Introduction to Data Communications
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Standard COM Port Assignment

Note that COM1 (DB9 on the back of the PC) and COM3 share IRQ4. This is allowed as long as only one device is active at a time. This means that if you are running a mouse on COM1 then you cannot use COM3 for an internal modem. You will run into a conflict.

Some communication packages will allow you to do this but most will choke or cause flaky operation. A common sympton is if you move the mouse, you see garbage on your terminal program.COM2 (DB25 on the back of the PC) and COM4 have a similar problem except that most people don’t use COM2. It is usually safe to configure an internal modem to COM4. If COM2 is used, it is typically used for an external modem or a plotter. Usually, both are not active at the same time.


	Port	IRQ	Function



	COM1	4	Mouse

	COM2	3	Not used or plotter or external modem

	COM3	4	Not used (conflicts with mouse)

	COM4	3	Not used or internal modem

DMA -Direct Memory Access

DMA stands for Direct Memory Access. This is a method that allows channels to be openned by the peripheral to read/write directly to memory without going through the CPU. This off-loads some of the work from the CPU to allow it to do more important tasks.

There are 8 DMA channels available in the PC: DMA0-7. They are divided into 8 bit channels and 16 bit channels based on the 8 bit ISA slot and 16 bit ISA slot. Here is a table that is used as a rule of thumb for selecting DMA channels:


DMA	Function		Physical Line	ISA Bus	     Channel Width

DMA0	Available			Yes	16 bit		8 bits

DMA1	Sound	card			Yes	8 bit		8 bits

DMA2	Floppy Disk controller		Yes	8 bit		8 bits

DMA3	ECP Parallel Port		Yes	8 bit		8 bits

DMA4	* - Not used			No	-		16 bit

DMA5	Sound	card			Yes	16 bit		16 bit

DMA6	SCSI				Yes	16 bit		16 bit

DMA7	Available			Yes	16 bit		16 bit

* - DMA4 is cascaded to the first 8 bit DMA controller and is not available.
Note: DMA0 is on the 16 bit ISA bus but is only 8 bits wide.

*** DMA conflicts are the NUMBER 2 source of PC problems! ***

Like IRQs, you are only allowed to assign one DMA channel to an active device at a time. Otherwise you will have a conflict appear and things will not work properly. You may have one DMA channel assigned to two devices ONLY if one device is active at a time.

Introduction to Data Communications
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