An error messages could be something like "No modem detected",
"Modem not responding". There could be no error message. If you
have installed an internal modem (serial port is builtin) or are using
an external one and don't know what serial port it's connected to then
the problem is to find the serial port. See
My Serial Port is Physically There but Can't be Found. This
section is about finding out which serial port has the modem on it.
There's a program that looks for modems on commonly used serial ports
called "wvdialconf". Just type "wvdialconf <a-new-file-name>".
It will create the new file as a configuration file but you don't need
this file unless you are going to use "wvdial" for dialing.
What is wvdialconf ? Unfortunately, if
your modem is in "online data" mode, wvdialconf will report "No modem
No response to AT
Your problem could be due to a winmodem (or the like) which usually
can't be used with Linux. See
Software-based Modems (winmodems). The "setserial program may
be used to detect serial ports but will not detect modems on them.
Thus "wvdialconf" is best to try first.
Another way try to find out if there's a modem on a port is to start
"minicom" on the port (after first setting up minicom for the correct
serial port --you will need to save the setup and then exit minicom
and start it again). Then type "AT" and you should see "OK" (or 0 if
it's set for "digit result codes"). If you don't immediately see "OK"
Some strange characters appear but they are not in response to
AT. This likely means that your modem is still connected to something
at the other end of the phone line which is sending some cryptic
packets or the like.
No response to AT
The modem should send you "OK" in response to your "AT" which you
type to the modem (using minicom or the like). If you don't see "OK"
(and in most cases don't even see the "AT" you typed either) then the
modem is not responding (often because what you type doesn't even get
to the modem).
A common cause is that there is no modem on the serial port you are
typing to. For the case of an internal modem, that serial port likely
doesn't exist either. That's because the PnP modem card (which has a
built-in serial port) has either not been configured (by isapnp or the
like) or has been configured incorrectly. See
My Serial Port is Physically There but Can't be Found.
If what you type is really getting thru to a modem, then the lack of
response could be due to the modem being in "online data" mode where
it can't accept any AT commands. You may have been using the modem
and then abruptly disconnected (such as killing the process with
signal 9). In that case your modem did not get reset to "command
mode" where it can interact to AT commands. "Minicom" may display
"You are already online. Hangup first." (For another meaning of this
minicom message see
You are already online! Hang up first.)
Well, you are sort of online but you are may not be connected to
anything over the phone line. Wvdial will report "modem not
responding" for the same situation.
To fix this as a last resort you could reboot the computer. Another
way to try to fix this is to send +++ to the modem to tell it to
escape back to "command mode" from "online data mode". On both sides
of the +++ sequence there must be about 1 second of delay (nothing
sent during "guard time"). This may not work if another process is
using the modem since the +++ sequence could wind up with other
characters inserted in between them or after the +++ (during the guard
time). Ironically, even if the modem line is idle, typing an
unexpected +++ is likely to set off an exchange of control packets
(that you never see) that will violate the required guard time so
that the +++ doesn't do what you wanted. +++ is usually in the string
that is named "hangup string" so if you command minicom (or the like)
to hangup it might work. Another way to do this is to just exit
minicom and then run minicom again.
What this means depends on what program sent it. The modem could
actually be in use (busy). Another cause reported for the SuSE
distribution is that there may be two serial drivers present instead
of one. One driver was built into the kernel and the second was a
In kppp, this message is sent when an attempt to get/set the serial
port "stty" parameters fails. (It's similar to the "Input/output
error" one may get when trying to use "stty -F /dev/ttySx"). To get a
few of these stty parameters, the true address of the port must be
known to the driver. So the driver may have the wrong address. The
setserial" command will display what the driver thinks but it's likely
wrong in this case. So what the "modem busy" often means is that the
serial port (and thus the modem) can't be found.
If you have a pci modem, then use one of these commands: lspci, or cat
/proc/pci, or dmesg to find the correct address and irq of the
modem's serial port. Then check to see if "setserial" shows the same
thing. If not, you need to run a script at boot-time which contains a
setserial command that will tell the driver the correct address and
irq. The reason that the driver has it wrong may be due to failure of
the kernel to understand the lspci data correctly. You might notice
this in a boot-time message.
