Thank you for your email.
It is possible to measure the RS485 signals with the requency that you stated. The units that are capable of this is the ADC-212/50, ADC-200/50 and PS3205 and PS3206. The maximum sampling time that you will be able to collect data is limited by the units buffer size. The units I have stated have differing buffer sizes and I would advise that you look at there specifciations.
The units above are all able to use external triggering.
Unfortunately, Picoscope does not have mathematical functionality, but it would be possible for you to take the details form Picoscope and save the readings as a text file, which can then be placed into excel or some other application to manipulate the results.
A second option would be to use Picolog in 'Fast Block' mode. In this application and mode you are able to sample at the units fastest time base and collect the maximum number of samples. It has the facility to do calculated paramesters so therefore, you could apply A - B.
The third option is to write your own software which will perform your requirements, the driver and prototype descriptions are available.
If you have any furhter question please do not hesitate to contact me again.