Hello Bardes
In our plant we make use of the 505-6851A and for the older 500 I/O bases the 500-2114A RBC controllers. They were connected to 555-1102/1103 cpu,s but since 2008 we replaced the 555 cpu,s with the newer CTI 2500 series CPU,s.
You are correct that the A and B versions are the newer ones and the difference at the front is that a 505-6851A has a thumbwheel for setting the Base number and with the 505-6851B this is a rotary switch.
But you want to know what the difference is in firmware and that I can,t tell you at the moment.
Next week on the job I can look for you if I can find some details in the older manuals.
Why do you need the information , are you planning to change the existing configuration by adding some extra modules in the remote base or do you want to replace the 555 cpu,s?
Kind regards
Henny