LOSTCIRCUITS		SHORTCUTS: IDE Cables: Wired RF Transmitters UDMA 33 vs UATA Cables Mechanical and RF Properties Real Cables Real World System Problems Tips and Conclusion Any Comments? Cheetahs and Cudas on the web
ATA Not-So-FAQs    Or: Why Ribbon Cables are unsuitable for RF transmission of data
(Article by snn47, March 17, 2003)          -.. ..-. ..... .--. -.--

The following article was written by snn47 to address some of the issues associated with standard ribbon cables and the use of e.g. removable drive racks as an attempt to share some insight into factors that can adversely affect the life or reliability of of desktop Hard Disk Drives. Specifically, issues like why some drives are working in some systems and not in others, the impact of cable routing and why is it that the drive manufacturers always recommend using their own cables (if supplied with the drive).

There is a ton of data in this article, some of which are of interest only for the EE-crowd or else some nerds but there is also some stuff of why sometimes simply rerouting the cable can solve the problems at hand.

In detail, those parameters are:

• the actual problem (e.g. data corruption vs. physical failure of the drive)
• under which circumstances, that is, setup and settings they occurred (e.g. system-type,
• clock speed)
• cable length and type (e.g. rounded, twisted pair etc.)
• number of connectors (master, slave)
• use of a drive bay or not.

Different types of connector are used in removable drive bays and most of those connectors (like this Centronics printer connector) were never meant for high frequency data traffic. In addition, PCB-routing adds some extra noise and sources for potential data corruption.

Any RF system has a limited tolerance for distortion of signals, which, in the worst case, can destroy some of the semiconductor components. While a certain amount of variation is part of any systems specification, one needs to remember that ATA was never intended to handle today's data rates. ATA or Advanced Technology Attachment started as the usual run of the mill or: "just a system at the lowest possible price point that will work most of the time without the need for huge financial investments". The problems started when the system was forced to handle higher and higher clock and data rates within the original design limitations. Keep in mind that the latest ATA-PI7 specifications allow data rates of 133 MB/sec, which is 44-times faster than the original ATA transfer of 3 MB/sec. This increase in speed makes it necessary to enforce minimum tolerances and detailed specifications to allow for the manufacturing of affordable systems with minimum compatibility problems.

In an earlier article we pointed out that reliability issues with hard disk drives can occur in situations where first-generation rounded cables or else swappable ATA drive bays are used, while the same drives worked without problem when connected directly to the 80 wire ribbon cable [1] and [2].

ATA66 or higher was defined to use a 80 wire flat ribbon cable (0,635mm between wires) with a maximal length of 45cm and up to 3 connectors (blue for controller, black for slave and master). Longer ribbon cable can be used if the slave connector is omitted as can be seen with 3ware IDE-RAID-Adapter that come with a 60cm long UDMA cable. While this is in violation to the specifications it seems to work flawlessly since the disturbance on the transmission line caused by the slave drive connector in the middle is deleted [3].

Even more disturbances are generated if a cheap HDD-drive-bay is used that use a mechanical robust connector commonly known as Centronics-connector and originally intended for use in e.g. parallel printers, which was decades ago, but never intended for today's high data-rates. Additional disturbance is created by the signal traces if the connector is mounted on a PCB.

next page:    => Specs and Specs and more Specs =>