Tài liệu Chapter-19-Serial communications ppt

2 April 2003, 17:00:47 The Complete FreeBSD (modems.mm), page 325

In this chapter:
• Terminology
• Asynchronous and
synchronous
communication
• Serial por ts
• Modems
• Modem commands

19
Serial
communications

UNIX has always had a high level of support for serial lines, but their purpose has
changed dramatically. In the early 70s, the standard ‘‘terminal’’ was a Teletype KSR35, a
10-character-per-second serial printer with keyboard. Early UNIX serial line support was
geared towards supporting these devices, either directly connected, or via a modem.
Even in the early 80s, when 4.2BSD introduced network support, things didn’t change
much: the network support used different hardware. By this time, the Teletypes had been
replaced with glass ttys, in other words serial terminals with a monitor instead of a
printer. The speeds had gone up from the 110 bps of the Teletype to 9600 bps, but the
underlying principles hadn’t changed.
It wasn’t until the last 10 years that the glass ttys were replaced by display boards directly
connected to the system bus, or by other machines connected by Ethernet. The role of the
serial port has changed completely: nowadays, they’re used mainly for mice and dialup
Internet connections.
This change in use has invalidated a few basic concepts. Only a few years ago, the
standard ‘‘high-speed’’ modem was a V.22bis 2400 bps modem, even then too slow for an
Internet connection. The standard data communication line was 56 kb/s, and it was

invariably a leased line. As a result, the Internet grew up assuming that connections were
leased lines, and therefore permanently connected. Even today, the Internet protocols do
not deal well with dialup access.
On the other hand, UUCP did use dialup access. As a result, provisions for dialup access
in UNIX tend to be derived from UUCP. This doesn’t make for smooth integration.
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In this chapter, we’ll look at the way FreeBSD handles serial communications, at how
modems work, and how the two fit together.

Terminology
Any serial connection has two ends, which may be computers, terminals, printers or
modems. In modem terminology, the computers are Data Terminal Equipment or DTE
(this terminology arose at a time when the device connected to a modem was usually a
terminal), and modems are Data Communication Equipment or DCE. You’ll also
sometimes hear the name dataset for a modem.

Asynchronous and synchronous communication
There are two different ways to transmit serial data, called synchronous and asynchronous communication. They grew up in different worlds:

Asynchronous communication

Asynchronous communication predates computers. It was originally developed to run
teletypewriters, electrical typewriters that were run off a serial data stream, the bestknown of which were made by the Teletype corporation. These machines were
frequently used to provide a remote transcript of what somebody was typing miles away,
so they would typically print one character at a time, stop, and wait for the next. In the
early days of UNIX, the standard terminal was a Teletype model KSR35, commonly just
called teletype or tty (pronounced ‘‘titty’’).
Here’s a picture of a typical byte encoding:

1

1

1

0

1

0

1

1

0

StartParity D7 D6 D5 D4 D3 D2 D1 D0 Stop Stop
Bit 1 Bit 2

Figure 19-1: Asynchronous byte encoding

This figure shows an encoding for the letter j, in binary 01101011. We’ll see a number
of things about it:

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Chapter 19: Serial communications

•

Before the character starts, the line is idle, which shows as a high level: this indicates
to the teletype that the line is still connected.

•

First comes a start bit. In the olden days, this bit started the motor of the teletype
mechanism. Now it signals that data is coming.

•

Next comes a parity bit. To detect any transmission errors, this character is encoded
with even parity. The parity bit is set to 1 if the character contains an odd number of
bits, and to 0 otherwise, which ensures that the character, including the parity bit,
always has an even number of bits. If a single bit is corrupted during transmission,
the character will arrive with odd parity, and the receiver will detect an error.

•

Next come the bits of the character, last bit first. We represent 1 with a low level and
0 with a high level.

•

Finally come one or two stop bits. The stop bits were originally intended to give the
teletype time to stop the motor, but they are now largely superfluous. You needed two
stop bits for a teletype, but nowadays you should always use one.

•

This example also shows something else of importance: there are a number of ways
to encode the character. How many bits? How many stop bits? Odd parity? Even
parity? No parity? Mark parity (always a 1 bit)? Space parity (always a 0 bit)?
How much time from one bit to the next (what bit rate)? They’re all set with the stty
program (see man page stty(1)), but if you set them wrongly, you’ll run into trouble.

