LEGO FAQ — P program & IR tower

LEGO's own customer-service answer, pulled out of the Wayback Machine. It is the primary source on what the Micro Scout's P slot was for, and it names the hardware LEGO expected you to use.

Verbatim quote

"All of the models are built around the 'Micro Scout'. This is a small computer with a built in motor and 7 programs. None of the programs can be over written or erased; the 'P' program is a blank program area where the user can 'download' commands to the Micro Scout using Visual Light Link (VLL). This is for more advanced users and requires other LEGO sets. The Robotic Discovery Set and Ultimate Accessory Set or Robotic Invention System 2.0 with the USB IR tower is required."

What it establishes

• Programming the Micro Scout P slot = VLL, delivered by either the Scout (Robotic Discovery Set) or the RIS 2.0 USB IR tower.
• → The tower fills the P slot over VLL on a visible-red LED (not its ~940 nm IR: that's the RCX channel). The sensor's IR-sensitivity (⇒ invisible IR VLL could work) is a separate fact, confirmed on-device → Micro Scout.
• Confirms the CD itself wasn't the programmer (Droid Developer Kit CD (_ddk.exe)).