scsi cable configuration guide for ultra ultra2 lvd differential devices and controllers

 

SCSI cable configurations


SCSI cable FAQ

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Cable configuration Diagrams and explanations:



Ultra Wide (not to be confused with Ultra2/Ultra160 or Ultra3)

Ultra Wide SCSI has two bytes (16 bits - each byte consists of 8 bits) of data. All 50-pin (or less) SCSI's have a single byte of data. Wide (devices with two bytes) devices can deliver two bytes of data at the same time, which effectively doubles their throughput. When using 50-pin (or less) devices, there are several options to make sure that the high (upper) byte of data gets properly terminated so that your Wide SCSI device(s) will perform as designed. When using only narrow devices with a wide controller care should be taken to terminate the high byte properly, even though with termination set to "Auto" things may apparently work without problems.
First it must be understood that SCSI is a "bus" architecture, which basically means that it has a beginning and an end, it's a straight line, one end to the other end with no "Y" type of configurations allowed! Second, both "ends" of the "bus" must be "terminated". In most cases, one end will be the SCSI controller "host adapter" which is usually nowadays terminated automatically. Many SCSI controllers have more than one "channel", which basically means you can think of their cable length and termination needs entirely seperately. Please read your manual! Many Ultra Wide SCSI controllers use the external and internal connectors on the same "chain", which means that you will have to not terminate the controller (auto-termination usually is a good setting here) and terminate both ends (no problem, usually, since any end must be terminated anyway).

Basically, make sure to terminate every end connector/device and no other!!! Also please note:"Ultra" SCSI cable lengths are severely limited! The maximum cable length is ten feet when four devices (including the host adapter) or less are on the bus. If five devices are used (four devices and your host adapter), then the maximum bus length is 1.5 meters (five feet!).

Here are some common configurations:

Many of these setups will work in Ultra2/Ultra160 setups, but there are some "gotchas", so Email support for help.

Mixing Ultra Wide (68-pin) and non wide (50-pin) devices on a single cable of a Wide controller.

Option #1 (cheaper)

Controller

(68-pin cable)----------------------|-------------------------------------------|

wide device(s) Wide Hard drive(s)

or terminated

Adapter 68-50

50-pin Device(s)

not terminated


Option #2 (like to waste money or want/have a high-byte terminator)

Controller

(68-pin cable)----------------------|-------------------------------------------|

Wide Hard drive(s) Adapter 68-50

not terminated (with high-byte termination)

50-pin Device


terminated




OK, say your using 68 and 50 pin internal connectors, you can try this:


Controller

(68-pin cable)---------------------|----------------------|------------------------|

Wide drive* Wide drive* Wide Hard drive

not term. not term. terminated



Controller


(50-pin cable)------------------|------------------------|------------------------|

50-pin Device(s) 50-pin Device(s) 50-pin Device

not term. not term. terminated



OK, say your controller has 68 and 50 pin internal connectors as well as a Wide external connector. Many controllers use the same channel for 68-pin internal and 68-pin external which does more than a little bit to confuse the termination/connection problems. In most cases you do not want to use both the 68-pin internal and 68-pin external connectors at the same time, due to length considerations even more than the termination headaches involved. A good rule to follow would be to keep your internal and external devices on seperate chains. Here are some examples of connecting 50-pin (or less) external devices and internal 68-pin and 50-pin devices.


Option #1

Controller

(68-pin cable)---------------|-------------------------------|-----------------------------|

Wide drive* Wide drive* Wide Hard drive(s)

or or last drive

68-50 Adapter* 68-50 Adapter* must be

50-pin Device* 50-pin Device* terminated



Controller

(50-pin cable)------Internal/External Adapter |

External cable(s)

External device(s)

Controller

(68-pin exteral connector)---|

no connection




Option #2

(68-pin cable)---------------|-------------------------------|-----------------------------|

Wide drive* Wide drive* Wide Hard drive(s)

or or last drive

68-50 Adapter* 68-50 Adapter* must be 
50-pin Device* 50-pin Device* terminated



Controller

(50-pin cable)------Internal/External Adapter |

External cable(s)

External device(s)


Controller

(68-pin exteral connector)---|

no connection


Option #3 (warning! read your manual! Terminations and cable lengths are big problems. You must know your SCSI "channels" etc.) Not recommended.

(68-pin cable)---------------|-------------------------------|-----------------------------|

Wide drive* Wide drive* Wide Hard drive(s)

or or last drive

68-50 Adapter* 68-50 Adapter* must be

50-pin Device* 50-pin Device* terminated



Controller

(50-pin cable)------------------|------------------------|------------------------|

50-pin Device(s) 50-pin Device(s) 50-pin Device

not term. not term. terminated


Controller

(68-pin cable)------Adapter |

External cable(s)

External device(s)


termination



Ultra2/U160 SCSI (Ultra3) (LVD- Low voltage differential)

Want to use your external connector? See here, then read on.