•

The encoding isn’t very efficient. For every character you send, you also send a start
bit and a stop bit. Most communications no longer use the parity bit, but this still
means that you have a 25% overhead on communication: for every 8 bits, you send
10, and you could send up to 12, as in this example. We’ll see that synchronous
communication doesn’t have this problem. Users of synchronous communication
protocols often refer to asynchronous communication as start-stop communication.

Synchronous communication
By contrast with asynchronous communication, synchronous communication comes from

the mainframe world, and it assumes that data does not come one byte at a time. Instead,
it transmits data in blocks. Each block is preceded by one or two SYN characters that tell
the receiver that data is coming, and that enable it to determine the correct orientation of
the bits in the data.
All modern modems use synchronous communication on the phone line, because it is
more efficient, and it’s the basis of protocols such as SNA and X.25, but you will almost
never see any other use of it in UNIX systems.

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Serial ports
Nowadays, all PCs come equipped with two serial ports, which are called COM1: and
COM2: in the DOS world. UNIX names are different, and FreeBSD calls these same
devices sio0 and sio1. It’s possible to connect up to four direct serial ports on a
standard PC, but due to the design of the board, each one requires a separate IRQ line. If
you put two serial ports on the same interrupt line, neither of them will work.
The first two devices, sio0 and sio1, normally use the default IRQs 4 and 3. By default,
however, PC manufacturers put COM3: and COM4: also at IRQs 4 and 3. How can this
work? It can’t, if you also have COM1: and COM2: enabled at those IRQs. However,
DOS tends to do only one thing at a time, so you can use different ports at different times
on the same IRQ, as long as the interrupts aren’t enabled on more than one of the ports at
a time. This restriction is unacceptable for UNIX, so we have to put them somewhere
else. The only unused interrupt available to 8-bit boards is IRQ 5, originally intended for

a second parallel printer port.
There’s a very good chance that IRQ 5 will already be occupied. What can you do? If
one of the boards has a 16-bit or better interface, you can check if one of the interrupts 10
to 15 is available. All EISA and PCI boards fit into this category, and so do ISA boards
with two connectors to the motherboard. Unfortunately, a lot of ISA serial cards only
have an 8-bit interface. The only alternative is an intelligent serial board that only
occupies a single interrupt. In this case, you will probably have to build a custom kernel.
See the man page sio(4).

Connecting to the port
Theoretically, a serial line can consist of only three wires: a Receive Data line, often
abbreviated to RxD, a Transmit Data line (TxD), and a Signal Ground line (SG). In fact,
it is possible to get a link to work like this, but there are a number of problems:

•

How do we know when the other end is able to accept data? It may be busy
processing data it has already received.

•

How do we know when it’s even switched on?

•

In the case of a modem, how do we know when it is connected to the modem at the
other end?

We solve these questions, and more, by the use of additional lines. The most common
standard is RS-232, also known as EIA-232, a standard for DCE to DTE connection. In

Europe, it is sometimes confused with the CCITT V.24 standard, though V.24 does not in
fact correspond exactly to RS-232. Most external modems display some of these signals
on LED, but modem manufacturers love to create alternative abbreviations for signal
names. Here are the signals that RS-232 defines, with some of the more common
abbreviations that you may see on external modems.

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Chapter 19: Serial communications

Table 19-1: RS-232 signals and modem LEDs
RS-232
name

Modem
LED

PG

Pin
1

TxD

2

TD D1

Transmitted data: data coming from the DTE to the
modem.

RxD

3

RD D2

Received data: data coming from the modem to the DTE.

RTS

4

Request to send. Indicates that the device has data to
output.

CTS

5

Clear to send. Indicates that the device can receive input.

DSR

6

SG

7

DCD

8

CD M5

Carrier detect. Indicates that the modem has connection
with another modem.

DTR

20

DTR S1

Data terminal ready. Indicates that the terminal or
computer is ready to talk to the modem.

RI

22

AA

Ring indicator. Raised by a modem to indicate that an

incoming call is ringing. The AA indicator on a modem will
usually flash when the incoming call is ringing.

