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Installing Terminals & Modems.

16 Installing Terminals & Modems.

A few facts about (ASCII or "dumb") terminals and modems:

16.1 Pins used for RS-232 serial connections.

Table 8: RS-232 pin assignments 
Pin  Name  Function                        Pin  Name  Function                        
1    FG    Field ground                    14   STD   Secondary transmitted data      
2    TD    Transmitted data                15   TC    Transmitted clock               
3    RD    Received data                   16   SRD   Secondary received data         
4    RTS   Request to send                 17   RC    Received clock                  
5    CTS   Clear to send                   18         (Not assigned)                  
6    DR    Data set ready                  19   SRTS  Secondary request to send       
7    SG    Signal ground                   20   DTR   Data terminal ready             
8    DCD   Data carrier detect             21   SQ    Signal quality detector         
9          Positive test voltage           22   RI    Ring indicator                  
10         Negative test voltage           23   DRS   Data rate selector              
11         (Not assigned)                  24   SCTE  Serial clock transmit external  
12   SDCD  Secondary data carrier detect   25   BUSY  Busy                            
13   SCTS  Secondary clear to send                                                    
Most of these pins have no practical use, except:

When installing cables in walls/conduit, it is best to use straight-through cables using all 25 pins and use smaller, specialized (e.g., null modem) cables or converters between the ends of the routed cable and the devices.

16.2 Equipment types.

Connecting a modem to a computer is different than connecting two computers together.

16.3 Hardware handshaking (RTS/CTS).

Tells computer that modem can accept data (ignored by many U.S Unix systems which use DCD instead).

Dropped signal means that modem lost connection to other system.

16.4 Software Handshaking (XON/XOFF).

Since hardware handshaking is implemented in the serial port hardware, it is far more efficient and reliable. This is because:

16.5 Steps in installing terminals/modems

16.6 /etc/ttys file

Here is an example /etc/ttys file from a BSD-style system (a NeXT). The uncommented lines with "on" in the fifth column are active and will have associated getty processes. The second field (between the " ") points to entries in the /etc/gettytab file.

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