I could use this unit that Kevin Timmerman designed! I have the same digital caliper that he designed the serial interface for.

Project Page.

"The caliper sends a burst of data approximately three times each second. Each burst contains 48 bits of data as shown in timing.pdf. The first 24 bits are the position relative to the zero point set on the caliper. The second 24 bits are the position relative to an arbitrary zero that does not change. This is sort of an absolute position, but the zero point changes each time the caliper loses power. Each 24 bit value is in ones compliment form (1 sign bit, 23 data bits). This allows for a 0 and a -0. The unit of measure is 1/20480th of an inch. The caliper is normally powered by a small 1.55 volt battery. The positive of the battery is connected to the metal parts of the caliper making it positive ground. The logic levels are -1.55 volts low and 0 volts high. The clock and data signals from the caliper are AC coupled by C1 and C2. The AC coupling effectively moves the logic levels above ground so they can be easily converted to standard CMOS logic levels (0 and +5 volts). Three gates of IC1 are used as a 3.3x linear amplifier. The amplifier is driven to saturation by the low logic levels from the caliper. The data and clock signals are then sent to the 12C509 microcontroller."