AA

‘‘Auto Answer.’’ Indicates that the modem will answer an
incoming call.

HS

‘‘High Speed.’’ Indicates that the modem is running at a
higher speed than its minimum. Individual modems
interpret this differently, but you can assume that something
is wrong if your modem has this indicator and it’s off
during transmission.

MNP

Indicates that error correction is active.

OH

‘‘Off hook.’’ Indicates that the modem has some
connection with the phone line.

PW

Indicates that modem power is on. May or may not imply
DSR.

MR
ON

PW

Purpose
Protective ground. Used for electrical grounding only.

Data set ready. Indicates that the modem is powered on
and has passed self-test. On some modems, PW indicates
that power is on, and MR indicates that it is operative.
Signal ground. Return for the other signals.

The line DCD tells the DTE that the modem has established a connection. We’ll look at
how to use this information on page 335.
In addition to these signals, synchronous modems supply clocks on pins 17 and 19. For
more details about RS-232, see RS-232 Made easy by Martin Seyer.
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When can I send data?
There are two ways to determine if the other end is prepared to accept data: hardware
handshaking and software handshaking. Both are also referred to as flow control. In
each case, the handshaking is symmetrical. We’ll look at it from the point of view of the

DTE, because this is the more common viewpoint.
In hardware handshaking, the DCE raises CTS (Clear to Send) when it’s ready to accept
input. The DTE only transmits data when CTS is asserted from the other end. You’ll
often see that the DTE asserts RTS (Request to send) when it wants to send data. This is
a throwback to the days of half-duplex modems, which could only transmit in one
direction at a time: RTS was needed to switch the modem into send mode.
Software handshaking is also called X-on/X-off. The DCE sends a character (X-off, which
corresponds to Ctrl-S) when the buffer is full, and another (X-on, corresponding to CtrlQ) when there is space in the buffer again. You can also use this method on a terminal to
temporarily stop the display of a lot of data, and then restart it. It’s no longer a good
choice for modems.
For hardware handshake to work, your modem must be configured correctly, and you
must have the correct cables. If it isn’t, the symptoms will be very slow response when
transferring large quantities of data: at a higher level, TCP can recover from these
overruns, but it takes at least a second to do so every time. We’ll see how to check that
your modem has the correct kind of flow control on page 333.

Modems
A modem is a device that transfers digital data into a form suitable for transmission over a
transmission line, which is usually a telephone line. Telephone lines are limited to a
frequency of about 3.6 kHz, and this limited the speed of older modems to about 1200
bits per second. Modern modems use many sophisticated techniques to increase the
speed way beyond this. Current modems transmit at 56 kilobits per second.
Let’s consider the modem connection in the reference network on page 294, which is
repeated in figure 19-2. As we can see, there are three connections:

•

The connection from the router gw to the local modem, connected at 57,600 bits per
second.

•

The connection between the modems, at 56,000 bits per second.

•

The connection from the ISP’s modem to his router, at 115,200 bits per second.

You’ll also note another value specified here: the connection between the modems is
2,400 baud. Isn’t a baud the same thing as a bit per second? No, not always. The term
baud is a representation of the frequency of data on a serial line. On the connections
between the systems and the modem, which handle raw digital data, it corresponds to the
bit rate. On the modem line, it doesn’t. Here, it indicates that 2,400 units of data are sent
per second.
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Chapter 19: Serial communications

Unfortunately, many people use the term baud where bit rate should be used. This didn’t
make any difference in the old days with simple modems where the bit rate and baud rate
were the same, but nowadays it’s confusing.

•

DTE

DCE

DCE

DTE

gw
Router

•
•
modem
•
•
modem
•
•
ISP

Local modem connection, 57,600 bps
PPP link, 56,000 bps, 2400 baud
ISP modem connection, 115,200 bps

router

•

Figure 19-2: Network modem connection

Modem speeds
Two factors determine the data transmission speed of a modem: the protocol and the use
of data compression. Table 19-2 on page 332 gives an overview of modem protocols and
their speeds.
Currently, the most popular modem protocol is V.90. V.90 pushes modem technology to
the limit, and it only works when the other end of the link is a digital (ISDN) connection.
You can’t get a 56 kb/s connection with any kind of analogue modem at the other end.
As a result, they’re really only suitable for connection to a large ISP site. In addition, the
actual speed of the connection depends greatly on the telephone line quality, so the
difference between a V.90 and a V.34bis modem may not be as much as it appears.

