This item is from the Yet Another Enhanced IDE/Fast-ATA/ATA-2 FAQ, by John Wehman and Peter den Haan with numerous contributions by others. (v1.92).
Last but not least, the usual limit of two devices was far too restrictive if CD-ROMs and tape drives were to be connected to the IDE interface in addition to the harddisk(s). Fortunately the solution was already known in the form of a so-called secondary harddisk interface. The possibility of such an interface has been there for a long time, but support was lacking.
There is nothing special about a secondary channel; it is an ordinary (E)IDE port that uses a different interrupt and I/O addresses to avoid clashes with the ordinary (primary) one. A secondary interface allows you to connect another ribbon cable with two more ATA devices (harddisk, ATAPI CD-ROM or ATAPI tape). Today, many interfaces combine both primary and secondary port on a single board to make a dual-ported interface that handles up to four devices.
To use harddisks on the secondary port with DOS and Windows 3.x, you will need BIOS (either system BIOS or adapter BIOS) or driver support. You can recognize a BIOS with four drive support by the fact that it allows for four sets of drive parameter in the BIOS setup.
There are two further (semi-)standard channels beyond the secondary port: the tertiary and quaternary ones. Some soundcard IDE interfaces can be configured as tertiary or quaternary. See section 10.4 for the I/O and IRQ assignments. Software support for these is still rare.