RS-232 port

I have noticed some inconsistencies with RS-232 connections on pentium and 486 based machines. I’ve had this issue with two different devices.. one was a link for a TI calculator and the other was a Haas vertical machiner that sends data in the form of text files back to the computer. The issue on both of them was that I had no problem getting the devices to communicate with a 486 or slower computer, but when connecting them to a pentium machine, no communication occurs. Can anyone explain the incsonsistency to me?