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Modem-HOWTO : Locating the Serial Port: IO address, IRQs Next Previous Contents

6. Locating the Serial Port: IO address, IRQs

6.1 IO & IRQ Overview

For a serial port to work properly, it must have both an IRQ and an IO address. Without an IO address, it can't be located and will not work at all. Without an IRQ it will need to use inefficient polling methods for which one must set the IRQ to 0. So every serial port needs an IO address and IRQ. In olden days this was set by jumpers on a serial port card. Today it's set by digital signals sent to the hardware and this is part of "Plug-and-Play (PnP).

The driver must also know both the IO address and IRQ so that it can locate the card. Modern drivers (kernel 2.4) determine this by PnP methods so one doesn't need to tell them (by using "setserial"). The driver uses PnP functions provided by Linux to determine what the IO and IRQ are and to set them if necessary. The driver also probes possible serial port addresses to see if there are any serial ports there. This works for the case of jumpers and sometimes works for a PnP port when the driver doesn't do PnP.

Locating the serial port by giving it an IRQ and IO address is low-level configuring. What follows repeats what was said above in more detail. This low-level configuring consists of assigning an IO address, IRQ, and name (such as ttyS2). This IO-IRQ pair must be set in both the hardware and told to the serial driver. We could call this "io-irq" configuring for short. The "setserial" program is one way to tell the driver. The other way is for the driver to use PnP methods to determine/set the IO/IRQ and then remember what it did. For jumpers you must always use "setserial". If you need to configure but don't understand certain details it's easy to get into trouble.

When Linux starts, some effort is made to detect and configure (low-level) a few serial ports. Exactly what happens depends on your BIOS, hardware, Linux distribution, etc. If the serial ports work OK, there may be no need for you to do any more low-level configuring.

If you're having problems with the serial ports, then you may need to do low-level configuring. If you have kernel 2.2 or lower, then you need to do it if you:

  • Plan to use more than 2 ISA serial ports
  • Are installing a new serial port (such as an internal modem)

For kernel 2.2+ you may be able to use more that 2 serial ports without doing any low-level configuring by sharing interrupts. All PCI cards should support this but for ISA it only works for some hardware. It may be just as easy to give each port a unique interrupt if they is available. See Interrupt sharing and Kernels 2.2+

The low-level configuring (setting the IRQ and IO address) seems to cause people more trouble than the high-level, although for many it's fully automatic and there is no configuring to be done. Until the serial driver knows the correct IRQ and IO address, the port will not usually not work at all. Also, PnP ports can be disabled so that they can't be found (except with PnP tools). Applications, and utilities such as "setserial" and "scanport" don't use PnP tools and thus can't detect disabled ports. For example, an IO enabling bit must be set in PCI serial port hardware by PnP so that it's IO address may be used.

Even if the port can be found by normal (non-PnP) software, it may work extremely slow if the IRQ is wrong. See Extremely Slow: Text appears on the screen slowly after long delays.

In the Wintel world, the IO address and IRQ are called "resources" and we are thus configuring certain resources. But there are many other types of "resources" so the term has many other meanings. In summary, the low-level configuring consists of enabling the device, giving it a name (ttyS2 for example) and putting two values (an IRQ number and IO address) into two places:

  1. the device driver (often by running "setserial" at boot-time)
  2. memory registers of the serial port hardware itself

You may watch the start-up (= boot-time) messages. They are usually correct. But if you're having problems, there's a good chance that the ones that look like "setserial" output don't show the true configuration of the hardware (and they are not necessarily supposed to). See I/O Address & IRQ: Boot-time messages.

6.2 PCI Bus Support

If you have kernel 2.4, then there should be support for PnP (either built-in or by modules). Some PCI serial cards can be automatically detected and low-level configured by the serial driver. Others will not be. Kernel 2.2 had no support for PCI serial ports (although some people got them working anyway). The 2.4 serial driver will read the id number digitally stored on the serial hardware to determine how to support it (if it knows how). It should assign the I/O address to it, determine it's IRQ, etc. So you don't need to use "setserial" for it ??

If you have a

but it will not work because the latest serial driver doesn't support it, you can help in attempting to create a driver for it. To do this you'll need to contact the maintainer of the serial driver, Theodore (Ted) Y. Ts'o.