Data compression
In addition, you usually have a choice of data compression: V.42bis or MNP-5. The
choice depends on what the modem at the other end of the line does. You can set most
modems to negotiate either protocol. These protocols include related error correction
standards, called V.42 or MNP2-4 respectively. If you believe the sales claims, these
compression protocols will give you up to 100% increase in data throughput. Consider
this the upper limit; a lot of data is binary, and when ftp’ing a typical gzipped tar archive,
you will probably get almost no speed improvement.

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Data compression has one negative side: it increases the data rate, but it also increases

latency, the time it takes for data to get from the local DTE to the remote DTE. The data
doesn’t take as long on the line, but it spends more time in the modems being compressed
and uncompressed. If you’re running a protocol like PPP that supplies optional
compression in the software, you may find it advantageous to turn off compression.
We’ll look at that again in Chapter 20, Configuring PPP.
Table 19-2: Modem protocols and speeds
Protocol
Bell 203
V.21
Bell 212
V.22
V.22bis
V.32
V.32bis
V.34
V.34bis
V.90

Speed (bps)
300
300
1200
1200
2400
9600
14400
28800
33600
56000

The link speed
The standard PC serial hardware can run at speeds that are a fraction of 115,200 bps (in
other words, 115200 divided by a small integer). This gives the following combinations:
Table 19-3: Serial line speeds
Divisor
1
2
3
4
5
6

Speed (bps)
115200
57600
38400
28800
23040
19200

You’ll notice that it can’t run at 33600 or 56000 bps. Also, looking at the example above,
you’ll note that all three links run at different speeds. How can that work? Only a few
years ago, it wouldn’t, but modern modems can buffer data. For example, the ISP can
send data to the modem far faster than the modem can send it to the other modem. It
stores the data in internal memory until it can be transmitted. This can also happen at the
other end. If you misconfigure your line so that the local link runs at 9600 bps, things
will still work, but of course the total speed is the speed of the slowest link, in this case
9600 bps.

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Chapter 19: Serial communications

This flexibility brings a problem with it: the modem can’t know in advance how fast the
connection to the computer is. It needs a way to find out. The modem solves the
question of local line speed by a trick: all commands start with AT or at (you’re not
allowed to mix cases, like aT or At). It can recognize these characters even if they arrive
at the wrong speed, and thus it can establish the speed of the connection.

Dialing out
Nowadays, all modems are capable of dialing. That wasn’t always the case, and in some
old documentation you may find references to an Auto-Call Unit or ACU, which is simply
the dialler part of a modem connected via a separate port. Typically, one ACU could
serve multiple modems.
Nearly every modern modem uses a command set designed by Hayes Corporation, which
is thus called the Hayes Command Set. We’ll look at it in the following section. It is also
sometimes called the AT command set, because nearly all the commands start with the
sequence AT. The CCITT also created an autodial recommendation, V.25, which was
occasionally implemented, but now appears to be dead.

Modem commands
Modern modems store their state in a number of registers, called S registers. The register
use varies somewhat from manufacturer to manufacturer, but most modems have a
number in common. They each store a one-byte value, ranging between 0 and 255.
Here’s a list of the more important ones for a Rockwell V.34 chip set. The name of the

chip set is not the same as the name of the modem. You’ll note that one of the commands
enables you to find out the chip set version, as we’ll see in the example that follows.
Table 19-4: Selected S registers
Register number
S0

Purpose
Number of rings until auto-answer. 0 disables auto-answer. Set to 0
for no automatic answer, or 1 for auto-answer.

S2

The escape character, which lets you return from online mode to
command mode. Normally, this character is a +. To return to
command mode, wait a second after any previous input, enter +++, and
wait a second, after which the modem should reply with OK.

S6

The time, in seconds, to wait before blind dialing. If you have set your
modem to not wait for a dial tone (maybe because it doesn’t
understand the dial tone), it will wait this long and then try to dial
anyway.

