Data Acquisition

• Line controlThis applies to half duplex links where the transmitter indicates to the receiverwhen it can ‘turn the line around’ and commence transmitting in the oppositedirection.• Error controlTypical techniques used here are block redundancy checks and cyclic redundancychecks.• Timeout controlThis applies to the transmitter when it doesn’t receive an acknowledgementwithin a pre-defined period of time and assumes the receiver never received theoriginal message.Just as there are a number of human languages, there are a considerable number ofcommunication protocols that have been defined at different times by computer vendors andinternational bodies. Protocols on serial communications hardware linking the PC to dataacquisition and control hardware are typically ASCII based. This allows much easiertroubleshooting of communication problems where the level of understanding of industrialcommunication systems may be fairly low. In addition, a high level of integrity of datatransfer is required where the PC controls critical equipment over the communications link.In an industrial environment where there may be a lot of electrical noise, a high degree oferror checking, such as cyclic redundancy checks, is used. Several of the most commonlyused protocols are discussed below.6 7.1 Flow control protocolsCooperative flow control, in which the transmitter and receiver operate under a common setof rules, is called a flow control protocol. Below are described the two most popular flowcontrol protocols.Character flow protocols (XON/XOFF)This is a popular flow control protocol that has two characters assigned as XON (start) orXOFF (stop). Typically, the ASCII characters DC3 (Ctrl-S) and DC1 (Ctrl-Q) are assigned toXOFF and XON respectively. For example, consider a transmitter, a PC, sending a stream ofcharacters to a printer. When the printer buffer fills to a certain predefined level, say 66%, ittransmits an XOFF character back to the PC, instructing it to stop transmitting characters.Once the printer buffer has emptied to a preset level, say 33%, the printer sends an XONcharacter to the PC, which then resumes transmission of the characters. A variation of thisprotocol is that the PC will resume transmission of the character stream when any character isreceived from the printer.Whole line protocols (ETX/ACK)The ETX/ACK protocol, designed by IBM, is based on the transmitter appending an ETXcharacter after each line of data and waiting for the receiver’s ACK, requesting the next lineof data.6.7.2 ASCII-based protocolsThe use of ASCII-based protocols is popular because of their simplicity and ease oftroubleshooting. Their main disadvantage is that they are slow and unwieldy, especially whenthe system requires considerable amounts of data to be transmitted at high speeds.