The Apple IIc Plus unit was a direct response to the Laser 128EX/2.
Apple retrofitted the IIc design to try to compete with Video Technology's high-speed Apple II clone. The retrofitting is evident in the design of the motherboard. The motherboard runs at 1Mhz, unlike the Laser 128EX/2. In order to run its programs faster, Apple used a 4Mhz 65C02 with 8k of high speed SRAM cache and licensed the accelerator design from Zip Technologies (makers of the ZipChip accelerator for the IIe and IIc). They built an accelerator into a 1Mhz motherboard rather than designing a faster motherboard from scratch.
The designers of the Apple IIc Plus originally wanted to make a portable IIGS. Apple's management, bent on promoting the Mac, balked at the idea. At the same time, the Laser 128EX/2 made a quick design cycle mandatory. The result was an updated IIc.
The Apple IIc Plus was the finest Apple 8-bit computer ever built, but due to a lack of marketing, the apparent backward step in technology compared to the 16-bit Apple IIGS, and the aggressive sales of the Laser 128EX/2, the IIc Plus also was to be the most short-lived of the Apple II models.
Chris Brown adds:
I used this computer for a long time. My parents bought it for us as our very first family computer. I do remember that the "foreward slash" button (next to the reset button) on the IIc+ had a different function than on the IIc. On the IIC+ it allowed you to switch between Dvoark and Standard keyboard layouts without any other software - it was a hardware switch.
Also, the IIc+ came with a keycode that, I guess, disabled the Zip Chip accelerator. While the IIc+ was compatible with much of the IIe and II+ software, oftentimes software programmed for earlier systems would be "hyperactive" on the fast new processor, and was virtually unusable unless it was slowed down. The keycode for a standard, full speed reset was open apple - control - reset. If you wanted "half speed" you hit open apple - control - esc -reset.
Also, the power supply on the IIc+ was internal. This caused heat problems which the IIc did not have, and occasionally (I have heard) caused problems. But I used it for 7 years without any problems whatsoever. A flawless machine.
APPLE IIc Plus
Full stroke keyboard
128 KB (Contains AppleSoft Basic and an ML
40 x 24 / 80 x 24
(only the full screen modes are listed here) : 40 x 48 (16 col) / 80 x 48 (16 col) / 280 x 192 (6 col) / 140 x 192 (16 col) / 560 x 192 (16)
16 (256 colors with a upgrade that was availble in 1989)
one channel (Because the Apple IIc Plus operates at 4Mhz, sounds from Apple II programs are distorted, but you can cut the speed down to 1Mhz on start up for games).