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:

These were among the first 4 MB drives available in the market. Each of them has a mode select input on pins 2 and 6. The polarity required for each different data rate is as shown below:

It is clear from above that DRATE0 = MODSEL0 and MODSEL1 = DRATE#. This would mean taking the drate signals onto the pins 2 and 6 of the FDC-FDD interface. Unfortunately this solution requires an inverting gate. TEAC has recently however, come out with a new version called TEAC 235J-3653. On this drive there are a number of possible configurations into which the drive can be put into, however, I will only discuss the best way to interface to the 82077AA/SL. The requirements are as shown below. This shows that HDIN = DENSEL (original signal for conventional drives) and EDIN = DRATE0. As suggested in the TEAC spec for method 1, the straps connected are MSC, HI2 (sets HDIN on pin2), DC34 and EI6 (sets EDIN on pin 6). Pins 4, 29, and 33 are left open. Since the pin 2 has the same polarity as the conventional drive requirement and the secondary input is connected via pin 6 (no connect on the conventional drives) daisy chaining this TEAC drive with a conventional drive does not cause any incompatibility. Figure 7. Shows how the TEAC can be connected to the 82077AA/SL. It also shows daisy chaining of the TEAC drive with a conventional drive.

This is Panasonic's new drive and has the HDIN signal on pin 2 and EDIN signal on pin 6. The requirements are shown below. This type of interface allows for daisy chaining the Panasonic drive.

with conventional drive. The DENSEL signal can be connected to pin 2 and the DRATE0 should be connected to pin 6.

This drive has DENSEL1 located on pin 2 and DENSEL0 on pin 33. Via jumpers it is configurable to different polarity requirements. The following shows the configuration in which jumpers 2MS = I/F and 4MS = I/F:

If the drive is used in the PS/2 mode, then DENSITY SELECT = DENSEL and DENSITY SELECT0 = DRATE0. To use the drive in AT mode, DENSITY SELECT1 = DRATE1 and DENSITY SELECT0 = DRATE0, as shown below. However, daisy chaining is not possible.

The correct connection requirement is: DENSEL (from 82077AA/SL) = DENSEL1 and DRATE0 = DENSEL0. Although there are other configurations, this provides the best one, since daisy chaining is possible without any problem. The only difference is that pin 33 is used instead of one of the available no connects such as pin 6.

This drive has 3 different modes of operation. Mode B is the best and is similar to Mitsubishi's as described above. In this mode, HDI signal is connected to pint 2 and EDI is connected to pin 33. Mode B is enabled by inserting jumpers across 3-4 and 7-8 (SS01 B block) and 1-2 and 3-4 (SS03 block) for the drive with the power separated type (i.e., a connector for the floppy signals and another one for power supply) of 34-pin connector.

As demonstrated by the table, HDI = DENSEL and EDI = DRATE0. These connections would ensure daisy chaining capability without any problems.

There are dash 14 and 15 are two new drives from Sony that handle 4 MB requirements. The MP-F40W-14 has the DENSITY SELECT 1, DENSITY SELECT 0 on pins 2 and 33 respectively, whereas the MP-F40W-15 has the DENSITY SELECT 1, DENSITY SELECT 0 on pins 2 and 6 respectively. As it is obvious from the table below, daisy chaining is easily done if the 82077AA/SL is connected in the PS/2 mode (by typing IDENT high) with either type of drive, the only difference being the location of DENSITY SELECT 0.

This Toshiba's latest 4 MB drive. It has the HD mode selection on pin 6 and ED mode selection on pin2. This causes daisy chaining problems with conventional drives as shown in the figure below:

The DENSEL from the 82077 is connected to pin 6 and DRATE0 is connected to pin 2.