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  1.                                                                                
  2.       ┌─────────────────────────────┐                                          
  3.       │  rfrsh out ┌───────┐        │                                          
  4.       │   ┌────── 8│qd 7   │7──│    │                                          
  5.       │   │       9│   4 qd│6───────┘                                          
  6.       │   │      10│   L   │5─         74ls393                                 
  7.       │   │      11│   S   │4─         generates                               
  8.       │   │   │──12│clr3   │3─         8th refresh bit                         
  9.       └───│──────13│   9clr│2─                                                 
  10.           │+5v───14│Vc 3   │1──────┬─────────<RFRSH                            
  11.           │        └───────┘       │                                           
  12.           │           U1           │                                           
  13.           │                        │                                           
  14.       33Ω │        ┌───────┐       │                                           
  15. RA7<──   ─│────── 9│3y     │8──│   │                                           
  16.           └───── 10│3b 7   │7─     │   74ls153                                 
  17.           ┌──────11│3a 4   │6─     │   selects                                 
  18.           │      12│   L   │5─     │   between                                 
  19.           │      13│   S   │4─     │   refresh &                               
  20.           │      14│   1   │3─     │   address                                 
  21.           │   │──15│   5   │2──│   │                                           
  22.           │+5v───16│   8   │1──────┘                                           
  23.                    └───────┘                                                             │            U2                                                      
  24.           │                                                                    
  25.           │         ┌───────┐                                                  
  26.           └─────── 9│       │8──│     33Ω                                      
  27. IO bit 1>─────┬───10│   7   │7────────   ─────>RA8                             
  28.               └───11│   4   │6────────────────<IO bit 2                        
  29. A14b>─────────────12│   L   │5─── +5v                                          
  30.                │──13│   S   │4────────────────<IO bit 3                        
  31. A15b>─────────────14│   1   │3─── +5v                                          
  32.                │──15│   5   │2────────────────<MUX select                      
  33.              +5v──16│   3   │1──│                                              
  34.                     └───────┘                                                  
  35.                        U3                                                      
  36.                                         (─│ = gnd)                             
  37.                    Figure 1                                                    
  38.                                                                                
  39.                                                                                
  40.                                                                                
  41.                                                                                
  42.                                                                                
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  45.