm card and a DH+ RIO card no processor to speed up data block read and writes to iFix I/O server to PLC5. I was told this would work from a controls salesman. I cannot see that it would make any difference as the DH+ between PLC5 and installed ctrlogix rack would still be same bus as before just shorter.

If you had multiple PLC-5's then adding the CLX bridge would allow you to split your DH+ network, with half on each channel of the DHRIO card, by reducing the number of nodes on each network, communication speeds should increase.

But, if you only have one processor, I doubt you would see any improvement.