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, I2C
Between instruments/computer:
RS232/RS422, USB, IEEE488
Parallel data: microprocessor bus, IEEE488
Serial data: RS232/RS488, I2C
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 I2C 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Designed in the 1970s it has been largely superseded by fast serial bus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
I2C
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.