Computer to External Equipment (Two Cables) Two  cables  will  makeup  one  channel.  As  stated with two cables, one cable will specifically handle input functions and the other cable will handle output functions.  Refer  again  to  figure  7-27.  Notice  the direction of information flow. Data request signals are always  sent  from  the  external  equipment  to  the computer.  The  acknowledge  signals  are  always  sent from the computer to the external equipment. INPUT  CABLE.  —The  input  cable  contains control lines and data lines. The number of lines will vary with the type of computer. They range from 8 to 64 lines. The operating mode (single, dual, and so on) has an effect on the number of lines affected. External devices send input data words and interrupt codes to the computer  via  input  data  (ID)  lines.  The  information carried over these lines is as follows: External Interrupt Enable (EIE) —The  computer sends the external interrupt enable signal to the external device to indicate it is ready to accept an external interrupt code word on that channel. Input  Data  Request  (IDR) —The input data request control signal accompanies each input data word sent to the computer from the external device. The external device informs the computer that it has placed an input data word on the lines. External Interrupt Request (EIR) —The external interrupt request control signal accompanies each interrupt code sent to the computer from the external device. It informs the computer that an interrupt code is on the data lines. Input Data Acknowledge (IDA) —The input data acknowledge  control  signal  informs  the  external equipment  that  the  computer  has  sampled  the  input word or interrupt code on the input data lines on that channel. OUTPUT  CABLE.  —The  output  cable  contains control lines and data lines. Again the number of lines will vary with the type of computer. They range from 8 to 64 lines. The operating mode (single, dual, and so on) has an effect on the number of lines affected. Output  data  words  and  external  function  words  are  sent to the external device via data lines. The information carried over these lines is as follows: External  Function  Request  (EFR)  —The external device sends the external function request signal to the computer indicating that it is ready to accept an EF code word on that channel. External  Function  Acknowledge  (EFA) —The computer  sends  the  external  function  acknowledge signal to the external device indicating that it has placed an EF code word on the OD lines of that channel. This signal accompanies each function code word sent to the external  device. Output Data Request (ODR) —The external device sends the output data request control signal to the computer indicating that it is ready to accept an output data word. Output   Data   Acknowledge   (ODA)  —The computer sends the output data acknowledge signal to the external device indicating it has placed a word of data on the OD lines of that channel. This signal accompanies  each  output  data  word  sent  to  the  external device. It informs the external device that an output data word is on the data lines. Two Cable Sequence of Events The sequence of events using an input, output, external function (buffered), and external interrupt operations is described from the computer’s point of view. We begin from the point that an input data (ID), output data (OD), an external function (EF), or an external interrupt (EI) has been established for a channel. The computer and the external equipment on that channel transfer data as described in the following paragraphs. Refer back to figure 7-27. INPUT   DATA   (ID)   SEQUENCE   OF EVENTS. —We begin from the point that an ID has been established for a channel. The computer and the external  equipment  on  that  channel  will  do  the following to transfer data: 1. 2. 3. 4. The external equipment places a word of data on the ID lines. . The external equipment sets the IDR line to indicate that a word of data is on the ID lines. The computer detects the setting of the IDR line in accordance with internal priorities. The computer samples the data word that is on the ID lines. 7-33