Page 242 - Mechanical Engineers' Handbook (Volume 2)

8 Data Communications 231

7.2 System Error Analysis
The errors that can occur at different stages in the data acquisition process must be analyzed,
as they can add up to make the data meaningless. For instance, one may have very accurate
sensors, but by the time the data reach the host computer they might have been converted
to integer data or from real to integer and back to real again. This can lead to dangerous
assumptions about the accuracy of the received numbers, because each conversion can cause
rounding or other errors. It is the responsibility of the person setting up the acquisition system
and the analyst to examine each source of potential error, discover its magnitude, and reduce
it to the point where it will not have a significant impact on the conclusions to be derived
from the data. Use of the filtering techniques described above under data collection can be
of use to eliminate random error. It is not within the scope of this chapter to cover system
error analysis, but Chapter 1 gives some foundation.

8 DATA COMMUNICATIONS
Data communications are involved in many aspects of data acquisition systems. The com-
munications between the sensing and control elements and data acquisition devices, as well
as the communications between the data acquisition system and other computer systems, can
be carried out in many ways. This section will cover some aspects of communications,
especially as they pertain to computer systems.

8.1 Serial Communications
A serial communication link means that data sent over a communications line is spread out
over time on one physical data path. For instance, if a character is sent from a sensor to a
computer, each bit making up the character (normally eight bits) will be sent one after the
other (Table 7). This is often useful for low-cost, low-speed (usually less than 10,000 cps)
rates of data transfer.

Table 7 Time Sequence of Bits Sent over Serial
Communication Line
Character ‘‘A’’ is (in bit form)
Bit number 6543210
Bit value 1000001
The communications are using the RS232C communications
standard and sending the ASCII character A.
Bit Value Time
Start bit 0 s after start
1 (bit 0 of A) 1/9600 s after start
0 (bit 1 of A) 2/9600 s after start
0 (bit 2 of A) 3/9600 s after start
0 (bit 3 of A) 4/9600 s after start
0 (bit 4 of A) 5/9600 s after start
0 (bit 5 of A) 6/9600 s after start
1 (bit 6 of A) 7/9600 s after start
parity bit 8/9600 s after start
stop bit 9/9600 s after start