The most common commands issued are 'I' for inches, 'M' for mm, or 'B' for binary. A '?' will be send to the host when the interface has seen the break and entered command mode. The host computer should send a 10ms break to get the attention of the interface. The first one to initiate communication gets the full attention of the interface. While idle the interface checks for data from both the host computer and the caliper. The asynchronous transmitter is implemented in the firmware like the synchronous receiver. Sign and decimal are handled within the BCD to ASCII conversion routine. The result is then converted to BCD, then converted to ASCII and sent to the host computer. The raw value is multiplied by 125/256 for inches or by 127/1024 for mm. For inches or mm mode the first 24 bits are scaled to real units. In binary mode the 48 bits are sent to the computer literally. The microcontroller firmware has a synchronous receiver that handles the burst of data from the caliper. The data and clock signals are then sent to the 12C509 microcontroller. The amplifier is driven to saturation by the low logic levels from the caliper. Three gates of IC1 are used as a 3.3x linear amplifier. 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). The clock and data signals from the caliper are AC coupled by C1 and C2. The logic levels are -1.55 volts low and 0 volts high. The positive of the battery is connected to the metal parts of the caliper making it positive ground. The caliper is normally powered by a small 1.55 volt battery. The unit of measure is 1/20480th of an inch. Each 24 bit value is in ones compliment form (1 sign bit, 23 data bits). This is sort of an absolute position, but the zero point changes each time the caliper loses power. The second 24 bits are the position relative to an arbitrary zero that does not change. The first 24 bits are the position relative to the zero point set on the caliper. Each burst contains 48 bits of data as shown in timing.pdf. The caliper sends a burst of data approximately three times each second.
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