PH4705 ET4305 Interface Standards A number of standard digital data interfaces are used in measurement systems to connect instruments and computers for data transfer and control or to connect between complex ICs. Standards usually have two parts: Data and control formats and protocols, Electrical specification.

PH4705 ET4305 Interface Standards Between ICs: microprocessor data bus, I 2 C Between instruments/computer: RS232/RS422, USB, IEEE488 Parallel data: microprocessor bus, IEEE488 Serial data: RS232/RS488, I 2 C

PH4705 ET4305 Interface Standards Using any of these standards is usually a matter of programming some configuration information into the measurement system software (or accepting default values). Or, at a more basic level, adapting standard circuit configurations and low level software to a specific application.

PH4705 ET4305 Interface Standards RS232/RS422 and USB are widely used serial data standards. IEE488 and I 2 C are well defined but of more limited usage. Microprocessor bus is widely used but each microprocessor family has different hardware and control detail.

PH4705 ET4305 Interface Standards RS232/RS422 Serial Data: Asynchronous data Originally designed for data over the telephone system Commonly reduced to 2 wires Used widely to communicate between instruments and a PC, standard software means only data rate (baud rate) and data format need be specified. Data is usually ASII embedded in an 11 bit (max) packet that includes parity, for error detection, and start and stop bits, for synchronisation. The electrical interface for RS232 is single ended (signals with a common ground), for RS422 signals are 2 –wire balanced (send and return). Data rate can be higher than 100kb though 9600 baud is common.

PH4705 ET4305 Interface Standards USB (high speed serial data) Modern data acquisition hardware often uses a USB interface to a PC. Once drivers are installed data and control signals are passed transparently to/from the application software. One advantage of USB is that it can provide power for external hardware. Examples: LabView USB pods, Velleman USB Experiment Interface Board.

PH4705 ET4305 Interface Standards IEEE488 or GPIB : A high speed parallel bus designed to link intelligent instruments, or automated test equipment. Enables up to 15 instruments to be connected by daisy-chained cables and exchange 8 bit parallel data at up to 8Mb/s. Designed in the 1970s it has been largely superseded by fast serial bus systems.

PH4705 ET4305 Interface Standards I2CI2C This bus is mostly used to interface between ICs as an alternative to the usual parallel microprocessor bus. Its implementation is the domain of the circuit designer. It’s a two wire, data and clock, (plus ground) serial bus at up to 100kHz. A key feature is that control of data transmission can be passed to any device in the chain. Device addressing is usually 7 bits so up to 128 devices can be connected.

PH4705 ET4305 Interface Standards Interface Directly to microprocessor or microcontroller bus This interface consists of bidirectional data and control lines with unidirectional addressing. Data will be 8 or 16 bits, addressing 16 or more bits. For specific data acquisition tasks inexpensive simpler single chip microcontrollers that include features such as A/D are commonly used, these will often come as a ready made device. Design for scratch is a specialist task.

PH4705 ET4305 Interface Standards You are most likely to find either USB or RS232 as the link between instruments and a PC, with some older systems using IEEE488. Individual (inexpensive) sensor systems increasingly are based on system ICs that include microcontrollers these are more and more likely to use USB.