The modem has its CD signal on. Either you are actually online
(a remote modem is sending you a carrier) or your modem has been setup
to always send the CD signal. In minicom, type at&v to see if
&C or &C0 is set. If so, CD will be on even if you are
offline and you'll erroneously get this error message. The fix is to
set &C1 in the init string or save it in the modem.
There must be very low noise on the line for it to work at even
close to 56k. Some phone lines are so bad that the speeds obtainable
are much slower than 56k (like 28.8k or even slower). Sometimes
extension phones connected to the same line can cause problems. To
test this you might connect your modem directly at the point where the
telephone line enters the building with the feeds for everything else
on that line disconnected (if others can tolerate such a test).
Flow control (both at your PC and/or modem-to-modem) may not be
enabled. For the uploading case: If you have set a high DTE speed
(like 115.2k) then flow from your modem to your PC may work OK but
uploading flow in the other direction will not all get thru due to the
telephone line bottleneck. This will result in many errors and the
resending of packets. It may thus take far too long to send a file.
In some cases, files don't make it thru at all.
For the downloading case: If you're downloading long uncompressed
files or web pages (and your modem uses data compression) or if you've
set a low DTE speed, then downloading may also be broken due to no
Make sure you are using the correct syntax for your version of
init. The different init's that are out there use different
syntax in the /etc/inittab file. Make sure you are using the
correct syntax for your version of getty.
Id "S4" is just an example. In this case look on the line which
starts with "S4" in /etc/inittab and calls getty. This line
causes the problem. Make sure the syntax for this line is correct and
that the device (ttyS4) exists and can be found. If the modem has
negated CD and getty opens the port, you'll get this error message
since negated CD will kill getty. Then getty will respawn only to be
killed again, etc. Thus it respawns over and over (too fast). It
seems that if the cable to the modem is disconnected or you have the
wrong serial port, it's just like CD is negated. All this can occur
when your modem is chatting with getty. Make sure your modem is
configured correctly. Look at AT commands E and Q.
If you use uugetty, verify that your /etc/gettydefs syntax is
correct by doing the following:
This can happen when your modem doesn't reset when DTR is dropped.
Greg Hankins saw his RD and SD LEDs go crazy when this happened.
You need to have your modem reset. Most Hayes compatible modems do
this with &D3, but for USR Courier, SupraFax, and other
modems, you must set &D2 (and S13=1 for USR Courier).
Check your modem manual (if you have one).
It means exactly what it says. Someone else may be using another
telephone on the same line. You also get this error if there is no
phone line plugged into the modem, or if the phone line is somehow
broken. Try plugging a real telephone into the phone cord used by the
modem. Check it for a dialtone.
If for some reason your modem doesn't detect a dialtone, then you can
force it to dial anyway by putting X3 in the init string.
This means that the analog sine wave (the carrier) from the other
modem isn't present like it should be. If you were already connected,
this means that the connection has been lost. There may have been
noise on the line or a bad connection. The other modem may have hung
up on you for some reason: Perhaps the automatic login process
didn't work out OK. Perhaps PPP didn't get started OK. Perhaps a
time limit was exceeded.
If you get this error before you get connected, it means that the
carrier of the other modem wasn't detected by your modem. This may
happen if there is there is no properly working modem on the other
end. For example, an answering machine could have picked up your
call instead of a modem. NO CARRIER will also happen if the modems
fail to negotiate a protocol to use. This can happen if you have an
early V.90 modem that first tries to negotiate a high speed X2 or
K56flex protocol. These two protocols are obsolete and some ISP
servers will drop the connection (hang up) when this happens since
they have no understanding of such protocols and don't wait around
long enough for the calling modem to fallback to V.90.
If you force your modem to drop the connection by dropping the DTR
signal or sending your modem the hangup signal (ATH) you may get this
error message. But in this case you (or your software) wanted to drop
the connection so there should be no problem. In this case you are
only supposed to get NO CARRIER if data was lost. So for most cases
of dropping a connection by hangup (or by dropping DTR) you only get
an OK message. Your modem dialer program may not even display that to
There is a DEBUG option that comes with getty_ps. Edit your
add DEBUG=NNN. Where NNN is one of the following
combination of numbers according to what you are trying to debug:
If you are running syslogd, debugging info
will appear in your log files. If you aren't running syslogd
info will appear in /tmp/getty:ttySN for debugging
and /tmp/uugetty:ttySN for uugetty, and in
/var/adm/getty.log. Look at the
debugging info and see what is going on. Most likely, you will need
to tune some of the parameters in your config
file, and reconfigure your modem.
