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As one of the oldest methods of storage tape drives have persisted and evolved into cost effective solutions of mass storage for the personal computers. Intel's floppy disk controller provides the means to take advantage of this technology for storage on the PCs.
The tape drive support is available on the 82077AA-l and S2077SL-l. This paper describes a way to configure 82077AA-1/SL-l so that it can be optimized for tape drive support.
PLL: Phase Locked Loop is a control circuit that is used to lock onto the incoming serial data stream. The PLL tracks fluctuations in the data frequency.
Optimizing 82077AA-1/SL-1 for Tape Drive Mode:
The floppy disk controller can be configured for the tape drive mode by both hardware and software.
Configuring the S2077AA-l/SL-l for the tape drive mode refers to optimization of the internal data separator in order to deal with the effect of ISV which is more pronounced on a tape drive than on a floppy disk controller.
Hardware selection is done by setting the PLL0 (pin 39) to 0 or GND. This optimizes the data separator for tape drives by changing the loop filter component values and loop gain. TDR selection is disabled under this mode.
Software selection of the tape drive mode for the FDC is implemented via setting of the appropriate bits in the tape drive register (TDR). This selection is enabled only while PLL0 is set high. This aids the user in configuring the particular drives as tape drive even when in floppy mode. As shown below the TDR contains two bits which can be utilized to assign tape support to a particular drive during initialization.
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| * | * | * | * | * | * | TAPE SEL1 | TAPE SEL0 |
Hardware resets clears this register but it remains unaffected by any software reset. TDR[2:7] remain in a tristated condition and are not readable. Drive 0 is reserved for the floppy boot drive and cannot be configured for tape drives using the TDR (software mechanism).
Hardware selection overrides any selection made by the software i.e. by setting PLL0 to GND, tape drive mode will be selected regardless of the changes made to the TDR. Although the software mechanism does not allow to select drive 0 for tape drive when PLL0 = 0 any drive can be supported for tape drive.
Installation of Multiple Tape Drives:
82077AA-1/SL-l has the capability to support up to a total of four drives. Most PC systems today have at least one floppy disk drive. This leaves the possibility of installing up to three tape drives.
The following describes a way to configure the floppy disk controller in a multiple tape drive environment. This also depends on whether the system manufacturer wishes to leave certain drive slots fixed for tape drives or variable by the user.
All tape drives are variable - If the drives chosen as tape drives are variable then the configuration mechanism used is strictly software. After strapping PLL0 high the bits TDR[0:l] can be programmed during initialization for various drives that can be selected as tape drives. It should be noted that in this case drive 0 cannot be selected as one of the tape drives.
Combination of fixed variable tape drives - If any drive can be determined to be fixed then either the motor enable pin or the drive select pin of that particular drive can be used to drive PLL0 to GND when selected. Fig.1 and Fig.2 show two scenarios where drives that are fixed for tape drive use their motor enable or drive select signals to drive PLLO to GND.
Fig. 3 shows by using jumpers flexibility can be incorporated in the system and the drive 5 to be fixed for tape drives can be left to the user.
This paper describes a clarification of the specification for tape drives. The data sheet gives typical performance curves for tape drive mode of the 820772AAA-1/SL-l . This report elaborates on Intel's range of testing.
ISV: Instantaneous speed variation or ISV is defined as the variation in instantaneous speed at the head measured as a percentage of the nominal. High frequency ISV occurs due to tension waves in the unsupported length of the tape between the reels.
Jitter Tolerance: It is the maximum allowable skew from the ideal position for bits in the incoming data stream of a PLL (Phase Locked Loop) for both floppy and tape drives.
Capture Range: This is the frequency range over which the PLL will acquire phase lock with the incoming data stream.
The 820772AA-1/SL-l has integrated all the components required to support tape drive. The internal data separator requires no external components yet allows for a very wide capture range with high levels of jitter and ISV tolerance.
The 82077AA-1/SL-1supports tape drive mode via the TDR (tape drive register) or via an external pin PLL0.
The current data sheet for 820772AA (October 1990 Order number: 290166-003) depicts typical data separator characteristics for tape drive mode at nominal conditions.
Unlike the floppy disk characteristics the graphs for tape drive do not show any test points at which the floppy disk controller is tested. The graphs for tape drive mode in the above stated data sheet are typical only and do not represent the testing specification.
Intel will clarify this by defining the tape drive specification over all operating conditions to be:
At 3% ISV the jitter tolerance will be greater than 30% over a capture range of -3% to +3% for 500 Kbps and 1
Mbps tape drive operation.
The above stated spec is established for the current stepping of the 82077AA-1/SL-1.
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