SCSI TERMINATORS and their role (Judgement Day :)

Q) What the HELL is a TERMINATOR and why is it called like this?

Q) What happens if I don't terminate my devices properly?

Q) What does a terminator look like?

Q) What types of terminators are there?

A) We all know (or maybe not ;) that for things to work flawlessly in a SCSI chain/bus we must have our first and last devices on the chain terminated properly; cool we say, but WHY? Well as you know, the SCSI standard allows us to connect contemporarily various different types of devices (HDs, Scanners, Printers, Samplers, CD-ROMs, MO, Zip Drives etc...) all on one single bus. This potentially means 'long' distances that can reach up to more than 5 meters of connection cable for the SCSI-1 standard and this means variable resistivity and thus signal drop and electromagnetical interference caused by external equipment (like monitors) and most important of all SIGNAL REFLECTION due to the distance which all ends up in data instability along the bus. With all these parameters in mind, there had to be a way to keep the signal clean or at least the cleanest possible along the bus and so the termination tecnique was adopted. Among the SCSI bus signals there is also one called Termination Power which is responsible for the termination of your bus. This is an active high lead which brings the term. power along the bus UNTIL the last and first device that must be terminated. So when you terminate the last device what you are doing is actually giving power to the data bus through the last device's connection so that the signal can be "helped" from the end or beginning of the chain so that there is no signal drop and no signal reflection. When you erroneously terminate a "midway" device that is not at the end of a chain, you can think of it as introducing signal interference towards the last device becuase your are hereby generating a sort of "signal bounce" on it by having terminated it too early. On the contrary, if you DON'T terminate the last device, you are not eliminating the signal reflections from the ends of the scsi bus generating interference with consequent data loss and corruption (system hang, drive malfunction etc).

There are actually two families of terminators: passive ones and active ones. The passive ones are the ones we are all more or less familiar with; those three little "mini combs" used directly on the device's logic and the bigger and bulkier standard 50pin male/female centronics ones for external use. They are called passive becuase they get the current from the term. power lead and route it to the scsi bus lines through resistors in a PUSH-PULL manner- infact that's what they actually are: a series of 2 different resistors. The active ones are more "hidden": they are electronically controlled and are integrated on the device's electronics itself. They are activated and deactivated through a jumper or a switch (like the zip drive) and are called active because they use an electronic voltage regulator to produce the right voltage (2.85V) to supply to the scsi bus lines instead of using passive resistors which actually drain much more current and "stress" more your host adaptor that supplies it.