Page 73 - Programmable Logic Controllers, Fifth Edition - Mobile version

0110 0111 0101 0100
1100
0010 Light source
1101
0011

0001 1111
1
1 1110
0000 0
1
1000 1010
1001 1011 Sensors
Figure 3-15 Optical encoder disk.
Source: Photo courtesy Baumer Electric.

angular position. In this example, the encoder disk is at- memory to represent the alphanumeric, function, or
tached to a rotating shaft and outputs a digital Gray code control data represented by the specific keyboard key
signal that is used to determine the position of the shaft. that was depressed. ASCII input modules convert
A fixed array of photo diodes senses the reflected light ASCII code input information from an external device
from each of the cells across a row of the encoder path. to alphanumeric information that the PLC can pro-
Depending on the amount of light reflected, each cell cess. The communication interfacing is done through
will output a voltage corresponding to a binary 1 or 0. either an RS-232 or RS-422 protocol. Modules are
Thus, a different 4-bit word is generated for each row of available that will transmit and receive ASCII files
the disk. and that can be used to create an operator interface.
The user writes a program in the BASIC language that
operates in conjunction with the ladder logic as the
3.8 ASCII Code program runs.

ASCII stands for American Standard Code for Informa-
tion Interchange. It is an alphanumeric code because it in- 3.9 Parity Bit
cludes letters as well as numbers. The characters accessed
by the ASCII code include 10 numeric digits; 26 lower- Some PLC communication systems use a binary bit to
case and 26 uppercase letters of the alphabet; and about check the accuracy of data transmission. For example,
25 special characters, including those found on a standard when data are transferred between PLCs, one of the bi-
typewriter. Table 3-8 shows a partial listing of the ASCII nary digits may be accidentally changed from a 1 to a 0.
code. It is used to interface the PLC CPU with alphanu- This can happen because of a transient or a noise or be-
meric keyboards and printers. cause of a failure in some portion of the transmission net-
The ASCII code is a seven-bit code in which the deci- work. A parity bit is used to detect errors that may occur
mal digits are represented by the 8-4-2-1 BCD code pre- while a word is moved.
ceded by 011. Uppercase letters are preceded by 100 or Parity is a system in which each character transmitted
101. Lowercase letters are preceded by 110 or 111. Char- contains one additional bit. That bit is known as a par-
acter symbols are preceded by 010, 011, 101, and 111. ity bit. The bit may be a binary 0 or binary 1, depend-
This seven-bit code provides all possible combinations of ing on the number of 1s and 0s in the character itself.
characters used when communicating with peripherals or Two systems of parity are normally used: odd and even.
interfaces in a PLC system. Odd parity means that the total number of binary 1 bits in
The keystrokes on the keyboard of a computer are the character, including the parity bit, is odd. Even parity
converted directly into ASCII for processing by the means that the number of binary 1 bits in the character, in-
computer. Each time you press a key on a computer cluding the parity bit, is even. Examples of odd and even
keyboard, a 7- or 8-bit word is stored in computer parity are shown in Table 3-9.

54 Chapter 3 Number Systems and Codes

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