Serial communications
Dialing out manually
In this section, we'll look at what needs to be done to establish a dial-out connection. You don't normally do this yourself: some software will do it for you automatically it’s useful to know what goes on, though: it can be of immense help in solving connection problems.
There are two distinct things that you want to do with the modem: first, you send commands to the modem to set up the link. Once the link is established, you don’t want to talk to the modem anymore, you want to talk to the system at the other end of the link.
In the old days, the system used a separate ACU to establish the connection, and the solution was simple: the system issued the dialing commands to the ACU and opened the modem in such a manner that the open did not complete until a connection had been established. Nowadays, the modem handles both dialing and the establishment of connection. But to do so, the system has to open the modem before communication has been established.
The terminal parameter clocal enables communication with a device that is not asserting DCD (such as a modem that hasn’t established a connection yet). When it starts, the software sets clocal. When it has finished talking to the modem and wants to wait for the connection to be established, it resets (turns off) clocal and waits for DCD. You can check this with the stty command:
stty -f /dev/cuaa2 -a
ppp disc; speed 57600 baud; 0 rows; 0 columns;
lflags: -icanon -isig -iexten -echo -echoe -echok -echoke -echonl
-echoctl -echoprt -altwerase -noflsh -tostop -flusho –pendin
-nokerninfo -extproc
iflags: -istrip -icrnl -inlcr -igncr -ixon -ixoff -ixany -imaxbel ignbrk
-brkint -inpck ignpar -parmrk
oflags: -opost -onlcr -oxtabs
cflags: cread cs8 -parenb -parodd hupcl -clocal -cstopb crtscts -dsrflow
-dtrflow –mdmbuf
cchars: discard=^O; dsusp=^Y; eof=^D; eol=<undef>;
eol2=<undef>; erase=^?; intr=^C; kill=^U; lnext=^V;
min=1; quit=^\; reprint=^R; start=^Q; status=^T;
stop=^S; susp=^Z; time=0; werase=^W;
This example, taken when the modem is connected, shows clocal reset. As you can see, this is indicated by the text -clocal.
There’s problem here: what if this line is also enabled for dialup? As we shall see on page 338, there will be a getty process in the process of opening the line. It won't succeed until DCD is asserted, so we can dial with no problem. But when the connection is established, how do we stop getty from being there first?
The FreeBSD solution is to create separate devices for each case. For the second serial port, sio1, the system creates a file /dev/cuaal for dialing out, and /dev/ttydl for dialing in. If cuaal is open, an open on ttydl does not complete when connection is established.
Dialing out—an example
For an example of what you might look at, let’s consider a manual dialup to an ISP. This assumes that you are using user PPP (see page 348) and that have an entry ISP in your /etc/ppp/ppp.conf. If you don't have an entry for an ISP, you can still test the modem, but in this case you won't be able to dial. In this case, simply omit the text ISP.
ppp ISP User Process PPP. Written by Toshiharu OHNO. Using interface: tun0 Interactive mode ppp ON freebie> term go into direct connect mode Enter to terminal mode. Type `~?' for help. at synchronize with the modem OK at&v look at the modem profile ACTIVE PROFILE: B0 E1 L0 Ml N1 Q0 T V1 W0 X4 Y0 &C1 &D2 &G0 &J0 &K4 &Q5 &R1 &S0 &T5 &X0 &Y0 S00:000 S01:000 S02:043 S03:013 S04:010 S05:008 S06:002 S07:060 S08:002 S09:006 S10:014 S11:090 S12:050 S18:000 S25:005 S26:001 S36:007 S37:000 S38:020 S44:020 S46:138 S48:007 S95:000 STORED PROFILE 0: B0 E1 L0 M1 N1 Q0 T V1 W0 X4 Y0 &C1 &D2 &G0 &J0 &K4 &Q5 &R1 &S0 &T5 &X0 S00:000 S02:043 S06:002 S07:060 S08:002 S09:006 S10:014 S11:090 S12:050 S18:000 S36:007 S37:000 S40:104 S41:195 S46:138 S95:000 STORED PROFILE 1: B0 E1 L1 M1 N1 Q0 T V1 W0 X4 Y0 &C1 &D2 &G0 &J0 &K3 &Q5 &R1 &S0 &T5 &X0 S00:000 S02:043 S06:002 S07:060 S08:002 S09:006 S10:014 S11:090 S12:050 S18:000 S36:007 S37:000 S40:104 S41:195 S46:138 S95:000 TELEPHONE NUMBERS: 0=T1234567890 1= 2= 3= OK
The term profile refers to a set of the complete configuration information for the modem. External modems can usually store two different profiles. Some modems may not have any stored profiles, or they may have a different number. The AT&V command shows the current configuration (" active profile ") and the two stored profiles. The first line refects the parameters set with AT commands (for example, L0 means that the command ATL0, turn off the speaker, has been issued). The next two or three lines refect the values of the Sregisters. In addition, this modem can store up to four telephone numbers, a feature that is seldom of great interest.
