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The PC Engine Exp Connector
The PC Engine expansion connector is a hacker's dream port. It includes pins for just about everything you'd ever want, including the entire cartridge port, RGB, audio, video and a few other goodies. Completely uselessly it also has access to the communications between the video chip and the CPU, a mess of pins you're almost certainly never going to use. Perhaps these were available for planned video upgrades like a SuperGrafx-like module, but that's irresponsible speculation.

[Update: Feb 18 2021] Thanks to Dave Shadoff for letting me know the data lines were reversed. Also thanks to Martin Samuelsson for making me look at the power circuit again, leading to a correction about the Vin pins.

[Update: June 27 2006] Thanks to Charles MacDonald for filling in most of the blanks in the video bus pinouts. [Update: April 29 2008] Thanks to Charles MacDonald again for correcting a couple of leftover errors.
[Update: May 08 2014] Updated and corrected some pins based on info from the TG16 Service Manual

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
C SNDR GNDa HuD CEK A17 A13 A11 GNDd D5 D3 D1 D0 A1 A3 A4 A6 GNDd A15 A19 GNDd Vout Sync Blu
B SNDin CDD HSM /WR XNMI A8 XRD D7 D4 D2 HSn A0 A2 VD6 A5 A7 A12 A16 A20 /RDY IRQ2 /RST Grn
A SNDL Vout CS A18 A14 A9 A10 D6 DCK VSn Vin SPBG VD7 Vin VD5 VD4 VD3 VD2 VD1 VD0 GNDa Vid Red
    Power     Audio/Video     Data Bus     Address Bus     System Functions     Video Bus (?)

The PC Shuttle Exp Connector
For reasons known only to themselves the PC Engine Shuttle had a cut-down expansion connector. The primary change is the removal of the video bus pins, but in addition it lacks the IRQ2 pin, which is the only thing preventing the Shuttle from using a CD ROM peripheral. The Shuttle is a baffling mystery of the ages.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
C GND Vout A07 A04 A01 GND D2 D5 GND A10 A08 A17 GND /OE CDD GND
B GND Vout A12 A05 A02 GND D1 D4 D7 A20 A09 A14 GND HSM Vout GND
A A19 A16 A15 A06 A03 A00 D0 D3 D6 GND A11 A13 A18 /WR Vout /RST
    Power     Data Bus     Address Bus     System Functions

The Details Explained
Some things might need a little clarification. As far as I know all the system functions are active-low. Most of the functions in the PCE chart are active low, but not labelled as such (they wouldn't fit). The descriptions for these pins are:
OE - Output Enable
CDD - CD Detect (AKA K7, Input Bit 7)
HuD - HuCard Detect
HSM - High Speed Mode
RST - Reset
R/W - Read / Write
/RDY - Ready
IRQ2 - Interrupt 2
A20 is often used as CE - Chip Enable
CS - Chip Select (Hu6260)
CEK - Chip Enable (Hu6260)

CD Detect obviously isn't the correct term, as the Shuttle has it too and no CD peripheral was released for this unit, however it seems to be used for this purpose in normal PC Engines, so there you go.
HuDetect indicates the presence of a HuCard in the slot (cart port pin 1 is low).
Reset resets the machine (Surprise!)
High Speed Mode either indicates or determines whether the system is running at full speed (7.2MHz)or low speed (1.8MHz).
Interrupt Request 1&2 for... requesting ... interrupts. Shut up.
Chip and Output Enable are both signals used with ROM chips to activate the device, and enable output respectively.
Read/Write when set low the data bus is set to WRITE mode.

GNDa - Ground (analogue)
GNDd - Ground (digital)
Vout - Voltage out. The PC Engine supplies +5v to peripherals from this port.
Vin - Voltage in. You can power the PC Engine through these pins.
Audio and video:
SNDR and SNDL - Audio Right and Audio Left output.
SNDin - Audio Input
Video Bus (VD):
HSn Horizontal Sync
VSn Vertical Sync
DCK Dot/Pixel Clock (5.36 / 7.16 / 10.73 MHz)
VS Unknown
XNMI - Non-maskable interrupt to CPU
XRD - Read from CPU
VD1 - VD8 ViDeo Bus bits 1-8, directly connected to the video encoder (HuSomething)
VD0-VD3 = four bit pixel value
VD4-VD7 = four bit palette value
SPBG = high for sprite/border pixels, low for background pixels (for genlock, or LaserActive use?)

Unqualified Musings
The revelation of the video bus output (Thanks Charles) is quite fascinating. It's clear to see that NEC had thought, on some level, that video mixing or video expansion might have been in the cards for our diminutive PCE. With the available data anyone could make a digital digitizer, a video overlay system or perhaps a video expansion system for the PCEngine. The SuperGrafx couldn't use this bus unfortunately, it actually replaces the original video chip with a new chip that in turn talks to two video chips.

Special Thanks
Nothing I do is produced in a vacuum. I put a lot of hard work into this data, but none of it would be possible if it wasn't for the work of those who've come before. Special thanks to bt, Dave and Joann.

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