Flexibility, speed and much greater flexibility in cable lengths are the best reasons to use Ultra2/Ultra160 LVD SCSI controllers. Ultra2/Ultra160 LVD SCSI is "Low Voltage Differential" not to be confused with regular older versions of differential properly known as "High Voltage Differential" or HVD. Differential in general uses two noise canceling lines per signal to give you the ability to run longer cables at higher speeds. Ultra2/Ultra160 SCSI can reach speeds of 80MBps/160MBps as upposed to the 40MBs for Ultra wide SCSI. You'll need at least a few very fast hard drives to even come close to filling the bandwidth of Ultra2/Ultra160! It's hard to generalize about these adapters, since they come in so many different configurations with not only "channels" but also "segments". Read the manual!!! All these companies generally have excellent online information about their devices, as well. Some of these adapters come with as many as 4 connectors which can all be used at once. Others are far more finnicky about keeping segments "pure", and connecting your legacy "older" SCSI device will cause the whole segment to slow down to 40Mbs Ultra Wide speeds.

Here are some general rules of thumb:



  1. Learn about your controller! Read the manual, check with your controller/motherboard manufacturer.

  2. Use each connector for what it's for. Connect LVD componants to the LVD connector, use the regular Ultra wide connector for Ultra wide devices, use the 50-pin connector for your 50-pin devices.

  3. Be certain which connectors are possibly sharing segments.

  4. Read the manual again!




Differential (HVD- High voltage Differential)

Connecting differential devices to non-differential "Single-ended" controllers or Single ended devices to differential controllers can damage either or both! Be careful! Many people get stuck with differential drives by purchasing them from online auctions, etc. Converters are usually more expensive than the drives themselves! How do you tell if you have a differential device or controller without looking it up? Get an ohmeter or multitester that can measure accurately under 1 ohm. For 50-pin connectors that are high density or Centronics style, measure the ohms between pins 2 and 24. If the resistance is several tenths of an ohm or less it means it's a single ended device. If the ohms are close to one ohm or greater then it's a differential device. For Wide 68-pin devices use pins 2 and 33. For DB50 connectors use pins 3 and 49. Always check with the manufacturer to be sure!


Q & A: Questions and Answers

# Is there a reason for a minimum distance between connectors? Or is it for just to make it easy to remove and install devices?

Yes there's a reason. There are basically two electrical factors involved. First you must understand what a "stub" is. A "stub" is basically the distance between the cable (bus) and the device. A bunch of short stub spacings, can cause signal problems much like those of a stub that's too long. It is recommended that stubs are no closer than 3 times the stub length, and since the maximum allowed stub length is 100mm, this means that 300mm should be the maximum stub spaceing, however, this can be reduced if shorter stubs are used. The other facor has to do with the capacitance of the cable, and varies with the cable type, this normally gives a "minimum" distance somewhere around 150-200mm.
Overall picture: Keep your adapters to a minimum. Never try some kind of "Y" cable configuration (there're more factors than stub spacing involved in why) and don't think that using cables with "crammed" together connectors will be a great idea.



# Will an Ultra2/Ultra160 LVD controller with slower SCSI-1 or SCSI-2 devices on the "chain" or "segment" slow down to the speed of the slowest device in the SCSI chain?

Yes and No!. SCSI talks only to one device at a time. The controller will talk to each device at the speed it's capable of, with certain exceptions. On many Ultra2/Ultra160 controllers you cannot run at 80MB/sec with even a single non-LVD device on the same chain as your LVD devices. The controller will run at the Ultra wide speed of 40MB/sec with any non Ultra2/Ultra160-LVD devices on the same chain. Some newer controllers won't do this as long as you keep the slower devices on a different "segment". Most of these controllers have seperate Ultra2/Ultra160 segments, Ultra wide segments and narrow segments.

Overall picture: Keep your Ultra2/Ultra160, Ultra wide and regular (Ultra, Ultra-fast, SCSI-2, SCSI-1) devices on the "segment" or channel they should be on. Read the manual or ask someone!

# Is it alright to turn off my device that has a terminator on it? Will I still be terminating the SCSI bus?

Yes/maybe, if and only if (and it's a big if) the device is set to get its Termination power (TERMPWR) from the bus and not the powered off device. For many devices passive termination will still work. In many cases active termination will work if the device gets its termination power from the bus rather than the device.

# Should I turn off parity with removable drives?

No, SCSI parity should always be on.

# Can you connect two internal SCSI cables together with adaptors or something?

No, trying to do this will cause problems with impedance mismatches which can cause wave reflections, similar to termination problems. Don't be cheap, buy a SCSI cable with enough connectors for the number of devices you need.


# Do I need an Ultra2/Ultra160 controller for a single hard drive and SCSI CDROM?

No. Two 10,000 RPM drives and other devices won't overburden a regular Ultra wide controller most of the time. A 15K drive can max out the bandwidth of these controllers occasionally. Unless you are dedicating a very fast drive for digital video or using the computer as a file server for very large files, a Ultra wide controller may be all you need. The current rage for Ultra2/Ultra160 upgrades in order to run a single drive "at" 80/160MBps is a misconception on the part of many people since unless you want to use RAID 0, or RAID 5 (etc.) striped drives you rarely use the bandwidth available with these controllers.

# Can I use an Ultra/Ultra2/Ultra160 wide drive with a regular SCSI or Ultra 50-pin controller?

Yes, you don't need that high byte for it to work. It will not have the throughput of wide/Ultra2/Ultra160 of course, but it will still work, and if the RPM's are better than your current drive you'll still get an increase in performance. Ultra2/Ultra160 drives must be set to run in single ended (SE) mode, check your jumper settings!

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SCSI Information:

SCSI connector pictures

The basics of SCSI for newbies and technical information

SCSI connection FAQ

SCSI cable Length guide



Thomas Steves