Look at Ted Ts'o's site for the details of what you need to do. Here's a summary of what you need to do to help him. You will need to email Ted Ts'o a copy of the output of "lspci -vv" with full information about the model and manufacturer of the PCI modem (or serial port). Then he will try to point you to a test driver which might work for it. You will then need to get it, compile it and possibly recompile your kernel. Then you will test the driver to see if it works OK for you and report the results to Ted Ts'o. If you are willing to do all the above (and this is the latest version of this HOWTO) then email the needed info to him at: mailto:tytso@mit.edu.

PCI ports are not well standardized. Some use main memory for communication with the PC. Some require special enabling of the IRQ. The output of "lspci -vv" can help determine if one can be supported. If you see a 4-digit IO port, the port might work by just telling "setserial" the IO port and the IRQ. For example, if lspci shows IRQ 10, I/O at 0xecb8 and you decide to name it ttyS2 then the command is:

setserial /dev/ttyS2 irq 10 port 0xecb8 autoconfig

Note that the boot-time message "Probing PCI hardware" means reading the PnP configuration registers in the PCI cards which reveals the IO addresses and IRQs. This is different that the probing of IO addresses by the serial driver which means reading certain IO addresses to see if what's read looks like there's a serial port at that address.

6.3 Common mistakes made re low-level configuring

Here are some common mistakes people make:

  • setserial command: They run it (without the "autoconfig" and auto_irq options) and think it has checked the hardware to see if what it shows is correct (it hasn't).
  • setserial messages: They see them displayed on the screen at boot-time (or by giving the setserial command) and erroneously think that the result always shows how their hardware is actually configured.
  • /proc/interrupts: When their serial device isn't in use they don't see its interrupt there, and erroneously conclude that their serial port can't be found (or doesn't have an interrupt set).
  • /proc/ioports and /proc/tty/driver/serial: People think this shows the actual hardware configuration when it only shows about the same info (possibly erroneous) as setserial.

6.4 IRQ & IO Address Must be Correct

There are really two answers to the question "What is my IO and IRQ?" 1. What the device driver thinks has been set (This is what setserial usually sets and shows.). 2. What is actually set in the hardware. Both 1. and 2. above should be the same. If they're not it spells trouble since the driver has incorrect info on the physical serial port. In some cases the hardware is disabled so it has no IO address or IRQ.

If the driver has the wrong IO address it will try to send data to a non-existing serial port --or even worse, to some other device. If it has the wrong IRQ the driver will not get interrupt service requests from the serial port, resulting in a very slow or no response. See Extremely Slow: Text appears on the screen slowly after long delays. If it has the wrong model of UART there is also apt to be trouble. To determine if both I0-IRQ pairs are identical you must find out how they are set in both the driver and the hardware.

6.5 What is the IO Address and IRQ per the driver ?


What the driver thinks is not necessarily how the hardware is actually set. If everything works OK then what the driver thinks is likely correct (set in the hardware) and you don't need to investigate (unless you're curious or want to become a guru). Ways to determine what the driver thinks include: boot-time messages I/O Address & IRQ: Boot-time messages, the /proc directory "files" The /proc directory and setserial, and the "setserial" command.

I/O Address & IRQ: Boot-time messages

In many cases your ports will automatically get low-level configured at boot-time (but not always correctly). To see what is happening, look at the start-up messages on the screen. Don't neglect to check the messages from the BIOS before Linux is loaded (no examples shown here). These BIOS messages may be frozen by pressing the Pause key. Use Shift-PageUp to scroll back to the messages after they have flashed by. Shift-PageDown will scroll in the opposite direction. The dmesg command (or looking at logs in /var/log) will show some of the messages but they seem to miss important ones from setserial. Here's an example of the start-up messages (as of mid 1999). Note that ttyS00 is the same as /dev/ttyS0.