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Modem commands

Register number
S7

S11

334

Purpose
The number of seconds to wait after dialing before DCD must be
asserted (before a connection is established). If this is set too short,
you will not be able to establish a connection. If it’s too long, you will
waste time when there is no answer or the line is busy.
The duration of DTMF (dialing) tones. If these are set incorrectly, the
telephone exchange may not understand the number you dial.

The AT command set tells the modem to do something specific. Here are some of the
more important ones.
Table 19-5: Selected AT commands
Command
A/
ATA

Meaning
Redial the last number.
Answer an incoming call manually. This is an alternative to auto-answer
by setting S0.

ATDnumber

Dial number. This command has a large number of options, but if your
modem is set up correctly, you probably won’t need any of them.

ATEnumber

Enable command echo if number is 1, disable it if number is 0. The
setting of this parameter can be important for some chat scripts, which
may not respond correctly otherwise.

ATH0

Disconnect the line.

ATInumber

Display modem identification. The values of number vary from one
modem to the next. See the examples below.

ATLnumber

Set the speaker volume. number ranges from 0 to 3. 0 means ‘‘speaker
off,’’ 3 is the loudest.

ATMnumber

Determine when the speaker is on. 0 means ‘‘always off,’’ 1 means
‘‘speaker on until connect,’’ 2 means ‘‘speaker always on,’’ and 3 means
‘‘speaker off during dialing and receiving.’’

ATO0

Go back online from command mode. You don’t need this command
when dialing: the modem automatically goes online when the connection
is established.

ATP

Select pulse dial. If your exchange doesn’t understand DTMF (tone)
dialing, you should set this mode. Never use it if your exchange
understands DTMF: pulse dialing (also known as steam dialing) is much
slower.

ATQnumber

If number is 0, suppress result codes (like OK after every command). If
number is 1, enable them. This value can be of importance for chat
scripts.
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Chapter 19: Serial communications

ATSr=n

Meaning
Set the value of S register r to n.

ATSnumber?

Display the contents of an S register. See the example below.

ATT

Set tone (DTMF) dialing.

ATVnumber

If number is 0, return result codes in numeric form. If it’s 1, return text.
Don’t rely on either form to be consistent from one modem to the next.

ATXnumber

Determine the form of the result codes. This depends a lot on the
manufacturer, but it’s important for chat scripts. If you run into trouble,
with chat scripts, check your modem documentation.

ATZ

Reset modem configuration to default values.

AT&Knumber

Select flow control method. Normally, 3 enables RTS/CTS flow control,
which is what you want.

AT&Rnumber

If number is 0, CTS is only asserted if the DTE asserts RTS, even if the
modem is able to receive data. If it’s set to 1, it behaves normally. Make
sure this value is set to 1.

AT&Tnumber

Perform modem-specific test number. This command is the origin of the
statement: ‘‘UNIX is a trademark of AT&T in the USA and other
countries. AT&T is a modem test command.’’

AT&V

View the current configuration. See the example below.

AT&Wnumber

Store the current configuration as profile number. Most external
modems can store two profiles, or configurations. If number is not
specified, write the profile specified in a previous AT&Y command. See
the example below.
Decide which profile (0 or 1) will be loaded when the modem is reset,
and which will be written by the command AT&W

Command

AT&Ynumber

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 any more, 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
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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 = ˆO; dsusp = ˆY; eof = ˆD; eol = <undef>;
eol2 = <undef>; erase = ˆ?; intr = ˆC; kill = ˆU; lnext = ˆV;
min = 1; quit = ˆ\; reprint = ˆR; start = ˆQ; status = ˆT;
stop = ˆS; susp = ˆZ; time = 0; werase = ˆW;

This example, taken when the modem is connected, shows clocal reset. As you can see,
this is indicated by the text -clocal.
There’s a 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/cuaa1 for dialing out, and /dev/ttyd1 for dialing
in. If cuaa1 is open, an open on ttyd1 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 M1 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
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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
2=

1=
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 reflects
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 reflect the values of the
S registers. 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
2=

1=
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
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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:
show contents of S86
Connection dropped by other end

ats86?
012

With this background, we can now proceed to establish a connection:
just dial

atd1234567
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 flag 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 fine 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:
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# kill -1 1

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Chapter 19: Serial communications