one second, needs to be at a frequency of several megahertz. Thus, the conversion speed of an ADC,
which is defined as the time taken to sample the analogue signal once and output the digital equivalent,
is as important a consideration as resolution in analytical applications. Typical conversion speeds are of
the order 5– 30 μs.
If several instruments in a laboratory are to be interfaced with a computer, individual ADC units can be
fitted to each one and the digitized signals from each transmitted to the computer. This has the
advantage of minimizing the effect of electrical noise from outside sources, especially where the
instruments and computer are not close together. An alternative arrangement is to use an analogue
multiplexer which consists of a set of analogue input channels, switches and an ADC with a single
digital output. The multiplexer can be controlled by the computer software to sample a single channel or
to monitor a series of channels sequentially and at selected time intervals.
A further requirement of an instrument computer interface and its controlling software is the provision
of an accurate and stable time-base. This is necessary to ensure the synchronization of data collection
with instrument scanning, to facilitate the exact matching of data from repetitive scans or runs, and for
the computation of real-time experimental parameters such as retention times in chromatography. The
generation of interrupt and control signals is also used in this context. Reference to the time-base
allows interrupt signals to be sent at precise intervals under software control and is an essential feature
of an analogue multiplexer system. Control signals passed from an instrument to the computer in the
form of voltage pulses are also utilized in some instances.
To replot stored analytical data such as chromatograms and spectra on a chart recorder, digital-to-
analogue converters (DAC) are used. These are similar in construction and performance to an ADC but
function in the reverse direction producing an analogue output signal varying between defined limits. A
particular advantage of replotting data from an earlier run is the ability to produce a permanent record
(hard copy) in a different or enhanced form. For example, a spectrum might be replotted after
smoothing, scale-expansion or the subtraction of solvent peaks, or the first derivative of a titration curve
might be drawn.
The transmission of data between an ADC or DAC and a computer is usually accomplished by sending
one byte at a time along a multiway ribbon cable (external bus). The IEEE 488 standard bus has been
widely used for this purpose, but other buses are also used. As all bits are handled simultaneously this is
known as parallel I/O transmission, the data entering or leaving the computer via a parallel port or by
direct connection to the computer's internal bus. Alternatively, one bit at a time can be transmitted
along a single line (serial I/O transmission). Data enter or leave the computer via a serial port, usually
an RS-232C. Serial I/O ports are used for