Correct, I'm only connecting the MSC-400 to my PC and Hyperterminal, there by taking the device out of the equation. I'm not going any further until I get things to work in Hyperterminal ;)  All that is needed for the reward is to get things working in Hyperterminal.  This way we both can effectively use the same test device.

I'll worry about tailoring things to the specific device (which is a complex Home Automation system interfacing with HVAC, an alarm system, lighting and many media devices) after it is proven that Hyperterminal sees the correct output from the MSC-400. Given this RS232 is a must for communication between the MSC-400 and the home auatomation system.

Single IR commands sent to the MSC-400 don't exhibit the issue, only smart macro triggers to send text out an RS232 port. Press/hold wouldn't work as I need single press functionality to work 100% of the time; an example would be toggling speeds for fast forward and reverse. If a user hits the reverse button 4 times to get the fastest reverse speed for a component, one RS232 command may drop.

If I use press/hold there's no way I can see to toggle the smart macro sent out for a single press since the MX-900 editor is limited compared to the other more advanced remotes. Without toggling the smart macro sent out in between presses, RS232 reliability goes out the window...

The caching idea of yours sounds very plausible, except why hasn't anyone else complained of this issue before? I called URC about the issue three times and they were useless...

I'll see about making a simple CCP test file if you have an MSC-400 and an MX-900 to try?  I'll use my toggle work around for the up arrow, and leave the down arrow alone...  Hopefully someone else can then reproduce this issue. I think this is the first step, proving that a completely different system has the same issue.

EDIT:

Forgot to mention that repeat buttons have an issue documented in other threads. They always give an extra command. There's the press hold work around, but it doesn't fully work. If you look at the RS232 output in Hyperterminal, you see that you always end up with an extra command on any hold operation. Kind of annoying.