ISA Adapter Card Unit (ACU) to install third-party expansion cards, and a $995 CD-ROM drive, based on a Western Digital SCSI chip, that fit underneath the case. Some of the lower-end PS/1 models suffered from very limited expansion capabilities, since they lacked standard ISA expansion slots. The 2121 series computers used the same form factor as the 2011 series, but included up to two ISA slots inside the case. Memory could be expanded from 2MB to 6MB using a proprietary 4 MB memory module. The higher-end 2121 featured an Intel 80386SX processor running at 16 or 20 MHz. The 2121 series PS/1 computers can be split into the following major hardware categories: Monitors IBM made the decision to put the DC power supply in the monitor, making use of third-party monitors difficult and essentially impractical and limiting the usefulness of the computer if the monitor needed service (similar to the problems of the Coleco Adam and Amstrad 1512 years earlier). Some models were sold with greyscale VGA monitors. However, some hobbyists could manage to remap the pinouts to allow for third-party monitors. "DOS in ROM" Similar to a few Tandy 1000 models, the early 2011 and 2121 had an operating system (PC DOS 4.01) built into ROM, rather than loading it from a hard drive. The ROM disk would then load a "4-quad" screen which allowed users to access help, rapidly launch pre-installed software, connect online, and access files on the hard drive. It was possible to have the computer boot from the hard drive if the operating system was upgraded, and IBM provided a DOS 6.22-compatible version of the "4-quad" program that could be launched from the hard drive if users wished to continue using it. Model 2133 The 2133 series PS/1 computers can be split into the following major hardware categories: Post-"DOS in ROM" models On May 11, 1993, IBM introduced a "new generation" of the PS/1 line. Later PS/1s featured standard LPX-architecture motherboard. Many of these later PS/1s shipped from the factory with MS-DOS and Microsoft Windows, rather than IBM's PC DOS or OS/2. An early 2133 model did come preloaded with OS/2 2.1. This was because IBM targeted OS/2 for high-end computing machines with more power. Discontinuation The PS/1 line was discontinued in 1994 and replaced with the Aptiva line, which was architecturally very similar to the later models of the PS/1, but with a more