DiscFerret:FAQ

1. Frequently Asked Questions

Why another disc analyser?

Because the other analysers weren't good enough! The Catweasel Controller (all four versions of it) are essentially unsupported by the manufacturer. Developer support has tended to be poor, with little hardware documentation available. On top of all this, the drivers only allow discs to be written on machines running Linux 2.4 (a very old Linux Kernel release which has been essentially abandoned since the mid-2000s) or MS-DOS.

Ultimately I wanted to create a hardware-based disc reader/writer which could be used on any system with a Full-Speed-capable (12Mbps) USB port. Whether that is a PC, a Mac or some other platform is entirely up to you, but in general, if it runs Linux, supports USB and has a libusb port, it'll most likely work fine. This even extends to embedded systems like the [BeagleBoard](http://beagleboard.org/), as long as you add some form of storage (e.g. a USB thumb-drive or hard drive).

So it's a Catweasel clone then?

Not at all -- the DiscFerret was engineered completely from the ground up, and provides a lot of features that the Catweasel is simply incapable of:

    * Significantly increased timing resolution. The DiscFerret acquires timing data with a resolution of 10 nanoseconds, for the first time allowing MFM and RLL hard disc drives to be imaged at transition level. On a Catweasel Mk3 or Mk4, the maximum clock frequency is 28.322MHz (about 35ns resolution), with 14.161MHz (70ns) and 7.080MHz (140ns) as options. DiscFerret uses a 100MHz sample rate, providing single-nanosecond timing resolution, and is accurate enough to write a disc with software precompensation (where write-precompensation is applied before the disc is written).
    * Maximum timing period limited only by available RAM. The DiscFerret can store up to 524288 bytes (that is, 512 binary kilobytes) of acquisition data, or a maximum count of over 67 million timer ticks. This equates to an acquisition time of nearly a second (a disc typically rotates in around a fifth of a second)! The Catweasel is always limited to 128 counts per period, meaning that the acquisition clock must be slowed down when imaging at lower bit rates, or the hardware counter will overflow.
    * Hardware-based reading and writing of hard-sector discs. Literally "set it and forget it" -- enable the track-mark detector, specify how many sectors to skip (if any), and how many sectors to read, then trigger the acquisition. Simple!
    * MFM sync-word detection with "don't care" bits. This is exactly what the name suggests: a module which allows the DiscFerret to scan the incoming data stream for a specific bit sequence, then start or stop the acquisition. Bits can be masked off (ignored) if necessary, which allows for ranges of sync-words to be specified. The start and stop masks and sync-words can be specified completely independently of each other.

What happens if I use the wrong power supply?

In most cases, nothing. The DiscFerret incorporates a reverse-polarity protection circuit, which reduces the risk of damage due to the use of an incorrect power supply. This circuit has been tested up to 20V, which exceeds the input voltage rating of the DC power connector, and the output voltage of most off-the-shelf DC power supplies.

In the event of a reverse-polarity event, the power light on the power supply (if present) will be lit, while the power light on the DiscFerret will not (as the protection circuit is preventing the power from reaching the DiscFerret).

It should be noted that the protection circuit is intended as a last-resort safety device, and should not be "tested" or "tried out" by the end user. In situations where the DiscFerret is exposed to both reverse polarity and extreme overvoltage, the protection circuit may fail to operate. Always use the power supply which was supplied with the DiscFerret, or an identically-specified replacement.

If a higher-voltage power supply is attached to the DiscFerret, the "12V" output on the power output connector will rise to the input voltage. The 5V output will remain at 5V (except in cases of extreme overvoltage). Once again, your DiscFerret will probably survive, but your disc drive almost certainly won't...

How much power does the DiscFerret need?

The DiscFerret requires a 12V power supply, with a maximum current consumption of around 1.5A if powering the DiscFerret in "stand-alone mode" (where the power output socket is unused). This increases to around 3.5A if an external drive is being powered from the DiscFerret's power output socket, assuming the drive is consuming the maximum allocation of 1.5A on both the 5V and 12V power outputs.

If the external +12V output is not being used, the input voltage can be increased to a maximum of 15V DC, making it possible to run the DiscFerret from a standard 13.8V battery pack. It is, however, inadvisable to use an automotive ("cigarette lighter") power socket as a power source, due to the large amount of electrical noise present on motor vehicle power supplies.

How do I buy a DiscFerret?

You can buy a DiscFerret through the DiscFerret Store.