If you look at this profile, you'll notice that the active profile includes the parameter &K4 This means "use XON/XOFF flow control " This is not desirable: it's better to use RTS/CTS flow control. To fix it,
at&k3 set RTS/CTS flow control OK at&w write the active profile OK at&v and check ACTIVE PROFILE: B0 E1 L0 M1 N1 Q0 T V1 W0 X4 Y0 &C1 &D2 &G0 &J0 &K3 &Q5 &R1 &S0 &T5 &X0 &Y0 S00:000 S01:000 S02:043 S03:013 S04:010 S05:008 S06:002 S07:060 S08:002 S09:006 S10:014 S11:090 S12:050 S18:000 S25:005 S26:001 S36:007 S37:000 S38:020 S44:020 S46:138 S48:007 S95:000 STORED PROFILE 0: B0 E1 L0 M1 N1 Q0 T V1 W0 X4 Y0 &C1 &D2 &G0 &J0 &K3 &Q5 &R1 &S0 &T5 &X0 S00:000 S02:043 S06:002 S07:060 S08:002 S09:006 S10:014 S11:090 S12:050 S18:000 S36:007 S37:000 S40:104 S41:195 S46:138 S95:000 STORED PROFILE 1: B0 E1 L1 M1 N1 Q0 T V1 W0 X4 Y0 &C1 &D2 &G0 &J0 &K3 &Q5 &R1 &S0 &T5 &X0 S00:000 S02:043 S06:002 S07:060 S08:002 S09:006 S10:014 S11:090 S12:050 S18:000 S36:007 S37:000 S40:104 S41:195 S46:138 S95:000 TELEPHONE NUMBERS: 0=T1234567890 1= 2= 3= OK
The active profile includes the parameter &Y0, so the AT&W command writes back to stored profile 0.
The AT&V command doesn't show all the S registers. Some of them relate to the current state of the modem, and aren’t part of the configuration. For example, my modem includes an S register S86, the Call Failure Reason Code If a call fails, it could be interesting to look at it. To do so:
ats86? show contents of S86 012 Connection dropped by other end
With this background, we can now proceed to establish a connection:
atd1234567 just dial CONNECT 57600 ppp ON freebie> PPP ON freebie>
Dialing in
Traditionally, UNIX distinguishes between local serial terminals and terminals connected by modem by whether they assert the DCD signal. It starts a getty (for Get TTY) process for each line getty opens the line, but for modems the line state is set in such a way that the call to open does not complete until the DCE asserts DCD. This is done by resetting the tag clocal. If you look at the line state with the stty program, it will show -clocal if the flag is reset.
To set up a line for dialing in, add information about the line in the file /etc/ttys. The default file contains a number of lines like:
ttyd0. "/usr/libexec/getty std 9600". unknown off secure
This information has the following meaning:
- The first column is the name of the terminal special file, relative to /dev In other words, this entry represents the file /dev/ttyd0.
- The next field consists of the text /usr/libexec/getty std 9600 This is the invocation for getty: the getty program is /usr/libexec/getty, and it is invoked with the parameter std 9600 This is a label in the file /etc/gettytab, and describes a standard 9600 bps connection. You'll probably want to upgrade to std 57600.
- unknown refers to the terminal type. This is the value to which getty sets the environment variable TERM. If you know that only people with VT100 terminals dial in, you might change this string to vt100, but you should do this with care. It can cause a real mess on the screen, and even make it impossible to work with it.
- The remaining fields can occur in any order off means "don't start the getty after all ". If you want to run a getty on this line, change this string to on.
secure means that only people you trust can access this line, so you can allow a root login on this line. That's fi ne for a direct connect terminal in the same room, for example. It's not a good idea for a modem to which anybody can dial up. If the line is not secure, just omit the string.
After changing /etc/ttys, send init (process 1) a HUP signal to tell it to re-read /etc/ttys:
# kill -1 1