At first you see what was detected (but the irq is only a wild guess):

Serial driver version 4.27 with no serial options enabled
ttyS00 at 0x03f8 (irq = 4) is a 16550A
ttyS01 at 0x02f8 (irq = 3) is a 16550A
ttyS02 at 0x03e8 (irq = 4) is a 16550A

Later setserial shows you  what was saved, but it's not necessarily
correct either:

Loading the saved-state of the serial devices...
/dev/ttyS0 at 0x03f8 (irq = 4) is a 16550A
/dev/ttyS1 at 0x02f8 (irq = 3) is a 16550A
/dev/ttyS2 at 0x03e8 (irq = 5) is a 16550A

Note that there is a slight disagreement: The first message shows ttyS2 at irq=4 while the second shows it at irq=5. dmesg may not display the second message. In most cases the second message is the correct one. But if your having trouble it may be misleading. Before reading the explanation of all of this complexity in the rest of this section, you might just try using your serial port and see if it works OK. If so it may not be essential to read further.

The second message is from the setserial program being run at boot-time. It shows what the device driver thinks is the correct configuration. But this too could be wrong. For example, the irq could actually be set to irq=8 in the hardware (both messages wrong). The irq=5 could be there because someone incorrectly put this into a configuration file (or the like). The fact that Linux sometimes gets IRQs wrong is because it doesn't by default probe for IRQs. It just assumes the "standard" ones (first message) or accepts what you told it when you configured it (second message). Neither of these is necessarily correct. If the serial driver has the wrong IRQ, the serial port is very slow or doesn't seem to work at all.

The first message is a result of Linux probing the serial port addresses but it doesn't probe for IRQs. If a port shows up here it exists but the IRQ may be wrong. Linux doesn't check IRQs because doing so is not foolproof. It just assumes the IRQs are as shown because they are the "standard" values. Your may check them manually with setserial using the autoconfig and auto_irq options but this isn't guaranteed to be correct either.

The data shown by the BIOS messages (which you see at first before Linux is booted) is what is initially set in the hardware. If your serial port is Plug-and-Play (PnP) then it's possible that "isapnp" or "setpci" will run and change these settings. Look for messages about this after Linux starts. The last serial port message shown in the example above should agree with the BIOS messages (as possibly modified by isapnp or setpci). If they don't agree then you either need to change the setting in the port hardware or use setserial to tell the driver what is actually set in the hardware.

Also, if you have Plug-and-Play (PnP) serial ports, they can only be found by PnP software unless the IRQ and IO has been set inside the hardware by Plug-and-Play software. Prior to kernel 2.4 this was a common reason why the start-up messages did not show a serial port that physically exists. A PnP BIOS may automatically low-level configure them. PnP configuring will be explained later.

The /proc directory and setserial

Type "setserial -g /dev/ttyS*". There are some other ways to find this info by looking at "files" in the /proc directory. Be warned that there is no guarantee that the same is set in the hardware.

/proc/ioports will show the IO addresses that the drivers are using. /proc/interrupts shows the IRQs that are used by drivers of currently running processes (that have devices open). It shows how many interrupts have actually be issued. /proc/tty/driver/serial shows much of the above, plus the number of bytes that have been received and sent (even if the device is not now open).

Note that for the IO addresses and IRQ assignments, you are only seeing what the driver thinks and not necessarily what is actually set in the hardware. The data on the actual number of interrupts issued and bytes processed is real however. If you see a large number of interrupts and/or bytes then it probably means that the device is (or was) working. But the interrupts might be from another device. If there are no bytes received (rx:0) but bytes were transmitted (tx:3749 for example), then only one direction of flow is working (or being utilized).

Sometimes a showing of just a few interrupts doesn't mean that the interrupt is actually being physically generated by any serial port. Thus if you see almost no interrupts for a port that you're trying to use, that interrupt might not be set in the hardware. To view /proc/interrupts to check on a program that you're currently running (such as "minicom") you need to keep the program running while you view it.

6.6 What is the IO Address & IRQ of my Serial Port Hardware?


If it's PCI or ISA PnP then what's set in the hardware has been done by PnP methods. Even if nothing has been set or the port disabled, PnP ports may still be found by using "lspci -v" or "isapnp --dumpregs". Ports disabled by jumpers (or hardware failures) are lost. See ISA PnP ports, PCI: What IOs and IRQs have been set?, PCI: Is the port enabled?

PnP ports don't store their configuration in the hardware when the power is turned off. This is in contrast to Jumpers (non-PnP) which remain the same with the power off. That's why a PnP port is more likely to be found in a disabled state than an old non-PnP one.

