730 Chapter 13. Data Acquisition Systems
provide the complete solution including signal conditioning, A/D conversion, and
computer interfacing. The user has to simply connect the inputs and configure the
software.
Since PCI modules are directly connected to the computer bus, they can handle
high data transfer rates. Fast data polling can therefore be easily implemented in
such systems. However one important point to note here is that the data transfer
rate is actually limited by how the PCI board (or DAQ^1 module) communicates with
the random access memory (RAM) of the computer. The microprocessor speed is not
an issue here since modern microprocessors have clock frequencies in the gigahertz
range. However since a microprocessor in a typical PC does multitasking therefore
it can delay the data transfer to the RAM, thus damping the data acquisition fre-
quency. This is the mode in which conventional DAQ boards work (Fig.13.3.1(a)).
The data acquisition frequency can be increased if the DAQ module performs the so
calledbus mastering. What it does is that it bypasses the microprocessor and sends
the data directly to the RAM. Microprocessor then acquires data from the RAM and
performs the required operations (see Fig.13.3.1(b)). Since modern computers have
big enough RAMs, data is generally not lost. It is, however, up to the programmer
to ensure that the routines do not allow complete filling of the data buffers at any
time.
The PCI based modules are good for building small scale data acquisition systems
since typical modules can handle up to only 30 input channels. This is good enough
for few channel detection devices but for large scale systems one should resort to
other solutions.
13.3.BPCSerialPortModules
Most PCs contain one or more serial ports. The most commonly available port is
the RS232. This port can be used to communicate with a data acquisition module.
However such systems can only be used for slow data acquisition due to the serial
nature of data transfer. The good thing is that the RS232 port can be used to
transmit data and control signals to large distances. The RS232 standard specifies
a maximum cable length of 50 feet but this does not mean that larger cables can
not be used. In fact, the maximum length of the cable depends on the environment,
baud rate, and the cable type. In an electromagnetically hostile environments even
a few feet long unshielded cable might not work. On the other hand at low baud rate
of about 110 one can use a shielded cable that is as long as 5,000 feet. Hence one
should not feel constraint by the RS232 50-feet specification as good shielding and
lower baud rates can assure perfect data transfer through very long cables. Another
option is to use the so called RS485 port, which is specified to transmit data to
about 5,000 feet.
According to the RS232 standard the devices should use 25-pin connectors. How-
ever only 9 of those are used for communication (plus one for ground). Therefore
most computers have 9-pin connectors instead. The pin configurations of 25-pin and
9-pin male connectors are shown in Fig.13.3.2. The pin assignments can be found
in table 13.3.1.
The most problematic things with serial (or parallel) communication is that it
need direct connection of a PC with the module. This may become impractical if
(^1) DAQ stands for data acquisition.