internal 3.5" or 5.25" floppy disk drive. The rear panel has the same ports as the 1000 SX, except that an RS-232C serial port replaces the light-pen port. The memory size is 640 KB, with sockets for an additional 128 KB devoted to the onboard video logic. This extra 128 KB can only be used for and by the on-board video controller, so it is impractical to expand the on-board memory beyond 640 KB if a VGA graphics card is installed. The computer comes bundled with Personal DeskMate 2. The TX is the last 1000-series computer to use DIP switches to store the system configuration parameters. All later 1000s rely on CMOS static RAM chips powered by a battery soldered to the circuit board. These batteries degrade with time (as well as often leaking acid which ruins the circuit board) and must be replaced to restore full functionality after any lengthy period of disuse. Tandy 1000 EX and HX The Tandy 1000 EX and HX were designed as entry-level IBM-compatible personal computers, and marketed as starter systems for people new to computing. They were offered in a compact, all-in-one chassis that features a 7.16 MHz 8088 (capable of clocking down to 4.77 MHz), 256 KB of memory (expandable to 640 KB with a PLUS memory expansion board), PCjr- and CGA-compatible Tandy Video graphics controller, a keyboard and, depending on the model, either a single 5.25" 360 KB floppy drive, or one to two 3.5" 720 KB floppy drives. An external floppy drive can be connected to a port on the back. The machine itself supplies power to the external drive, so only Tandy's floppy drive unit is usable with the EX and HX. The external drive is the standard 360 KB 5.25" format; in 1988 a 720 KB 3.5" model was offered. The EX and HX are upgradable via Tandy PLUS cards, and these systems have bays for three cards. A PLUS card connector is electrically identical to an ISA slot connector, but uses a Berg-style 62-pin connector instead of a 62-contact ISA card-edge connector. Other PLUS cards can be installed to add serial ports, a 1200-baud modem, a clock/calendar and bus mouse board, or a proprietary Tandy network interface. Radio Shack later sold an adapter card allowing installation of a PLUS card into a standard ISA slot, such as those in the larger Tandy 1000 models. Like the original