PCI: What IOs and IRQs have been set?

For PCI, the BIOS almost always sets the IRQ and may set the IO address as well. To see how it's set use "lspci -vv" (best) or look in /proc/bus/pci (or for kernels <2.2 /proc/pci). The modem's serial port is often called a "Communication controller". If more than one IO address is shown, the first one is more likely to be it. You can't change the IRQ (at least not with "setpci") This is because if one writes it in it's hardware register no action is taken on it. It's the BIOS that should actually set up the IRQs and then write the correct value to this register for lspci to view. If you must, change the IO address with "setpci" by changing the BASE_ADDRESS_0 or the like. The _0 (or _1) after BASE_ADDRESS must be the correct register.

PCI: Is the port enabled?

If the port communicates via an IO address "then lspci -vv" should show "Control: I/O+ ..." with + meaning that the IO address is enabled. If it shows "I/O-" then you may need to use the setpci command to enable it. For example "setpci -d 151f:000 command=101". The "command" means the command register which is the same as the "Control" register displayed by lspci. The 101h sets two bits: the 1 sets I/O to + and the 100 part keeps SERR# set to +. In this case only the SERR# bit of the Control register was initially observed to be + when the lspci command was run. So we kept it enabled to + by setting bit 8 (where bit 0 is I/O) to 1 by the first 1 in 101. My apologies if setting bits is confusing. Bit 8 is actually the 9th bit since we started counting bits from 0.

ISA PnP ports

For an ISA Plug-and-Play (PnP) port one may try the pnpdump program (part of isapnptools). If you use the --dumpregs option then it should tell you the actual IO address and IRQ set in the port. It should also find an ISA PnP port that is disabled. The address it "trys" is not the device's IO address, but a special address used for communicating with PnP cards.

Finding a port that is not disabled (ISA, PCI, PnP, non-PnP)

Perhaps the BIOS messages will tell you some info before Linux starts booting. Use the shift-PageUp key to step back thru the boot-time messages and look at the very first ones which are from the BIOS. This is how it was before Linux started. Setserial can't change it but isapnp or setpci can. Starting with kernel 2.4, these are built into the serial driver.

Using "scanport" will probe all I/O ports and will indicate what it thinks may be serial port. After this you could try probing with setserial using the "autoconfig" option. You'll need to guess the addresses to probe at (using clues from "scanport"). See What is Setserial.

For a port set with jumpers, the IO ports and IRQs are set per the jumpers. If the port is not Plug-and-Play (PnP) but has been setup by using a DOS program, then it's set at whatever the person who ran that program set it to.

Exploring via MS Windows (a last resort)

For PnP ports, checking on how it's configured under DOS/Windows may (or may not) imply how it's under Linux. MS Windows stores its configuration info in its Registry which is not used by Linux so they are not necessarily configured the same. If you let a PnP BIOS automatically do the configuring when you start Linux (and have told the BIOS that you don't have a PnP operating system when starting Linux) then Linux should use whatever configuration is in the BIOS's non-volatile memory. Windows also makes use of the same non-volatile memory but doesn't necessarily configure it that way.

6.7 Choosing Serial IRQs

If you have Plug-and-Play ports then either a PnP BIOS or a serial driver may configure all your devices for you so then you may not need to choose any IRQs. PnP software determines what it thinks is best and assigns them (but it's not always best). But if you directly use isapnp (ISA bus) or jumpers then you have to choose. If you already know what IRQ you want to use you could skip this section except that you may want to know that IRQ 0 has a special use (see the following paragraph).

IRQ 0 is not an IRQ

While IRQ 0 is actually the timer (in hardware) it has a special meaning for setting a serial port with setserial. It tells the driver that there is no interrupt for the port and the driver then will use polling methods. Such polling puts more load on the CPU but can be tried if there is an interrupt conflict or mis-set interrupt. The advantage of assigning IRQ 0 is that you don't need to know what interrupt is set in the hardware. It should be used only as a temporary expedient until you are able to find a real interrupt to use.

