This console supported a wide range of accessories, some of which completely revamped its internal capabilities (by providing more RAM, for instance) and external functionality (e.g., by adding more ports or additional storage media).
The Expansion Port at the back of the PC Engine.
This was largely due to the inclusion of the Expansion Port at the back of the console, which gave access to the following components :
- A +5 Volts line to power the accessory.
- A Voltage input line to bypass the original PSU and power the console through the accessory.
- The Audio Input and Output buses, allowing to send and/or receive stereo audio.
- The CPU data and address lines, enabling the CPU to access the accessory and vice versa.
- The VDC data lines, allowing the accessory to tap into the video encoder's input. I'm not sure what for though.
- The CPU control lines. These expose miscellaneous information, such as the HuCard, a CD detection flag, the speed mode used, and various interrupt lines.
- The Analogue video out, including RGB + Vertical Sync and Horizontal Sync + Composite sync.
I'm curious to know the extent to which NEC or Hudson envisioned this console. Were they attempting to build some sort of 'modular console' that could be upgraded to seamlessly transition into the next generation?
The rest
I have yet to mention the controller (or Joypad, as some sources call it). It is very similar to other offerings. The console provides only one controller port, and games check a single address in memory to retrieve key presses. That address returns a 4-bit value .