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Active and Pssive SCSI Termination

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    Active and Pssive SCSI Termination

    Hi Guys,
    I see SCSI cable terminators listed as active and passive. What is meant by that term? Why is one chosen over the other?

    Thanks...Joe
    Last edited by Grandcheapskate; December 30, 2016, 02:52 PM.

    #2
    From my limited understanding... In both cases a resistor network exists within the terminator on each signal pin to a) return the signal to an intermediate voltage when not driven to allow for faster slew rates, b) provide a deterministic load impedance for the drivers themselves, and c) to dampen the effects of having a long un-terminated wire hanging in space - which EEs typical call an antenna!

    ~5V termination power is typically sent down the wire on the center two pins. A passive terminator just has two resistors on each signal - one tied from each signal wire to +5 termination voltage and the other from each signal to ground. This creates a simple voltage divider. The problem with passive termination is the resistance of the cable itself. The resistor values are low which means the termination current draw is high. As the length of the cable increases, so does the voltage drop at the far end. So 5V isn't 5V typically.

    Active termination uses a DC/DC regulator at the far end to drop the 5V (or sometimes much less) termination voltage to 2.65V. It then uses smaller resistor networks to more tightly regulate the characteristic line impedance and undriven settling voltage.

    Just my $.02
    "Good engineers keep thick authoritative books on their shelf. Not for their own reference, but to throw at people who ask stupid questions; hoping a small fragment of knowledge will osmotically transfer with each cranial impact." - Me

    Comment


      #3
      https://forums.anandtech.com/threads...inator.814339/

      Choose either 'active' or 'forced-perfect' (FPT) terminators and scrap the passive ones. The active and forced-perfect types utilize active circuitry to better control the transmission line end impedance to prevent reflections from the end of the bus interfering with the data on the bus. This is similar to termination of old 10Base-5 and 10Base-2 coaxial ethernet, but on a parallel bus instead of a relatively simple coax cable. The terminators on SCSI also have to deal with providing the source or sink current for the bus drivers, and thus they are a bit more complex than just a simple resistor.

      Note that single-ended, differential, and low-voltage-differential (LVD) each have specific terminators and they are not interchangeable (except that the LVD terminator can work in single-ended mode).

      Skip the passive; use active or forced-perfect.
      --
      Thus spake Tandy Xenix System III version 3.2: "Bughlt: Sckmud Shut her down Scotty, she's sucking mud again!"

      Comment


        #4
        Really, Joe, a simple google search will give you far more detail on this. Basically, in a nutshell:

        Passive terminators (we're talking about a SCSI-2 single-ended (i.e. not differential) bus) are simply 220/330 resistor packs (220 to TERMPWR and 330 to ground) that attempt to hold the SCSI lines at the SCSI-2 spec of 2.85V. Usually, not very successfully.

        Active terminators use voltage regulators to enforce the 2.85V which are then in series with 110 ohm resistors (which matches the characteristic impedance of the SCSI bus).

        The whole object is to reduce line signal reflections, which can compromise the signal integrity of the SCSI bus.

        There is a variation of active termination called Forced Perfect Termination (FPT) which is a more complex form of active termination and occurs in several flavors. You mostly see FPT on IBM gear.
        Reach me: vcfblackhole _at_ protonmail dot com.

        Comment


          #5
          Originally posted by Chuck(G) View Post
          Really, Joe, a simple google search will give you far more detail on this...
          Why trust Google when you trust Chuck(G)?
          Daniel P. Cayea - The Lyon Mountain Company - Plattsburgh, New York 12901
          Vintage Equipment: IBM 5150 * IBM 5161 * ThinkPad 770ED
          Modern Equipment: MacBook Pro 13 * Alienware M15R3

          Comment


            #6
            Excellent explanations, guy's!

            I'll add in my own .02 just because i was a SCSI nut back in the day: Ensure your only terminating on the end of the cable, and if your using the older 50pin drives, check and see that they are not terminating the bus themselves. You'll see a resistor pack on the pcb, easily removable. Or check the docs on the drive and disable it via jumper (my preferred method after losing a number of them) Double termination can lead to strange issues.

            I don't recall exactly whether or not termpower should be enabled or disabled. IIRC, it was used primarily in cases when there were more then 3 or 4 devices on the chain and the adapter card couldn't be counted upon to provide the necessary current. Maybe the last device in the chain it would be prudent to enable it...

            There used to be a scsifaq floating around the internet, google it. Had the best answers for everything relating to scsi. Look's like I'm googleing to reflash my memory cells. Darn NMOS chips, i didn't provide enough juice while burning them
            -------------------------------------------------------------------------------------------------------------
            Currently Looking for Samsung Sens Pro Laptop's (500, 520,800,820) and accessories, specifically a docking station.

            Comment


              #7
              Most SCSI host adapters and devices are set up so that if more than one device provides TERMPWR, there's no harm.