Interrupt sharing, Kernels 2.2+

Sharing of IRQs is where two devices use the same IRQ. As a general rule, this wasn't allowed for the ISA bus. The PCI bus may share IRQs but one can't share the same IRQ between the ISA and the PCI bus. Most multi-port boards may share IRQs. Sharing is not as efficient since every time a shared interrupt is given a check must be made to determine where it came from. Thus if it's feasible, it's nicer to allocate every device its own interrupt.

Prior to kernel 2.2, serial IRQs could not be shared with each other except for most multiport boards. Starting with kernel 2.2 serial IRQs may be sometimes shared between serial ports. In order for sharing to work in 2.2 the kernel must have been compiled with CONFIG_SERIAL_SHARE_IRQ, and the serial port hardware must support sharing (so that if two serial cards put different voltages on the same interrupt wire, only the voltage that means "this is an interrupt" will prevail). Since the PCI bus specs permit sharing, any PCI card should allow sharing.

What IRQs to choose?

The serial hardware often has only a limited number of IRQs. Also you don't want IRQ conflicts. So there may not be much of a choice. Your PC may normally come with ttyS0 and ttyS2 at IRQ 4, and ttyS1 and ttyS3 at IRQ 3. Looking at /proc/interrupts will show which IRQs are being used by programs currently running. You likely don't want to use one of these. Before IRQ 5 was used for sound cards, it was often used for a serial port.

Here is how Greg (original author of Serial-HOWTO) set his up in /etc/rc.d/rc.serial. rc.serial is a file (shell script) which runs at start-up (it may have a different name or location). For versions of "setserial" after 2.15 it's not always done this way anymore but this example does show the choice of IRQs.

/sbin/setserial /dev/ttyS0 irq 3        # my serial mouse
/sbin/setserial /dev/ttyS1 irq 4        # my Wyse dumb terminal
/sbin/setserial /dev/ttyS2 irq 5        # my Zoom modem
/sbin/setserial /dev/ttyS3 irq 9        # my USR modem

Standard IRQ assignments:

        IRQ  0    Timer channel 0 (May mean "no interrupt".  See below.)
        IRQ  1    Keyboard
        IRQ  2    Cascade for controller 2
        IRQ  3    Serial port 2
        IRQ  4    Serial port 1
        IRQ  5    Parallel port 2, Sound card
        IRQ  6    Floppy diskette
        IRQ  7    Parallel port 1
        IRQ  8    Real-time clock
        IRQ  9    Redirected to IRQ2
        IRQ 10    not assigned
        IRQ 11    not assigned
        IRQ 12    not assigned
        IRQ 13    Math coprocessor
        IRQ 14    Hard disk controller 1
        IRQ 15    Hard disk controller 2

There is really no Right Thing to do when choosing interrupts. Try to find one that isn't being used by the motherboard, or any other boards. 2, 3, 4, 5, 7, 10, 11, 12 or 15 are possible choices. Note that IRQ 2 is the same as IRQ 9. You can call it either 2 or 9, the serial driver is very understanding. If you have a very old serial board it may not be able to use IRQs 8 and above.

Make sure you don't use IRQs 1, 6, 8, 13 or 14! These are used by your motherboard. You will make her very unhappy by taking her IRQs. When you are done you might want to double-check /proc/interrupts when programs that use interrupts are being run and make sure there are no conflicts.

6.8 Choosing Addresses --Video card conflict with ttyS3

Here's a problem with some old serial cards. The IO address of the IBM 8514 video board (and others like it) is allegedly 0x?2e8 where ? is 2, 4, 8, or 9. This may conflict with the IO address of ttyS3 at 0x02e8. Your may think that this shouldn't happen since the addresses are different in the high order digit (the leading 0 in 02e8). You're right, but a poorly designed serial port may ignore the high order digit and respond to any address that ends in 2e8. That is bad news if you try to use ttyS3 (ISA bus) at this IO address.

For the ISA bus you should try to use the default addresses shown below. PCI cards use different addresses so as not to conflict with ISA addresses. The addresses shown below represent the first address of an 8-byte range. For example 3f8 is really the range 3f8-3ff. Each serial device (as well as other types of devices that use IO addresses) needs its own unique address range. There should be no overlaps (conflicts). Here are the default addresses for commonly used serial ports on the ISA bus:

ttyS0 address 0x3f8
ttyS1 address 0x2f8
ttyS2 address 0x3e8
ttyS3 address 0x2e8

Suppose there is an address conflict (as reported by setserial -g /dev/ttyS*) between a real serial port and another port which does not physically exist (and shows UART: unknown). Such a conflict shouldn't cause problems but it sometimes does in older kernels. To avoid this problem don't permit such address conflicts or delete /dev/ttySx if it doesn't physically exist.

