The FDCX4, in addition to using an analog phase-locked-loop data separator in all modes, also used write-precompensation.
2.
Later Cromemco disk controllers such as the 16FDC and 64FDC contained both single and double density data separators and the 64FDC also supplied write pre-compensation.
3.
Although demand for Model I drives greatly exceeded supply at first, since the interface lacked a separate external data separator, it was very unreliable in practice.
4.
The Percom Doubler added the ability to boot and use Double Density Floppies ( they provided their own modified TRSDOS called DoubleDOS ), and included the Data Separator.
5.
Due to the second fact, an unmodified 4FDC can not be used with 8-inch drives that don't have single-density data separators on the drive electronics.
6.
A data separator and a double density disk controller ( based on the WD 1791 chip ) were made by Percom ( a Texas peripheral vendor ), LNW, Tandy and others.
7.
ESDI improved on ST-506 by moving certain parts that were traditionally kept on the controller ( such as the data separator ) into the drives themselves, and also generalizing the control bus such that more kinds of devices ( such as removable disks and tape drives ) could be connected.
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