SCSI Termination FAQ

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1. What is termination ?
2. What is Active ?
3. What is Passive ?
4. What is Differential ?
5. What is Low Voltage Differential ?
6. What is High Byte Termination ?
7. What is Forced Perfect Termination ?
8. What is Active Negation Termination ?

What is Termination?

Termination causes a SCSI bus to think it has infinite length. An in depth technical explanation would not be appropriate here, as I am not an electronics engineer. I can tell you that termination is required to be at both ends of each SCSI bus. Sometimes the termination can actually take place in 4 different places, but that is in the realm of High Byte Termination only. If you are ever terminating your SCSI bus in more than 2 places, you need to exercise extreme caution. There are many different forms of termination a SCSI bus can use. Passive, Active, Differential, Forced Perfect Termination, Active Negation, and Low Voltage Differential.

What is Passive termination?

Passive termination is the lowest form of termination known to man. It boggles the mind that people would spend so much on SCSI and cheap out on the terminator (which will decrease performance). The difference between passive and active terminators is 5-10 dollars in most cases.

Passive termination is recommended in low performance applications only. A scanner would probably work fine on a passively terminated bus with a cable length of 6 feet or less.

Rules of thumb:

1. Don't use if you have more than 2 devices.
2. Don't use if you have any Ultra devices.
3. Don't use if you want the highest performance for your system.

Some scanners use passive termination for their onboard termination. Watch out for that. It can render a UW HD screamer silent because the entire SCSI chain is connected together. We sell active DB-25 terminators to correct this situation.

What is Active Termination?

Active termination is currently the most common type of termination being used. It is passive termination + voltage regulation. You can't go wrong with active termination unless you have a true SCSI-3 spec device. See Active Negation Termination for more details.

What is Differential Termination?

Differential termination is used in differential systems. They are not compatible with single-ended or LVD. Differential termination is easy - Always use differential termination for differential devices, and it works. There is no active or passive differential termination.

What is Low Voltage Differential Termination?

Low Voltage Termination is fast becoming the new standard in SCSI. It is downward compatible with existing Single-ended systems, so it is very popular for gradual upgrades. It is the only standard that supports Ultra2 Wide 80MB/S. If you aren't sure whether you need LVD or Active, you don't have to choose! LVD has Diffsens, which automatically chooses between LVD and Single-ended when your devices first power up.

note : Some LVD devices, terminators, and host adapters are NOT multimode LVD/SE. If your equipment is not multimode, then the downward compatibility is lost, but you still get the speed and distance of LVD.

What is Active Negation Termination?

This is a sensitive spot because not allot of information is readily available on active negation termination. What I have found is that it is the required termination for true SCSI-3 spec single-ended devices. Active Negation Termination may get passed over as a technology because most people going to SCSI-3 are using LVD to do it. Active Negation is fully downward compatible with Active, so it doesn't hurt, it can only help. When in doubt, spend hours researching or spend an extra 5-10 bucks.

What is Forced Perfect Termination?

FPT is a technology that got passed up. Very few people actually implemented this style of terminator, and it's day is certainly past. It is better than active, and it extends the length of single-ended devices. They are hard to find.

What is High Byte Termination?

a.k.a. high 9, high G, high bit.

High Byte Termination is used when the SCSI bus must be terminated in multiple locations. It is traditionally the most complex topic in termination. If you have all narrow devices, then it is not an issue.

The concept is simple - If you are running a wide system, then the wide bus is actually transmitting 16 bits of information every time a data transmission is sent. That's 16 bits or 2 bytes. The low byte, which take up the first 50 pins along with all the control signals, and the high byte, which has only 8 bits, 1 parity bit, and 9 grounds. (50 + 18 = 68 That's where 68 comes from. ).

Everything is simple and happy until we introduce a narrow (50 pin) device to the bus. Why is that complex? Well, because we still don't need high byte termination in every instance. Let me run some scenarios by you....

W = wide device
N = narrow device
C = controller
* = termination
# = adapter

1) Internal :

*C<----------->W<------------------>
*C<----------->W<-------->#N*

BAD - We have an unterminated bus here. As you can see, the wide byte is unterminated and doomed to fail.

2) Internal :

*C<----------->#N<------------->W*
*C<---------------------------->W*

GOOD - We have a fully terminated bus here. The narrow device is using a 68 to 50 converter, and it is not interfering with anything.

3) Internal :

*C<------------------->W#*
*C<------------------->W##<----------------->N*

GOOD - We used a 68 to 50 adapter with high byte termination, so the bus is now terminated in 3 places, but it is terminated properly.

4) Internal :

*C<----->W<------>W#*
*C<----->W<------>W##<------>N<------>N<------>N*

GOOD - Same as #3, but we added a few drives for the heck of it.

5) External :

*C<---------->W<---------------->
*C<---------->W<------->N*

BAD - It's the same sad story as #1. The upper byte is not terminated.

6) Internal and External :

<--#------>W<------->C<------->W<----------->
 *N#<----->W<------->C<------->W<------> N*

BAD BAD - This scenario has two bad points. The high byte is incorrectly terminated internally and externally.

7) Internal and External :

     *##------>W<------>C<------>W<------>##*
*N<--->#N<---->W<------>C<------>W<------>#N<------->N*

GOOD - This bus is terminated in 4 places, but it still works properly.

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