Data Acquisition

A duplex system is designed for sending messages in both directions. Two types of duplexsystems exist:• Half duplex occurs when data can flow in both directions but only in onedirection at a time as shown in Figure 6.2. First one end transmits and the otherend receives. After what is called the line turnaround time, the roles are reversed.Figure 6.2Half duplex communications• In a full duplex system, shown in Figure 6.3, the data can flow in both directionsat the same time.Figure 6.3Full duplex communications6.1.2 Coding of messagesTo transfer a message across a communications interface, both the sender and receiver must,among other things, agree on the meaning of the binary digital patterns transferred, or thecode. Encoding is the process of converting the message data into a standard binary code fortransfer over the data communications link. The number of bits in a code determines the totalnumber of unique characters that are possible.The most common character set used for digital data communications in the Western worldis the American Standard Code for Information Interchange, or ASCII code (see AppendixE).This code assigns a binary field of 7-bits to represent each character, giving 128 (2 7 ) uniquecharacters made up of:• Upper and lower case letters, and numerals 1 to 9• Various punctuation marks and symbols• A set of control codes (the first 32 characters) that are used by the communicationslink itself and are not printableOn a computer keyboard, ASCII control codes are generated by pressing the Control key[Ctrl] and another key. For example, [Ctrl]-[A] generates the ASCII code SOH.A communications link set up for 7-bit data strings can only handle hexadecimal valuesfrom 00 to 7F. For full hexadecimal data transfer, an 8-bit link is needed, with each packet ofdata consisting of a byte (two hexadecimal digits) in the range 00 to FF. For this reason, an8-bit link is often referred to as transparent because it can transmit any value. In this case, a