Linux Step By Steps


In general,  you will not experience winmodem issues with these internal modems.

From : Guidry, Toby <>

>  I am the "proud" owner of a Smart Rapid Transit "modem"
> which is an ISA card.  It is most _definitely_ a winmodem (even identifies
> itself as an "LT Win Modem" to ISAPNP).


Most isa modems are isa-pnp and you are advised to either read the isapnp documentation first, or, take it out of plug and play mode if you can. The choice is yours.

There are four accepted serial ports on an ibm-pc.

Com1 /dev/ttyS0 IRQ4 0x3F8    traditionally a mouse
Com2 /dev/ttyS1 IRQ3 0x2F8    traditionally an external modem
Com3 /dev/ttyS2 IRQ4 0x3E8    traditionally not used
Com4 /dev/ttyS3 IRQ3 0x2E8    traditionally an internal modem

Note that interrupts are shared. This is accepted practice, BUT, you cannot share them at the same time. For this reason, it is 'traditional', to put a modem in the box on Com4 to avoid mouse conflicts. With the advent of PS/2 style, and, USB mice, this is obviated, and the practice now is to use Com3. Be warned that although setserial uses the above convention, the linux kernel does not. Read on.

Typically, a motherboard already comes with the first two ports on the board as genuine serial uarts.

Typically, a modem, will allow you to choose between these four port addresses and one of the following interrupts.

IRQ 3,4,5 or 7

The reason for the small number of IRQ's available is that isa modems, being 8 bit devices, only use the (now) ancient 8 bit bus. That bus only has provision for these lower interrupts (IRQ's 8 to 15 are not available)

The use of IRQ 5 or 7 is very bad practice (tm). Interrupts are so scarce in a pc, that these numbers are now (normally) assigned to either sound cards, or printers. Avoid the temptation. You have been warned.

The real choice you have left, is to use IRQ 4 or 3, and select a COM port as per tradition.

By far and away, the easiest method is to simply disable either com1 or com2 in the cmos of the motherboard and utilise it's freed resources for the modem.

The second method is to place the modem on com3. Note this carefully. NOT com4, despite what has been said above. As mentioned above, if you have a serial mouse, move it to com2. The DB25 connector for com2 is unsuitable, so, either, swap the internal 10 pin grey ribbons around, OR, use a db25 to db9 adaptor (very very common component).

The reason for this is that  linux in general uses linear, sequential, logical, addressing. What this means is, it will assign ttyS2 to the 3rd detected serial port. Your modem! Linux does not care WHAT  resources are used for /dev/ttyS2, only that it has been detected as the third serial device (from 0).

If you had four serial devices, the table above, and linux, and setserial, would match. But your modem, *must* be assigned as ttyS2 if it is the 3rd serial device (and most are).

You can override this default behaviour. Considering the unecessary pain involved to do so, I won't waste my time describing a pointless excercise.