Eaton Cutler-Hammer Durant PRESIDENT Series Installation And Operation Manual

Contents

SERIAL COMMUNICATIONS36Several types of information may be transmitted orreceived by the control. The serial communi-cations capability allows any combination of countand preset values to be printed, remotely dis-played, or sent to a host computer or other periph-eral device for processing. The characteristics ofthe communication are controlled by functioncodes.COMMUNICATION FORMATThe control uses a 20 milliamp current loop typeof electrical interface for serial communications.The control has a separate 20 milliamp currentloop for incoming communications and anotherloop for outgoing communications.Since serial communication (either in or out) isdone through only two wires, each character trans-mitted or received must be generated by a seriesof on and off states called bits. Each character hasits own unique code or sequence of bits thatallows the receiving device to understand whatcharacter it is receiving. The character “5”, forexample, has a series of bits which are differentfrom the series of bits for the character “6”. In fact,eight individual bits are needed to express a singlecharacter. Seven bits identify the character itselfand the eighth is used for error checking to allowthe receiving device to make sure that the previ-ous seven are correct when they are received.This eighth bit is called the parity bit and shows“even parity” to the receiving device when trans-mitting data. When the counter receives serialdata, it ignores the parity bit.There are several different standard rates at whichserial communications occur. Each is a function ofthe number of bits transmitted per second. Theterm which defines transmission rate is “Baud”which is understood to mean “bits per second.”The standard transmission rates the control canbe set up to use are 110 Baud, 300 Baud, and1200 Baud.While each character requires eight individual bitsto be uniquely expressed, a few additional bitsmust be sent between characters. These arecalled “start” and “stop” bits. The “start” bit signi-fies that this is the beginning of the character andthe next eight bits are the character itself. After thecharacter is transmitted, either one or two “stop”bits are sent to indicate that the character hasbeen completely transmitted. When the control isoperating at 110 Baud, two “stop” bits are sent andat 300 or 1200 Baud one is sent. Thus, at 300Baud, for example, each character requires tenbits to be transmitted: one “start” bit, eight data bitsand one “stop” bit. If information is being communi-cated at 300 Baud, 30 characters per second arecommunicated since a total of ten bits per charac-ter are required.The standard set of codes used by the control forcommunicating information serially is called theASCII character table. ASCII stands for AmericanStandard Code for Information Interchange. Thecontrol uses ASCII codes for all its communica-tions.A typical character transmitted or received isshown in Figure 30. In this figure, the character isshown with the “start” bit, seven data bits, the evenparity bit, and one “stop” bit.SENDING DATAData transmission can be initiated by either of twomethods. The first is by connecting the PRINTREQUEST terminal (terminal #16) to DC Com-mon. The second is by a special code transmittedto the control via the serial communications.Once a transmission has been initiated, thecounter will first transmit the “Carriage Return” and“Line Feed” characters (described in the followingparagraphs and illustrated in Figure 30) followedby the numeric information selected for printing.The “Carriage Return” and “Line Feed” characterscause the printer to provide spacing between print-outs.When the control transmits the actual value oreither preset value through the SERIAL DATAOUTPUT (SDO) terminals, it sends the characters“0” through “9” as necessary to express the value.It transmits the most significant digit (MSD) first.For example, if the current value of the counter is1357, the control sends the ASCII code for “0”since the most significant digit is blank and has avalue of zero, then the code for “1”, then the codefor “3”, then “5”, and finally “7”.