              Andy Johnson-Laird, back in the day, famously referred to "SCSI Voodoo". He was right.
              Reach me: vcfblackhole _at_ protonmail dot com.

              Comment


                #8
                Originally posted by Chuck(G) View Post
                ...
                Andy Johnson-Laird, back in the day, famously referred to "SCSI Voodoo". He was right.
                It's not that hard, really, even when doing oddball stuff like running a CAMAC crate controlling a large scanner (I mean large as in 7,000 pounds large) off of a VAXstation 4000/90's secondary SCSI bus. You just need the right reference. I always liked 'The Book of SCSI' from No Starch Press myself. I have the second edition here somewhere. There was a Slashdot review about 15 years ago on the 2nd ed.... lessee: https://hardware.slashdot.org/story/...si-2nd-edition
                --
                Thus spake Tandy Xenix System III version 3.2: "Bughlt: Sckmud Shut her down Scotty, she's sucking mud again!"

                Comment


                  #9
                  Both Andy and I were playing with state-of-the-art tape and disk drives back in the day (circa 1995). This was the "we're getting ready to release this thing, how would you like an evaluation unit?" era.

                  You really could get in the situation where a specific device would refuse to work with a specific controller, ANSI standards notwithstanding. Even today, I'll occasionally run into a vintage bit of kit that doesn't do what the ANSI specs says it should.
                  Reach me: vcfblackhole _at_ protonmail dot com.

                  Comment


                    #10
                    Thanks guys...and Check is right, there is a lot of stuff out there about passive vs. active (and all say forget using passive).

                    Here's a follow-up question...if I plug a device into every connector on the SCSI cable and none of those devices has a termination jumper, should that work since there is no "loose end" on the cable to act as an "antenna"?

                    Thanks...Joe

                    Comment


                      #11
                      It could very well work, particularly for low-speed devices and short cables. But work reliably? That's another question entirely.
                      Reach me: vcfblackhole _at_ protonmail dot com.

                      Comment


                        #12
                        Originally posted by Grandcheapskate View Post
                        ...if I plug a device into every connector on the SCSI cable and none of those devices has a termination jumper, should that work since there is no "loose end" on the cable to act as an "antenna"?
                        No, or at least it won't work well. It's not about the end of the cable acting as an antenna, but the electrical signals reflecting at the end of the electrical run; whether that is the end of a cable or the PC traces of a device doesn't matter.

                        When you say 'termination jumper' do note that most of the jumpers labeled 'termination' are for termination power injection, not selecting an on-board termination network. I've not seen (but I reserve the right to be wrong) any drives with built-in termination selectable by jumper only; most drives with on-drive termination (50-pin narrow SCSI) have a socket for a resistor network. What kind of drives are you wanting to use?
                        --
                        Thus spake Tandy Xenix System III version 3.2: "Bughlt: Sckmud Shut her down Scotty, she's sucking mud again!"

                        Comment


                          #13
                          I just checked the manual for the Adaptec 2940UW controller (which I will be using) and it explicitly says to use an active terminator and termination must be done even if all the connectors on the cable are plugged into a device. Since none of the drives I just purchased have any type of on-board termination, I will have to either get active termination blocks or a cable with built in termination. I am leaning toward buying cables with built in termination.

                          The drives I will be using are SCA 80 pin drives (U320) connected via either a 50 or 68 pin adapter (depending on which cable I use).

                          Thanks...Joe

                          Comment


                            #14
                            Originally posted by Grandcheapskate View Post
                            I just checked the manual for the Adaptec 2940UW controller (which I will be using) ...

                            The drives I will be using are SCA 80 pin drives (U320) connected via either a 50 or 68 pin adapter (depending on which cable I use).
                            You'll have to watch out with some U320 drives. Some U320 drives won't work in single-ended mode. Most are supposed to work UW, but SGI O2 owners have found out the hard way that that is not always true (I have an O2, and a later-model 300GB U320 drive worked fine in it, but I did my homework first). See the Nekochan O2 Hardware Aggregator post's FAQ entry 1a on the subject.... ( http://forums.nekochan.net/viewtopic.php?f=3&t=8113 and you'll have to scroll down to it).

                            If you can get a 29160 or 39160 series you'll be a lot happier. These cables show up on eBay a lot; you do need to get an SE-compatible cable (most LVD cables are, but I have one or two here that aren't; look specifically for LVD/SE on the description) for the 2940UW. Something like: http://www.ebay.com/itm/Amphenol-497...8AAOSwOyJX5WhG should work ok.
                            --
                            Thus spake Tandy Xenix System III version 3.2: "Bughlt: Sckmud Shut her down Scotty, she's sucking mud again!"

                            Comment


                              #15
                              Also, it should be noted that some (almost always external devices or host adapters) are "smart" in the sense that they can automatically provide termination. Usually, this is restricted to devices of the same manufacturer and often, to devices of the same type. Most modern host adapters automatically provide termination on the host end.
                              Reach me: vcfblackhole _at_ protonmail dot com.

                              Comment

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