There is currently no industry-wide standard for the FDC to FDD interface. There are numerous floppy drive vendors, each with their own modes and interface pins to enable 4 MB perpendicular mode. The drive interface does not only vary from manufacturer to manufacturer but also with in a manufacturer's product line. The differences on the interface mainly originate from configuring the floppy drive into the 4 MB mode. Depending on the drive, the differences can create problems of daisy-chaining a 4 MB drive with the standard 1 MB and 2 MB drives. Of course, for laptops this is not a problem since most of them use a single floppy drive. With IBM adopting the 4 MB drives, however, a standard may emerge in the future. In the meanwhile, it is necessary to look at each drive and build an interface for that particular drive.
The following is a brief discussion about some of the floppy drives available in the market and how these can be interfaced with the 82077AA/SL. It is important to note that although a manufacturer's name may be given in connection with the interface described, Intel does not guarantee that the interface discussion will apply to all drives from that manufacturer. The main goal is to introduce to the reader how to interface the 82077AA/SL with a 4MB floppy drive.
Previously, for the conventional 1 MB and 2 MB AT mode drives, a single Density Select input was used by floppy drives to select between high density and low density drives. A high on this input enabled high density operation (500Kbps) whereas a low enabled low density operation (300Kbps/250Kbps). This signal was asserted by the floppy disk controller depending on the data programmed. For the 4 MB drives with conventional drives. Another problem is that the second density select pin varies on its location on the FDC-FDD interface from drive to drive.
The way that the BIOS determines what type of diskette is in what type of drive is by trial and error. The system tries to read the diskette at 250Kbps, if it fails then it will set the data rate to higher value and retry. The BIOS does this until the right data rate is selected. This method will still be implemented for the 4 MB drives by some BIOS vendors, however, the 4 MB drives by some BIOS vendors, however the 4 MB drives available today also have two media sense ID pins that relate to the user what type of media is present in the floppy drive. This information will also require two pins on the FDC-FDD interface. The location of these pins is once again variable from drive to drive.
Some manufacturers have circumvented the entire standardization problem by including an auto configuration in the drive. In these cases, the type of floppy put into the drive is sensed by hole (each 4/2/1 MB diskette has a hole in different locations identifying it) on the diskette. Then the drive automatically sets itself up for this mode. The BIOS must obviously set up the floppy disk controller for the correct data rate which could be done if the media sense ID was read and decoded as to the data rate. Due to lack of extra pins on the even side of the floppy connector the newer locations of some of the functionalities is migrating to the odd pins (previously all grounded). Some drive manufactures have even made this configurable as the rest of 4 MB specific selection jumper configurable. For instance, the new TEAC drives has a huge potuporri of configurations that would satisfy the appetite of some of the most finicky system interfaces.
The 82077AA/SL currently has two output pins DRATE0 and DRATE1 (pins 28 and 29 respectively) which directly reflects the data rate programmed in the DSR and CCR registers. These two pins can be used to select the correct density on the drive. These two can also be sued with the combination of DENSEL to select the correct data rate. At the present time the 82077AA/SL does not support media sense ID. However, the user could easily make it readable directly the BIOS. The following is a discussion on what the combination of DRATE0, DRATE1, and DENSEL could be used to interface to some of the currently available floppy drives: