|
Serial drivers need to know two things about a serial port in order to operate the port correctly - its base address and its interrupt number (IRQ).
Most PCs have two serial ports fitted, COM1 and COM2. All software works correctly when just these ports are fitted. There is also de facto standard for two more serial ports - COM3 and COM4. The addresses of these ports are unique, but the IRQ numbers are the same as for COM1 and COM2.
Port
|
Base address
|
Interrupt
|
Standard?
|
COM1
|
3F8
|
4
|
Yes
|
COM2
|
2F8
|
3
|
Yes
|
COM3
|
3E8
|
4
|
De facto
|
COM4
|
2E8
|
3
|
De facto
|
COM5...
|
|
|
No
|
Some serial port cards are capable of sharing an interrupt, and some software is capable of driving a serial port that is sharing an interrupt, but it is likely that either hardware or software cannot handle interrupt sharing. If you install additional COM ports as COM3 or COM4, using the default IRQs, these may interfere with devices connected to COM1 and COM2. For example, an ADC-16 connected to COM3 may cause a mouse connected to COM1 not to function correctly.
The better quality serial port cards make it possible to use other interrupts, such as IRQ5 or IRQ10. There are no hard and fast rules about what IRQs are used by specific devices, so it is necessary to study the technical information supplied with your computer and the proceed by trial and error. The following table of typical IRQ usages may be helpful.
If you wish to use non-standard IRQ values with Windows, you should find the following lines in C:plw.ini, and modify the details to the values that you have selected:
COM4Base=02E8
COM4Irq=3
COM3Base=03E8
COM3Irq=4
It is possible to buy multi-channel serial port cards (normally four or eight channels) where the serial ports use the same interrupt. There is no standard for these cards. These cards are normally supplied with Windows drivers, and so the ADC-16 DLL should work correctly once the Windows drivers for the serial card are installed.
| 
