The Macintosh Plus was introduced in 1986 with SCSI support, a redesigned motherboard, and more RAM. A few years later, a minor revision changed the case color from Beige to Platinum. Apple offered an official upgrade for Mac 128k and 512k users that involved swapping out the motherboard and the rear "bucket".
For common issues, upgrades and accessories, see the above links.
You can put up to 4MB of RAM in a Mac Plus, installed via 30-pin SIMMs.
If you wanted to stop dealing with swapping floppy disks, you could get an absolute mad hack of a hard drive upgrade installed. This upgrade gave an expansion board for interfacing with an MFM hard drive, a drive, and a second power supply. It did work though! You could get one in an original mac as well, but it was also available for the Plus!
The Macintosh Plus, unlike the original macs, was available with a full extended keyboard with a number pad. It used a phone jack cable to connect, but it is wired differently to a normal phone cable. Using a regular phone cable will fry the keyboard controller. The mouse used a different proprietary connector exclusive to the Mac Plus and original macs.
A second external floppy drive was available to help get the constant disk-swapping down a bit. It plugged into a special floppy connector on the back of the mac.
These macs ran HOT thanks to the lack of a fan. Over time, this could cause a variety of different problems inside the computer. Multiple different companies introduced add-ons that added a fan, usually on the outside of the mac near vent holes. This would reduce the temperature and increase reliability.
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