Sub second time delay RPL HP48GX/HP50g

[roadrunner]

This from the 50g AUR manual, page 3-272 might be what you are looking for:

WAIT
Type: Command
Description: Wait Command: Suspends program execution for specified time, or until a key is pressed.
The function of WAIT depends on the argument, as follows:
• Argument x interrupts program execution for x seconds.
• Argument 0 suspends program execution until a valid key is pressed (see below). WAIT then
returns xkey, which defines where the pressed key is on the keyboard, and resumes program
execution.
xkey is a three-digit number that identifies a key’s location on the keyboard. See the entry for
ASN for a description of the format of xkey.
• Argument –1 works as with argument 0, except that the currently specified menu is also
displayed.