4 serial ports required to work simultaneously

The application i have is one in which i want to use 4 serial ports and may be simultaneously..
This could go upto 8.

Is  there any hardware card which can help me do this. Conceptually i am told that there are only two interrupts which can be used for the serial ports
 i.e. IRQ 1 & 4 are the same and IRQ 2 & 3 are the same and cannot be used simultaneously.  Will this kind of a card along with the drivers be available anywhere .. where can i get it..

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Some errors :

Generally :
One Com port => 1 irq,1 I/O Address

Standart assignation :

COM1 and COM3 share IRQ 4
COM2 and COM4 share IRQ 3

I/O for COM1 : 3F8
I/O for COM2 : 2F8
I/O for COM3 : 3E8
I/O for COM4 : 2E8

With standart serials ports you can't use, in the same time, COM1 and COM3 or COM2 and COM4.

Possible with "industrial card" :

Look links :
You cannot really do this with standard PC hardware.  You can have up to  4 serial ports but you must be able to configure them to all use a DIFFERENT IRQ in order to have them work simultaneously.  If you have COM 1 and 3 both on IRQ4 and BOTH are receiving data at the same time, the system will get confused about which one has data and data will be lost.

For your application, you should consider a multi-port serial card that handles this internally.  I've been happy with the RocketPort cards.  See:


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www.equinox.com definitely the best cards out there (as in "fastest, most reliable, and with the lowest CPU usage, up to 128ports per IRQ)

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> Conceptually i am told that there are only two interrupts which can be used for the serial ports

That's incorrect.

Most modern motherboards have two COM ports: 'COM1' and 'COM2', and also will support 'COM3' and 'COM4'.

Each active COM port requires a unique IRQ.
By default, COM1 is associated with IRQ4, and COM2 is associated with IRQ3.

Also, by default, COM3 is associated with IRQ4.  So, by "default", you cannot use COM1 and COM3 at the same time; you'll need to assign a "free" IRQ to COM3.

The same caveat applies to COM2/COM4.

The big problem you'll face is a lack of IRQs, because your PC uses so many:

IRQ 1 KBD_0 Keyboard Controller
IRQ 3 SERIAL_2 Serial Controller
IRQ 4 SERIAL_0 Serial Port Controller
IRQ 5 Sound Blaster 16 Device Driver
IRQ 6 FLOPPY_0 Floppy Controller
IRQ 7 PARALLEL_0 Parallel Port Adapter
IRQ 11 Display Adapter
IRQ 12 PS/2 Mouse
IRQ 14 IDE_0 ST506/IDE Controller
IRQ 15 IDE_1 ST506/IDE Controller

I just answered a question on a "standard" card that could be used in a 4 port system, an ISA multi I/O card that could have the UARTS set to use IRQs 5 and 9. So if you were lucky enough to stumble across one of those for a couple of bucks second hand, it could help you out until you HAVE to buy a multiport device. Though I believe there are some of those devices that can be upgraded later to more ports.

see http://www.experts-exchange.com/jsp/qShow.jsp?ta=hardgen&qid=10268403   (5 points) for details on that ISA i/o card


Road Warrior
dbruntonQuid, Me Anxius Sum?  Illegitimi non carborundum.Commented:
This isn't answering the question but there is another misconception here.  There is nothing to stop two devices sharing the same interrupt.  It is possible but the coding to enable  it is horrendous.  So you could have two serial ports using the same IRQ.
Alloy make some PCI cards which allow multiple serial ports to share an IRQ - this is commonly used as a device to hook up terminals etc to a UNIX host.

I have used a couple of these to run serial printers, plotters etc and haven't found any problems with them yet......

You can get some info at:


then select Products / Serial Devices..

Hope this helps

trips001Author Commented:
Thanks .. I finally found one in the market.. from a company called MOXA Canada 8 port card with drivers
> There is nothing to stop two devices sharing the same interrupt. It is possible but the coding to enable it is horrendous. So you could have two serial ports using the same IRQ.

Sometimes, you don't need _ANY_ coding.
My digital-camera uploads via COM2/IRQ3 and my modem uses COM4/IRQ3.

As long as I don't try to use both devices at the _SAME_ time, IRQ3 can be used by both devices -- like sharing one crayon between two siblings.
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