4. 5. 6. 7. 8. In accordance with internal priorities, computer B detects the setting of the EFA line of computer A (which will be recognized as the EIR line) and samples the ID lines. Computer B clears the EIE line. Computer B sets the IDA line. Computer A detects the setting of the IDA line of computer B (which will be recognized as the resume line). Computer A clears the EFA line before placing the next word on the OD lines, and computer B clears the IDA line before reading the next word on the ID lines. NOTE:  Whenever   the   transmitting computer does not have an EFR line, or the receiving computer does not have an EIE line, a command will be transferred with force. For forced transfers, step 3 and step 7 are not used. Computer A and computer B repeat this sequent for  each  successive  command  word  until  they  have transferred the block of command words specified by computer B’s EF buffer control words. INTERCOMPUTER  DATA  TRANSFER.  — Whenever  an  OD  buffer  has  been  established  in computer A and an ID buffer has been established in computer B for the same channel, computer A and computer B transfer data. Again refer to figure 7-29 with  computer  A  as  the  winding  computer  and computer B as the receiving computer. The sequence is performed as follows: 1. 2. 3. 4. 5. Computer A places a word of data on the OD lines. The OD word is held on the data lines until computer B sets the resume line, or until computer A’s program intervenes to resolve the no resume condition. Computer A sets the ready line to indicate that a word of data is on the OD lines. In accordance with internal priorities, computer B  detects  the  setting  of  the  ready  line  of computer A (which will be recognized as the IDR line). Computer B samples the ID lines. Computer B sets the IDA line. 7-36 6. 7. Computer A detects the setting of the IDA line of computer B (which will be recognized as the resume line). Computer A clears the ready line before placing the next word of data on the OD lines, and computer B clears the IDA line before sampling the next word of data on the ID lines. Computer A and computer B repeat this sequence until  they  have  transferred  the  block  of  words  specified by the buffer control words. Buffer lengths specified by each computer are the same. SUMMARY—INPUT/OUTPUT (I/O) AND INTERFACING This chapter has introduced you to how computers communicate  with  and  control  other  computers  and external   devices. The   following   information summarizes important points you should have learned: I/O  ORGANIZATION  —All  computers  are capable of I/O operations. Some rely on the CPU to handle I/O operations. Others have an I/O processor (IOC). An I/O processor enables the computer to perform  other  operations  while  still  performing  I/O operations. I/O PROCESSOR  —An I/O processor (IOC) controls  the  transfer  of  information  between  the computer’s  main  memory  and  the  external  equipments. IOCs are packaged in (1) IOC/IOA modules or multiple IOC/IOA pcb’s, and (2) I/O pcb’s. The IOC relieves the CPU of the necessity to perform the time consuming functions  of  establishing,  directing,  and  monitoring transfers with external equipments. Data and control signals  are  exchanged  with  external  equipments  via  the IOA. The IOA changes the input and output control and data signal voltages to the voltage requirements of the computer or external equipments. Communication between  the  IOC  and  the  IOA  is  by  means  of  a bidirectional bus. I/O DATA ARRANGEMENTS —The types of information exchanged between the computer and the external equipments fall into two basic categories: data words  and  control  words.  Data  words  represent  the alphabetic  and  numeric  information  exchanged. Control words specify an action to be accomplished by an  external  equipment. I/O DATA FORMATS —Computers  exchange data  in  either  parallel  or  serial  format.  When  the computer uses a parallel configuration, all bits of information represented by a byte or word are input or