5 . 6 . 7 . The computer sets the IDA line, indicating that it has sampled the data word on the ID lines. The external equipment detects the setting of the IDA  line.  The  external  equipment  may  clear  the IDR line anytime after detecting the setting of the IDA line, but will clear the IDR before the computer will recognize the next IDR. The computer clears the IDA line before reading the next word on the ID lines. — OUTPUT   DATA   (OD)   SEQUENCE   OF EVENTS.—  We begin from the point that an OD 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. 5 . 6. 7 . When the external equipment is ready to accept data, it sets the ODR line (this may already have happened   before   the   OD   buffer   was established). The computer detects the setting of the ODR line in accordance with internal priorities. The computer places a word of data on the OD lines. The computer sets the ODA line to indicate that a word of data is on the OD lines. The external equipment detects the setting of the ODA lines. (The external equipment may clear the  ODR  line  anytime  after  detecting  the  setting of the ODA, but clears the ODR line before the computer will recognize the next ODR). The external equipment samples the data word that is on the OD lines. The  computer  clears  the  ODA  line  before placing the next word on the OD lines. EXTERNAL  FUNCTION  (EF)  SEQUENCE OF EVENTS (NORMAL).—  We begin from the point that an EF has been established for a channel. The computer and the external equipment on that channel will do the following to transfer: 1 . 2. 3 . When  the  external  equipment  is  ready  to  accept an EF code word, it sets the EFR line (this may have already happened before the EF buffer was established). The computer detects the setting of the EFR line in accordance with internal priorities. The computer places an EF code word on the OD  lines. 4. 5 . 6. 7 . The computer sets the EFA line to indicate that the EF codeword is on the OD lines. The external equipment detects the setting of the EFA line. The external equipment may clear the EFR line anytime after detecting the setting of the EFA line, but clears the EFR line before the computer will recognize the next EFR. The external equipment samples the EF code word that is on the OD lines. The computer clears the EFA line before placing the next word on the OD lines. Forced external functions are the same as normal external  functions  except  the  computer  does  not  require an  external  function  ready  signal  from  the  external equipment, so the computer will not be delayed by steps 1 and 2. EXTERNAL INTERRUPT (EI) SEQUENCE OF  EVENTS.—  The  computer  and  the  external equipment do the following to transfer an EI code word: 1  . 2. 3. 4. 5 . 6. 7. 8 . 9. 10. The computer, under program control, sets the EIE line when ready to accept an EI. The  external  equipment  detects  the  state  of  the EIE line. When the status requires that the computer be interrupted, the external equipment places an EI code word on the ID lines. The external equipment sets the EIR line to indicate that the EI codeword is on the ID lines. The computer detects the setting of the EIR line in accordance with internal priorities. The computer samples the EI codeword that is on the ID lines. The computer clears the EIE line. The computer sets the IDA line. The external equipment detects step (8) or both steps (7) and (8). The external equipment may clear the EIR line anytime after detecting the setting of the IDA line, but clears EIR line before the computer will recognize the next EIR. The  computer  clears  the  IDA  line  before sampling the next word on the ID lines. NOTE: Not all computers have the EIE lines;  consult  your  computer’s  technical manual. 7-34