From NEC Retro
The PC-6600 series (PC-6600シリーズ) is a range of computers manufactured between 1985 and 198x by NEC.
The PC-6600 series is something of an oddball, as the hardware is virtually identical to the PC-6000 series, just with minor changes. It is backwards compatible with all PC-6000 series software, but also had its own line due to the inclusion of a 3½-inch floppy disk drive which the PC-6000 series lacked.
For information, see PC-6600 series.