From NEC Retro
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The X1 Twin allows consumers to play PC Engine games and a range of X1 computer software. It was the last in the X1 line to be released, and so is theoretically backwards compatible with X1, X1 Turbo and X1 Turbo Z software dating back to 1982, however its design excludes it from using many PC Engine add-ons that would be developed in the years which followed.
The two "halves" operate independently, so the PC Engine side cannot access any X1 functionality and vice versa. Likewise aside from convenience, the X1 Twin offers nothing which cannot be found elsewhere - there is no special software making use of the X1 Twin's capabilities, and there is no means of using both platforms at the same time.
It is not currently known why the X1 Twin exists. While deals were drawn up between Sharp, Hudson Soft, and NEC Home Electronics, NEC already had its own range of computers in the form of the PC-8800 series and PC-9800 series. Nether of them ever received full PC Engine compatibility (there is, however, PC Engine technology in some systems - the PC-88 VA borrows some of the sound hardware and the PC-8801 MC can interface with a CD-ROM²).
|PC Engine (1987) | CoreGrafx (1989) | CoreGrafx II (1991)|
X1 Twin (1987) | PC-KD863G (1988) | Shuttle (1989) | GT (1990) | LT (1991)
|AV Booster (1988) | Interface Unit (1988) | Ten no Koe 2 (1989) | Backup Booster (1989) | Backup Booster II (1989) | Ten no Koe Bank (1991) | Memory Base 128 (1993)|