Page 243 - Mechanical Engineers' Handbook (Volume 2)

232 Data Acquisition and Display Systems

8.2 Parallel Communications
A serial communication link may not require very many wires, but the time spent to transfer
data can add up. A way to improve the speed of communications is to use parallel com-
munication links. This is done by having a number of wires to carry data. For instance,
sending the same ‘‘A’’ over a nine-wire bus would only require one transfer (Table 8).

8.3 Networks
Ethernet with Transmission Control Protocol/Internet Protocol (TCP/IP) has become the
dominant communications network protocol for data collection. There are proprietary process
control and data acquisition networks that serve special purposes, but Ethernet has proven
to be versatile for everything from office communications to data collection from smart
sensors. Many computers can be connected to the same network segments. The use of
switches and routers provides ways to isolate and limit communications to improve perform-
ance and security. Firewalls provide filters and protection for entire classes of messages and
sources.
While Ethernet cannot guarantee delivery (being based on a collision detection and
retransmit strategy), it has been shown to provide excellent response to moderate network
activity. Communications speeds are regularly being improved to provide an even greater
range of applicability.

8.4 OSI Standard
The International Standards Organization has developed a set of standards for discussing
communications between cooperating systems called the Open Systems Interconnect (OSI)
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model (see Table 9). This defines communications protocols in terms of seven layers. While

Table 8 Time Sequence of Bits Sent over Parallel
Communications Interface
Character ‘‘A’’ is (in bit form)
Bit number 6543210
Bit value 1000001
The communications are using a hypothetical nine-line
parallel communications bus sending the ASCII character A.
Bit Value Time
Start bit 0 s after start
1 (bit 0 of A) 0/9600 s after start
0 (bit 1 of A) 0/9600 s after start
0 (bit 2 of A) 0/9600 s after start
0 (bit 3 of A) 0/9600 s after start
0 (bit 4 of A) 0/9600 s after start
0 (bit 5 of A) 0/9600 s after start
1 (bit 6 of A) 0/9600 s after start
parity bit 0/9600 s after start
If the bus could handle the same rate of change of bits as the
serial interface, then the next character could be sent 1/9600
s after the first character (the A).