You could also try mgetty. Some people have better luck with
If a physical device (such as a modem) doesn't work at all it's
often because it's disabled and has no address (PnP hasn't enabled it)
or that it is enabled but is not at the I/O address that setserial
thinks it's at. Thus it can't be found.
First check BIOS messages at boot-time (and possibly the BIOS menu for
the serial port). Then for the PCI bus use lspci. If this shows
something like "LPC Bridge" then your port is likely on the LPC bus
which is not well supported by Linux yet (but the BIOS might find it)
?? If it's an ISA bus PnP serial port, try "pnpdump --dumpregs"
and/or see Plug-and-Play-HOWTO. If the port happens to be enabled
then the following two paragraphs may help find it:
Using "scanport" (Debian only ??) will scan all enabled bus ports and
may discover an unknown port that could be a serial port (but it
doesn't probe the port). It could hang your PC. You may try probing
with setserial. See
If nothing seems to get thru the port it may be accessible but have a
bad interrupt. See
Extremely Slow: Text appears on the screen slowly after long delays. Use setserial -g
to see what the serial driver thinks and check for IRQ and I0 address
conflicts. Even if you see no conflicts the driver may have incorrect
information (view it by "setserial") and conflicts may still exist.
If two ports have the same IO address then probing it will erroneously
indicate only one port. Plug-and-play detection will find both ports
so this should only be a problem if at least one port is not
plug-and-play. All sorts of errors may be reported/observed for
devices illegally "sharing" a port but the fact that there are two
devices on the same a port doesn't seem to get detected (except
hopefully by you). In the above case, if the IRQs are different then
probing for IRQs with setserial might "detect" this situation by
failing to detect any IRQ. See
It's likely mis-set/conflicting interrupts. Here are some of the
symptoms which will happen the first time you try to use a modem,
terminal, or serial printer. In some cases you type something but
nothing appears on the screen until many seconds later. Only the last
character typed may show up. It may be just an invisible
<return> character so all you notice is that the cursor jumps
down one line. In other cases where a lot of data should appear on
the screen, only a batch of about 16 characters appear. Then there is
a long wait of many seconds for the next batch of characters. You
might also get "input overrun" error messages (or find them in logs).
For more details on the symptoms and why this happens see
the Serial-HOWTO section: "Interrupt Problem Details".
If it involves Plug-and-Play devices, see also Plug-and-Play-HOWTO.
As a quick check to see if it really is an interrupt problem, set the
IRQ to 0 with "setserial". This will tell the driver to use
polling instead of interrupts. If this seems to fix the "slow"
problem then you had an interrupt problem. You should still try to
solve the problem since polling uses excessive computer resources.
Checking to find the interrupt conflict may not be easy since Linux
supposedly doesn't permit any interrupt conflicts and will send you a
/dev/ttyS?: Device or resource busy error
message if it thinks you are attempting to create a conflict. But a
real conflict can be created if "setserial" has told the kernel
incorrect info. The kernel has been lied to and thus doesn't think
there is any conflict. Thus using "setserial" will not reveal the
conflict (nor will looking at /proc/interrupts which bases its info on
"setserial"). You still need to know what "setserial" thinks so that
you can pinpoint where it's wrong and change it when you determine
what's really set in the hardware.
What you need to do is to check how the hardware is set by checking
jumpers or using PnP software to check how the hardware is actually
set. For PnP run either "pnpdump --dumpregs" (if ISA bus) or run
"lspci" (if PCI bus). Compare this to how Linux (e.g. "setserial")
thinks the hardware is set.
An obvious reason is that the baud rate is actually set too slow.
It's claimed that this happened by trying to set the baud rate to a speed
higher than the hardware can support (such as 230400).
Another reason may be that whatever is on the serial port (such as a
modem, terminal, printer) doesn't work as fast as you thought it did.
