Page 125 - Mechanical Engineers Reference Book

3/8 Microprocessors, instrumentation and control

establish a means of interfacing a variety of instruments for 3.4 Interfacing of computers to systems
data-logging applications. The most common standard for the
integration of automated test systems, developed by Hewlett- The serial and parallel communication standards are the basic
Packard, is referred to as the IEEE-488 interface bus, and has interfacing links between computers and their associated
achieved wide recognition among instrument manufacturers peripheral devices, or between computers and a comprehens-
since the start of the 1980s. ive range of measurement instrumentation. In general com-
The bus consists of 24 lines, accommodated within standard puter control applications, however, two other common inter-
stacked type connectors. The eight bi-directional data lines faces are fundamental. These are the digital interface which
carry information as 7-bit ASCII codes between the micro- implements the controller output and the analogue interface
computer (controller), and an instrument (listener) on the associated with the measured variable input.
bus. The roles may be reversed when data are being logged.
To process the information on the data bus, up to eight control
and status signals are available. 3.4.1 Digital interfacing
The bus is designed to interface with up to 15 instruments,
within a localized area, involving a total cable length of not The computer output port may be used to transmit control
more than 20 m. Each instrument is uniquely numbered signals on any one of the available lines by writing the
within the range of &30 and the overall activity is controlled appropriate number to the port address. When a line (or bit) is
by one of the devices, termed the Controller. This is usually set ‘high’ (i.e. a logic level of 1) the voltage on the line is
the microcomputer with an appropriate interface. Each device approximately 5 V. The current available, however, is fairly
number is switch selectable within the instrument. Other minimal (of the order of 1 mA) and no load can be connected
functional aspects of the devices on the bus are that they must directly to the port. There is in fact a danger of causing
be capable of acting as a ‘Listener’ or a ‘Talker’. A ‘Listener’ is extensive damage to the computer by connecting a load
a device that can receive data over the bus and a ‘Talker’ is directly to the port. An interface must therefore be provided
capable of transmitting data. There may be several Listeners to enable the computer to switch in power loads using the logic
active on the bus at any one time, but there can only be one level control signals from the output port. The most common
Talker. Most devices, including the microcomputer Con- interface device used for this purpose is the power transistor.
troller, can act as either Listeners or Talkers. In typical applications the power transistor, operating on logic
When setting up an instrument to measure some physical level control signals, switches in a mechanical relay which in
variable, codes devised by the instrument manufacturer are turn switches in the load.
sent on the bus, in ASCII format, as a data string to the The ‘Darlington Driver’ is a popular power transistor avail-
numbered device. In the case of a multichannel DVM, this able as an integrated circuit and which normally includes a
could take the form of the channel number to be monitored, number of separate stages. Figure 3.6 shows the wiring
voltage range to be selected and a terminating character. An diagram for a single stage in a Darlington Driver. The
example of the corresponding string to be put on the bus is Darlington Driver can switch up to 500 mA at 50 V and each
stage is diode protected for the switching of inductive loads.
‘C9R2T’ To provide a total isolation from high voltages, port output
which denotes channel 9, range number 2 (say, 0-10 V) and signals can be coupled through an ‘opto-isolator’, which,
‘T’ is the string terminating character recognized by the interfaced between the computer output port and the power
instrument. control device, is not an essential element in the digital
Manufacturers of add-on cards, to give IEEE-488 facilities interface. Opto-isolation, however, ensures that no hardwired
with microcomputers, usually supply software for initializing connections are made between the computer and the power
the bus, setting it up for transmitting data from controller to device. An added advantage is that the opto-isolator acts as a
instrument and returning data from instrument to controller. buffer to spurious noise signals which can corrupt the digital
The measured quantity is also sent to the computer in the form logic values being transmitted on the buses.
of an ASCII string from which the actual numerical value can The opto-isolator (Figure 3.7) transmits signals by means of
be extracted. infrared radiation, emitting from a source and sensed at a
One of the most important management control lines is the photo-transistor. When a computer based on transistor/
service request (SRQ). This is a type of interrupt line that is transistor logic (TTL) is powered up the state of the lines of
activated low by a device residing on the bus and needing the output port ‘float high’. That is, each output line becomes
service from the controller. It is used as a means of indicating set to a logic value of 1. Since a logic 1 is normally associated
that the instrument is ready to transmit the current reading with the function of switching a device ON, then a port which
onto the bus. floats high could inadvertently activate some power device.
Thus, a typical software sequence for implementing the Obviously, this is a dangerous precedent, which requires an
control of an instrument on the IEEE-488 bus for data additional element in the digital interface to counteract the
acquisition is: effect. The device commonly used is the ‘inverter’ (or NOT
1. lnitialize bus and set instrument as a Listener. gate) which has the simple function of inverting all logic
signals from 1 to 0 and vice versa (Figure 3.8). Following
2. Put control string on the bus to set up the instrument as power up, a logic 0 must then be sent to the relevant line of the
required. output port, to become a logic 1 after inversion and to operate
3. Check for SRQ line to go low indicating that data can be the control function.
read. The composite digital interface for a computer output port
4. Set instrument as a Talker. suitable for power switching is depicted in Figure 3.9. In
5. Read returned string and convert into a numerical value. high-frequency switching applications electromechanical re-
When operating in high-level BASIC, high data-collection lays are not suitable. Semiconductor devices such as silicon
rates are not possible. However, since most instrument manu- controller rectifiers (SRCs, alternatively called thyristors or
facturers offer the standard as an option it provides an triacs) may be more appropriate. Also particularly suitable are
intelligently controlled flexible arrangement for test and mea- the various solid-state relays which can operate directly from
suring instruments. logic level signals.