732 Chapter 13. Data Acquisition Systems
Table 13.3.1: Pin assignments of standard 25-pin and 9-pin RS232 male connectors
(see Fig.13.3.2).
Pin Number
Signal Description 25-Pin Connector 9-Pin Connector
TD Transmitted Data 2 3
RD Received Data 3 2
RTS Request To Send 4 7
CTS Clear To Send 5 8
DSR Data Set Ready 6 6
SGND Signal Ground 7 5
CD Carrier Detect 8 1
DTR Data Terminal Ready 20 4
RI Ring Indicator 22 913.3.CPCParallelPortModules
Just like the serial ports, the PC parallel ports can also be used to communicate
with data acquisition modules. Parallel ports support much faster data transfer
rates than the serial ports and are therefore more suitable for high rate applications.
The biggest disadvantage of such systems is that the generally available parallel
ports can not transmit signals beyond a few feet.
13.3.DUSBBasedModules
This is a relatively new development in small scale data acquisition. Instead of
installing a module into a PCI slot, the user simply connects the module to the
computer through a USB board.
USB based modules have two advantages over PCI boards. One is their ease in
operation since they are simply plug and play devices. The other advantage is that
in most cases there is no need for an additional power supply since the USB port
takes power from the PC. Of course the detector must be biased through a separate
power supply. All the signal conditioning units can generally run on the USB power.
13.3.ETCP/IPBasedSystems
Over the last few years a new technology, using the TCP/IP protocol, has emerged.
The main idea is to simply connect the data acquisition modules to the existing
network through Ethernet connectivity. This makes the data and control available
to users on any point on the local network. If the local network is connected to the
Internet, the user can access the system from any geographical location.
The most widely used and well established PC based data acquisition is through
the RS232 communication port where the data acquisition hardware is directly con-
nected to computer’s serial port. The main problem with this scheme is the direct
link between the two hardware, which becomes difficult to establish if there is sub-