6.9 Set IO Address & IRQ in the hardware (mostly for PnP)

After it's set in the hardware don't forget to insure that it also gets set in the driver by using setserial. For non-PnP serial ports they are either set in hardware by jumpers or by running a DOS program ("jumperless") to set them (it may disable PnP). The rest of this subsection is only for PnP serial ports. Here's a list of the possible methods of configuring PnP serial ports:

  • Using a PnP BIOS CMOS setup menu (usually only for external devices on ttyS0 (Com1) and ttyS1 (Com2))
  • Letting a PnP BIOS automatically configure a PnP serial port See Using a PnP BIOS to I0-IRQ Configure
  • Doing nothing if the serial driver recognized your card OK
  • Using isapnp for a PnP serial port non-PCI)
  • Using setpci (pciutils or pcitools) for the PCI bus

The IO address and IRQ must be set (by PnP) in their registers each time the system is powered on since PnP hardware doesn't remember how it was set when the power is shut off. A simple way to do this is to let a PnP BIOS know that you don't have a PnP OS and the BIOS will automatically do this each time you start. This might cause problems in Windows (which is a PnP OS) if you start Windows with the BIOS thinking that Windows is not a PnP OS. See Plug-and-Play-HOWTO.

Plug-and-Play (PnP) was designed to automate this io-irq configuring, but for Linux it initially made life much more complicated. In modern Linux (2.4 kernels --partially in 2.2 kernels), each device driver has to do it's own PnP (using supplied software which it may utilize). There is unfortunately no centralized planning for assigning IO addresses and IRQs as there is in MS Windows. But it usually works out OK in Linux anyway.

Using a PnP BIOS to I0-IRQ Configure

While the explanation of how to use setpci or isapnp for io-irq configuring should come with such software, this is not the case if you want to let a PnP BIOS do such configuring. Not all PnP BIOS can do this. The BIOS usually has a CMOS menu for setting up the first two serial ports. This menu may be hard to find. For an "Award" BIOS it was found under "chipset features setup" There is often little to choose from. Unless otherwise indicated in a menu, these first two ports normally get set at the standard IO addresses and IRQs. See Serial Port Device Names & Numbers

Whether you like it or not, when you start up a PC a PnP BIOS starts to do PnP (io-irq) configuring of hardware devices. It may do the job partially and turn the rest over to a PnP OS (which Linux is in some sense) or if thinks you don't have a PnP OS it may fully configure all the PnP devices but not configure the device drivers. This is what you want but it's not always easy to figure out exactly what the PnP BIOS has done.

If you tell the BIOS that you don't have a PnP OS, then the PnP BIOS should do the configuring of all PnP serial ports --not just the first two. An indirect way to control what the BIOS does (if you have Windows 9x on the same PC) is to "force" a configuration under Windows. See Plug-and-Play-HOWTO and search for "forced". It's easier to use the CMOS BIOS menu which may override what you "forced" under Windows. There could be a BIOS option that can set or disable this "override" capability.

If you add a new PnP device, the BIOS should PnP configure it. It could even change the io-irq of existing devices if required to avoid any conflicts. For this purpose, it keeps a list of non-PnP devices provided that you have told the BIOS how these non-PnP devices are io-irq configured. One way to tell the BIOS this is by running a program called ICU under DOS/Windows.

But how do you find out what the BIOS has done so that you set up the device drivers with this info? The BIOS itself may provide some info, either in its setup menus of via messages on the screen when you turn on your computer. See What is set in my serial port hardware?. Other ways of finding out is to use lspci for the PCI bus or isapnp --dumpregs for the ISA bus. The cryptic results it shows you may not be clear to a novice.

6.10 Giving the IRQ and IO Address to Setserial

Once you've set the IRQ and IO address in the hardware (or arranged for it to be done by PnP) you also need to insure that the "setserial" command is run each time you start Linux. See the subsection Boot-time Configuration

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