16 Installing Terminals & Modems.

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

• Terminals and modems are asynchronous devices
  
• Most use RS-232 standards (such as they are)
  
• Terminals and modems used for logins must deal with getty
  
• Modems used for both dial-in and dial-out have special needs
  
• Asynchronous data transfer is usually much slower than ethernet
  
• Official limit for RS-232 lines is 50 feet
  
• Most workstations and desktop Unix systems only have two async ports
  

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:

• Ground pins 1 (FG) and/or 7 (SG) are used
  
• If only one ground is used, it should be 7
  
• Pins 2 (TD), 3 (RD) and one ground pin must be used
  
• Pins 4 (RTS) and 5 (CTS) may be used for HW handshaking
  
• Modems commonly use pins 6 (DSR) and 20 (DTR)
  

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.

• DTE - Data Terminal Equipment (computers are usually DTE)
  
• DCE - Data Communications Equipment (modems are always DCE)
  

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

16.3 Hardware handshaking (RTS/CTS).

• DTR - "Ready to communicate"
  
• Tells modem that computer is ready/able to communicate - if this signal drops, modem should hang up.
  
• DSR - "Ready to receive data"
  

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

• DCD - "Have remote carrier"
  

Dropped signal means that modem lost connection to other system.

• RTS (Request to Send) and CTS (Clear to Send) lines used to "throttle" transmission (lines not shown)
  

16.4 Software Handshaking (XON/XOFF).

• XOFF (ASCII Control-S)
  Used to tell other side to stop transmitting data.
  
• XON (ASCII Control-Q)
  Used to tell other side that it is OK to continue transmitting data.
  

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

• the time delay between transmission of XOFF character and actual halting of transmission can cause buffer overflow and lost data (especially at very high baud rates)
  
• transmitted data may contain XON/XOFF characters which fool the hardware
  

16.5 Steps in installing terminals/modems

• Physically connect the terminal/modem to the computer (DTEDTE or DTEDCE)
  
• Determine the special file in /dev that communicates with the terminal/modem
  
• With terminals, make sure a termcap or terminfo entry exists for this terminal (you may need to create one)
  
• Add/modify an entry in the proper configuration files
  
• Force init to reread the terminal configuration information (kill -HUP 1 or init -q)
  
• Dial-in/dial-out modems are a special case(May need to send modem initialization codes and save to non-volatile memory using tip or cu)
  

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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