Basic Principles of Digital Communication-4

CODING:

An agreed standard code allows a receiver to understand the messages sent by a transmitter. The number of bits in the code determines the maximum number of unique characters or symbols that can be represented. The most common codes are described on the following paras.

Baudot code:

Although not in use much today, the Baudot code is of historical importance. It was invented in 1874 by Maurice Emile Baudot and is considered to be the first uniformlength code. Having five bits, it can represent 32 (25) characters and is suitable for use in a system requiring only letters and a few punctuation and control codes. The main use of this code was in early teleprinter machines. A modified version of the Baudot code was adopted by the ITU as the standard for telegraph communications. This uses two ‘shift’ characters for letters and numbers and was the forerunner for the modern ASCII and EBCDIC codes.

ASCII:

The most common character set in the western world is the American Standard Code for Information Interchange, or ASCII.
This code uses a 7-bit string giving 128 (27) characters, consisting of:
• Upper and lower case letters
• Numerals 0 to 9
• Punctuation marks and symbols
• A set of control codes, consisting of the first 32 characters, which are used by theBCommunications link itself and are not printable For example: D = ASCII code in binary 1000100.

A communications link setup for 7-bit data strings can only handle hexadecimal values from 00 to 7F. For full hexadecimal data transfer, an 8-bit link is needed, with each packet of data consisting of a byte (two hexadecimal digits) in the range 00 to FF. An 8-bit link is often referred to as ‘transparent’ because it can transmit any value. In such a link, a character can still be interpreted as an ASCII value if required, in which case the eighth bit is ignored.

The full hexadecimal range can be transmitted over a 7-bit link by representing each hexadecimal digit as its ASCII equivalent. Thus the hexadecimal number 8E would be represented as the two ASCII values 38 45 (hexadecimal) (‘8’ ‘E’). The disadvantage of this technique is that the amount of data to be transferred is almost doubled, and extra processing is required at each end.

ASCII control codes can be accessed directly from a PC keyboard by pressing the Control key [Ctrl] together with another key. For example, Control-A (^A) generates the ASCII code start of header (SOH).

The ASCII Code is the most common code used for encoding characters for data communications. It is a 7-bit code and, consequently, there are only 27 = 128 possible combinations of the seven binary digits (bits), ranging from binary 0000000 to 1111111 or hexadecimal 00 to 7F.

Each of these 128 codes is assigned to specific control codes or characters as specified by the following standards:
• ANSI-X3.4
• ISO-646
• ITU alphabet

EBCDIC:

Extended binary coded data interchange code (EBCDIC), originally developed by IBM, uses 8 bits to represent each character. EBCDIC is similar in concept to the ASCII code, but specific bit patterns are different and it is incompatible with ASCII. When IBM introduced its personal computer range, they decided to adopt the ASCII Code, so EBCDIC does not have much relevance to data communications in the industrial environment.

4-BIT BINARY CODE:

For purely numerical data a 4-bit binary code, giving 16 characters (24), is sometimes used. The numbers 0–9 are represented by the binary codes 0000 to 1001 and the remaining codes are used for decimal points. This increases transmission speed or reduces the number of connections in simple systems.

GRAY CODE:

Binary code is not ideal for some types of devices because multiple digits have to change every alternate count as the code increments. For incremental devices, such as shaft position encoders, which give a code output of shaft positions, the Gray code can be used.

The advantage of this code over binary is that only one bit changes every time the value is incremented. This reduces the ambiguity in measuring consecutive angular positions. 

BCD:

Binary coded decimal (BCD) is an extension of the 4-bit binary code. BCD encoding converts each separate digit of a decimal number into a 4-bit binary code. consequently, the BCD uses 4 bits to represent one decimal digit. Although 4 bits in the binary code can represent 16 numbers (from 0 to 15) only the first 10 of these, from 0 to 9, are valid for BCD.

BCD is commonly used on relatively simple systems such as small instruments, thumbwheels, and digital panel meters. Special interface cards and integrated circuits (ICs) are available for connecting BCD components to other intelligent devices. They can be connected directly to the inputs and outputs of PLCs.

A typical application for BCD is the setting of a parameter on a control panel from a group of thumbwheels. Each thumbwheel represents a decimal digit (from left to right; thousands, hundreds, tens and units digits). The interface connection of each digit to a PLC requires 4 wires plus a common, which would mean a total of 20 wires for a 4-digit set of thumbwheels. The number of wires, and their connections to a PLC, can be reduced to 8 by using a time division multiplexing system.

Each PLC output is energized in turn, and the binary code is measured by the PLC at four inputs. A similar arrangement is used in reverse for the digital display on a panel meter, using a group of four 7-segment LCD or LED displays.