AppleCD is a range of early SCSI-based CD-ROM drives for Apple Macintoshpersonal computers, manufactured and sold by Apple Computer. Earlier AppleCD drives required a CD caddy in order to be used, while later models used a tray-loading mechanism. The original model introduced in 1988 was simply called the AppleCD SC.  There was also a version on the CD drive that did not have the Apple logo.
The AppleCD SC was the first CD-ROM by Apple Computer Company, introduced in 1988. It originally contained a fan but in 1990 they removed it because it was unneeded and attracted a lost of dust onto the optical disk head which could cause problems. It uses a front-loading caddy 1x CD-ROM and is only capable only of Read Only Media. This accessory device was only able to read compact disks up to a 630 MB capacity in five formats, CD-Audio, CD-ROM, HFS, ProDOS, and High Sierra. On the front of the device it has an eject button, mini-phono sound out jack, and a volume knob. On the rear it has a power switch, power input, two Centronics 50-pin SCSI outputs, and an audio RCA connector. With appropriate software it would run on any Macintosh with a DB-25 connector, or a Apple II with an Apple II SCSI interface card.
The AppleCD SC Plus was Apple Computer Company's second CD-ROM as a replacement for the AppleCD SC which was introduced in 1991. It had the model number M3021, and it had just like it's predecessor, the AppleCD SC and it had a 1x Read Only Media CD-ROM. The Plus could read a CD with up to 750 MB of data over the 650 MB of the AppleCD SC. It had indicator lights, an eject button, mini-phone audio jack, volume knob, and the CD caddy drive on the front of the accessory. On the back there was two 50-pin Centronics SCSI connectors, audio RCA connectors, power input, and a power switch. The AppleCD SC Plus had only minor improvements over the AppleCD SC and it was relatively the same.
Several other models were made, including the 150, 300, 300e, 300 plus, 600i and 600e plus (model number also represents data reading speed in KB/s). They all include two Centronics 50-pin SCSI ports, and require mains power.