Data Set Ready (DSR) —A data set ready is sent from the computer to the external device to notify the external device that the computer is ready to transmit data when HIGH. Data  Terminal  Ready  (DTR) —A data terminal ready is sent from the external device to the computer to indicate that the external device is ready to receive data when HIGH. Request  to  Send  (RTS) —A request to send is sent from the external device to the computer to indicate that  the  external  device  is  ready  (HIGH)  or  busy (Low). Clear  to  Send  (CTS) —A clear to send is sent from the computer to the external device as a reply to the  RTS  signal. TRANSMIT   BUFFER/TRANSMIT   CON- TROL. —The transmit control logic converts the data bytes stored in the transmit buffer into an asynchronous bit  stream.  The  transmit  control  logic  inserts  the applicable start/stop and parity bits into the stream to provide the programmed protocol. A start bit is used to alert the output device, a printer for instance, to get ready for the actual character (bit). The signal is sent just prior to the beginning of the actual character coming down the line. A stop bit is sent to indicate the end of transmission. The parity bit is used as a means to detect errors; odd or even parity maybe used. RECEIVE  BUFFER/RECEIVE  CONTROL.— The receive control logic accepts the input bit stream and strips the protocol signals from the data bits. The data bits are converted into parallel bytes and stored in the   receive   buffer   until   transmitted   to   the microprocessor. Line Drivers/Receivers We discussed line drivers/receivers in chapter 4. Their basic function is to drive and receive (detect) the digital signal sent or received over a cable to other external equipments (including computers). The line drivers/receivers are designed to send and receive signals over short and long distances using serial or parallel  format. Large  voltages  or  currents  are generated from small voltage or current using TTL or MOS circuitry. The two types most commonly used include  single-ended  and differential.  The  voltage levels  and  current  amounts  sent  and  received  are dictated by the interface. The voltage and current characteristics  required  are  also  dictated  by  the interface. We discuss the voltage levels and some of the  characteristics  when  we  cover  I/O  channel/port configurations that include the various interfaces. I/O INTERFACE FORMATS There is a variety of serial and parallel I/O channel formats that you may encounter as a technician. Do not take for granted the type of interface a computer uses. A single different pin in a connector or a different voltage level used by a computer can make a vast difference when you are performing maintenance. Your  computer’s  technical  manual  will  provide  the standards to be used with the cabinet and cable con- nectors. They will match the standards that govern the requirements for parallel and serial interfacing. Table 7-1, from MIL-STD-2036,  General Requirements For Electronic  Equipment  Specifications,  provides you with some of the accepted standard external interfaces. We do not cover the General-Purpose Interface Bus (GPIB), Fiber   Distributed   Data   Interface   (FDDI),   and TACTICAL. Other interfaces used but not listed in the table  include  RS-449,  Centronics  Parallel,  ST-506/412, Enhanced Small Device Interface (ESDI), Integrated Drive  Electronic  (IDE),  and  Enhanced  Integrated  Drive Electronics (EIDE). We discuss signal designations in more detail later in this topic under serial and parallel I/O  operations.  First,  let’s  look  at  the  various  interfaces and  some  of  their  applications  and  any  unique characteristics.  As  stated,  each  interface  is  governed  by a  standard. Table  7-1.—Standard  External  Interfaces  from MIL-STD-2036 7-21