Data Acquisition

In summary, the optional settings for asynchronous transmission of characters are:• Start bits 1• Data bits 5, 6, 7, 8• Parity bits even, odd, mark, space or none• Stop bits 1, l½ or 2As there cannot be half a bit, 1½-stop bits means that the mark length is 50% longer thanfor one stop bit.6.1.4 Data transmission speedThe maximum rate at which data can be transferred from the source to the receiver on a communicationsinterface depends on a number of factors:• Type and complexity of the circuitry at each end (interface)• Communication link (twisted-pair, coaxial cable, radio etc)• Distance between the sender and receiver• Amount of data being transferred• The overhead associated with the data transfer• The acceptable rate of errorThe lower the data rate, the less complex are the requirements of the communication link,the source and receiver circuitry and the lower the errors due to timing and noise problems.Data transfer rates are usually measured in bits per second or bps. This is an indication ofthe useful data that has been transmitted to the receiver. For example, in Figure 6.4 the usefuldata is only seven bits, whilst the total number of bits transmitted was ten. The additionalthree bits are viewed as overhead bits for the data communication.The baud rate (named in recognition of Maurice Emile Baudot) can be considered the physicalrate, or signaling speed at which data bits can be transmitted and correctly received onthe communications interface. Referring to Figure 6.4, if each bit occupied a time of 1 millisecond(ms), the total baud rate would be 1 / 1 ms = 1000 baud. This is the signaling speed.The data transfer rate, on the other hand, can be calculated as:7 data bits / 10 ms = 700 bits per second (bps)The actual data transfer rate is therefore 30% less than the baud rate for this example of atotal of 10 bits in the frame. Baud rates are usually quoted in standard values such as 50, 110,300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, and 115200 baud. It is commonpractice in industry to use the terms baud rate and data transfer rate interchangeably, unless itis specifically noted that they are not equal.When using modems and sophisticated encoding techniques, a single signal change on theline can indicate several encoded bits. This means that the data rate (in bps) is far greater thanthe baud rate (the reverse of the situation quoted in the previous example).The Electronic Industries Association (EIA) RS-232 interface standard is probably the mostwidely known of all serial data interface standards. It was developed for a single purposeclearly stated in its title, and defines ‘Interface between Data Terminal Equipment (DTE) andData Communications Equipment (DCE) employing serial binary data interchange’. It wasissued in the USA in 1969 by the engineering department of the EIA. Bell Laboratories andleading manufacturers of communications equipment also cooperated to clearly define the