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Previous Chapter 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
Table 8: RS-232 pin assignments
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Pin Name Function Pin Name Function
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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
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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.
- 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.
- 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)
- 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
- Physically connect the terminal/modem to the computer (DTE<->DTE or DTE<->DCE)
- 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)
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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