A 56k Modem seldom works at 56k and the Internet
often has congestion and bottlenecks that slow things down. If the
modem on the other end does not have a digital connection to the phone
line (and uses a special "digital modem" not sold in most computer
stores), then speeds above 33.6k are not possible.
Another possible reason is that you have an obsolete serial port: UART
8250, 16450 or early 16550 (or the serial driver thinks you do). See
"What are UARTS" in the Serial-HOWTO.
Use "setserial -g /dev/ttyS*".
If it shows anything less than a 16550A, this may be your problem.
If you think that "setserial" has it wrong check it out. See
What is Setserial for more info. If you
really do have an obsolete serial port, lying about it to setserial
will only make things worse.
For non-PnP ports, Linux does not do any IRQ detection on startup.
When the serial module loads it only does serial device detection.
Thus, disregard what it says about the IRQ, because it's just assuming
the standard IRQs. This is done, because IRQ detection is unreliable,
and can be fooled. But if and when setserial runs from a start-up
script, it changes the IRQ's and displays the new (and hopefully
correct) state on on the startup screen. If the wrong IRQ is not
corrected by a later display on the screen, then you've got a problem.
So, even though I have my ttyS2 set at IRQ 5, I still see
ttyS02 at 0x03e8 (irq = 4) is a 16550A
at first when Linux boots. (Older kernels may show "ttyS02" as
"tty02" which is the same as ttyS2). You may need to use
setserial to tell Linux the IRQ you are using.
Check the file permissions on this port with "ls -l /dev/ttyS?"_
If you own the ttyS? then you need read and write permissions: crw
with the c (Character device) in col. 1. It you don't own it then it
will work for you if it shows rw- in cols. 8 & 9 which means that
everyone has read and write permission on it. Use "chmod" to change
permissions. There are more complicated (and secure) ways to get
access like belonging to a "group" that has group permission. Some
programs change the permissions when they run but restore them when
the program exists normally. But if someone pulls the plug on your
PC it's an abnormal exit and correct permissions may not be restored.
This means that an operation requested by setserial, stty, etc.
couldn't be done because the kernel doesn't support doing it.
Formerly this was often due to the "serial" module not being loaded.
But with the advent of PnP, it may likely mean that there is no modem
(or other serial device) at the address where the driver (and
setserial) thinks it is. If there is no modem there, commands (for
operations) sent to that address obviously don't get done. See
What is set in my serial port hardware?
If the "serial" module wasn't loaded but "lsmod" shows you it's now
loaded it might be the case that it's loaded now but wasn't loaded
when you got the error message. In many cases the module will
automatically loaded when needed (if it can be found). To force
loading of the "serial" module it may be listed in the file:
/etc/modules.conf or /etc/modules. The actual module should reside
When a port is "opened" by a program a lockfile is created in
/var/lock/. Wrong permissions for the lock directory will not allow a
lockfile to be created there. Use "ls -ld /var/lock" to see if the
permissions are OK. Giving rwx permissions for the root owner and the
group should work, provided that the users that need to dialout belong
to that group. Others should have r-x permission. Even with this
scheme, there may be a security risk. Use "chmod" to change
permissions and "chgrp" to change groups. Of course, if there is no
"lock" directory no lockfile can be created there. For more info on
lockfiles see the Serial-HOWTO subsection: "What Are Lock
This means that someone else (or some other process) is supposedly
using the serial port. There are various ways to try to find out what
process is "using" it. One way is to look at the contents of the
lockfile (/var/lock/LCK...). It should be the process id. If the
process id is say 100 type "ps 100" to find out what it is. Then if
the process is no longer needed, it may be gracefully killed by "kill
100". If it refuses to be killed use "kill -9 100" to force it to be
killed, but then the lockfile will not be removed and you'll need to
delete it manually. Of course if there is no such process as 100 then
you may just remove the lockfile but in most cases the lockfile should
have been automatically removed if it contained a stale process id
(such as 100).
This means that the device you are trying to access (or use) is
supposedly busy (in use) or that a resource it needs (such as an IRQ)
is supposedly being used by another device and can't be shared.
