This circuit is a Parallel interface between 89C52 of Atmel with 7135 of Intersil. With This circuit you can read analog data of both polarities. You can change the range scale with extra circuits, you can store data on a EEPROM or send them to PC thru RS232 or Comm port.
The Circuit Is Shown for One Anode Drive and one Segment Drive for Display. In similar Fashion connect rest, all 5 anodes and 7 segments and one dp decimal point. The +5 V of 2N2907 and gnd of BC547 must be directly from regulator with a big cap or even a separate supply.
Parallel interface is faster that a serial interface.It is more “Real Time” for a given processor and clock speed. The code and accompanying hardware also can speed up the data acquisition.
AT89C52 Parallel Interface to ICL7135 – Complete Page
A single maxim chip can act as a good interface between the uC and Serial Port. You could also do this with Logic chips but correct levels and isolation cannot be achieved, Hence better to use use these interface chips.
These days USB and Wireless Interfaces are being used and Tablet Computers becoming Popular. The Interfacing of the future is “Device Networking” and Wireless may be common.