This message is easy to understand if it only means that the device is
busy (in use). But it sometimes means that a needed resource is already
in use (busy). What makes it even more confusing is that in some cases
neither the device nor the resources that it needs are actually
In olden days, if a PC was shutdown by just turning off the power, a
bogus lockfile might remain and then later on one would get this bogus
message and not be able to use the serial port. Software today is
supposed to automatically remove such bogus lockfiles, but as of 2003
there is still a problem with the "wvdial" dialer program related to
lockfiles. If wvdial can't create a lockfile because it doesn't have
write permission in the /var/lock/ directory, you will see this
The following example is where interrupts can't be shared (at least
one of the interrupts is on the ISA bus). The ``resource busy'' part
often means (example for ttyS2) ``You can't use ttyS2 since
another device is using ttyS2's interrupt.'' The potential interrupt
conflict is inferred from what "setserial" thinks. A more accurate
error message would be ``Can't use ttyS2 since the setserial data
(and kernel data) indicates that another device is using ttyS2's
interrupt''. If two devices use the same IRQ and you start up only
one of the devices, everything is OK because there is no conflict yet.
But when you next try to start the second device (without quitting the
first device) you get a "... busy" error message. This is because the
kernel only keeps track of what IRQs are actually in use and actual
conflicts don't happen unless the devices are in use (open). The
situation for I/O address (such as 0x3f8) conflict is similar.
This error is sometimes due to having two serial drivers: one a module
and the other compiled into the kernel. Both drivers try to grab the
same resources and one driver finds them "busy".
There are two possible cases when you see this message:
There may be a real resource conflict that is being avoided.
Setserial has it wrong and the only reason ttyS2 can't be
used is that setserial erroneously predicts a conflict.
What you need to do is to find the interrupt setserial thinks
ttyS2 is using. Look at /proc/tty/driver/serial. You should
also be able to find it with the "setserial" command for ttyS2.
Bug in old versions: Prior to 2001 there was a bug which wouldn't let
you see it with "setserial". Trying to see it would give the same
"... busy" error message.
To try to resolve this problem reboot or: exit or gracefully kill all
likely conflicting processes. If you reboot: 1. Watch the boot-time
messages for the serial ports. 2. Hope that the file that runs
"setserial" at boot-time doesn't (by itself) create the same conflict
If you think you know what IRQ say ttyS2 is using then you may
look at /proc/interrupts to find what else (besides another serial
port) is currently using this IRQ. You might also want to double
check that any suspicious IRQs shown here (and by "setserial") are
correct (the same as set in the hardware). A way to test whether or
not it's a potential interrupt conflict is to set the IRQ to 0
(polling) using "setserial". Then if the busy message goes away, it
was likely a potential interrupt conflict. It's not a good idea to
leave it permanently set at 0 since it will put more load on the CPU.
This means that communication with the serial port isn't working
right. It could mean that there isn't any serial port at the IO
address that setserial thinks your port is at. It could also be an
interrupt conflict (or an IO address conflict). It also may mean that
the serial port is in use (busy or opened) and thus the attempt to
get/set parameters by setserial or stty failed. It will also happen
if you make a typo in the serial port name such as typing "ttys"
instead of "ttyS".
This is an overrun of the hardware FIFO buffer and you can't
increase its size. Bug note (reported in 2002): Due to a bug in some
kernel 2.4 versions, the port number may be missing and you will only
see "ttyS" (no port number). But if devfs notation such as "tts/2" is
being used, there is no bug. See
"Higher Serial Thruput" in the Serial-HOWTO.
This paragraph is for the case where a modem is used for both
dial-in and dial-out. If the modem generates a DCD (=CD) signal, some
programs (but not mgetty) will think that the modem is busy.
This will cause a problem when you are trying to dial out with a modem
and the modem's DCD or DTR are not implemented correctly. The modem
should assert DCD only when there is an actual connection (ie someone
has dialed in), not when getty is watching the port. Check to
make sure that your modem is configured to only assert DCD when there
is a connection (&C1). DTR should be on (asserted) by the
communications program whenever something is using, or watching the
line, like getty, kermit, or some other comm program.
There could be some other program running on the port. Use "top"
(provided you've set it to display the port number) or type "ps
-alxw". Look at the results to see if the port is being used by
another program. Be on the lookout for the gpm mouse program which
often runs on a serial port.