Image Map Image Map
Page 1 of 2 12 LastLast
Results 1 to 10 of 11

Thread: Mixing Passive and Active Terminatiors - Good, bad, or indifferent?

  1. #1

    Default Mixing Passive and Active Terminatiors - Good, bad, or indifferent?

    Let's say you have both internal and external devices on your SCSI bus. The internal HDD is old and has passive termination resistor packs. The CD drive is external and has a toggle switch to enable the internal active termination. Is mixing them bad? If not, do you gain anything by having at least one active terminator on the bus? Or would it be no different than just flipping the active termination off and using a passive terminator on the centronics connector?

  2. #2
    Join Date
    May 2009
    Location
    Connecticut
    Posts
    4,694
    Blog Entries
    1

    Default

    How are the cables being run? IIRC, most SCSI adapters would need two terminators: one on the internal cable and a second on the external cable. Generally, active termination is better but the terminator needs to be with the last device or end of the cable. I think in your case, both the CD and HD would need their terminators turned on unless you also have other terminators.

    I prefer an active terminator that plugs into the external device. That way, adding a device merely requires moving the terminator to the new final device instead of changing the device termination or redoing a lot of cable to put a new device in the middle of the chain.

  3. #3

    Default

    I'm fully aware that termination needs to be applied at both extremes of the bus. For this example we are considering a single external CDROM connected to the centronics port on the back of the card and a single HDD connected to the 50 pin ribbon cable internally. Both devices are at the bus terminuses, and both must be terminated

  4. #4
    Join Date
    Jan 2007
    Location
    Pacific Northwest, USA
    Posts
    34,147
    Blog Entries
    18

    Default

    Internal HDDs usually/often? have a termination on the drive PCB, enabled either by removable resistor packs or by jumper. Since that's the case, use the drive's termination. Use whatever floats your boat for the external drive. Make sure that the terminators on the SCSI PCB aren't enabled in this case.

  5. #5

    Default

    Quote Originally Posted by Chuck(G) View Post
    Internal HDDs usually/often? have a termination on the drive PCB, enabled either by removable resistor packs or by jumper. Since that's the case, use the drive's termination. Use whatever floats your boat for the external drive. Make sure that the terminators on the SCSI PCB aren't enabled in this case.
    Right, in this example the internal HDD has passive termination by way of installed resistor packs. The question is whether it's better to use passive termination on the external device to keep the termination styles alike, or if using an active terminator on the external device is better or worse.

  6. #6
    Join Date
    Jan 2007
    Location
    Pacific Northwest, USA
    Posts
    34,147
    Blog Entries
    18

    Default

    Active termination works better than passive for long (>1m) cables. It doesn't sound as if you're using long cables, so passive termination all around is fine.

    So what's the difference? SCSI passive terminators are usually +5->220 ohm-+-330 ohm->gnd networks with the signal tied to the "+". This is a compromise; it sets the inactive/high voltage line level to +3v and the line impedance is about 130 ohms, which is a "good enough" match to SCSI's 110 ohm line impedance. This works for short runs; each line termination draws current (5v/(220+330)), which amounts to a couple hundred mA total. But it's good enough for devices run at standard speeds on short lines.

    Active termination uses a 2.85V LDO voltage regulator off the +5 line with output to 110 ohm series resistor to each signal line. This presents a "stiff" termination matching the characteristic SCSI line impedance and owing to the "active" voltage regulator is quite "stiff"--and uses less power. It's pretty much a must on long signal lines running at full bus speed.

  7. #7
    Join Date
    May 2009
    Location
    Connecticut
    Posts
    4,694
    Blog Entries
    1

    Default

    I can think of no benefit in not using the external CD drive's active termination.

  8. #8

    Default

    Quote Originally Posted by Chuck(G) View Post
    Active termination works better than passive for long (>1m) cables. It doesn't sound as if you're using long cables, so passive termination all around is fine.

    So what's the difference? SCSI passive terminators are usually +5->220 ohm-+-330 ohm->gnd networks with the signal tied to the "+". This is a compromise; it sets the inactive/high voltage line level to +3v and the line impedance is about 130 ohms, which is a "good enough" match to SCSI's 110 ohm line impedance. This works for short runs; each line termination draws current (5v/(220+330)), which amounts to a couple hundred mA total. But it's good enough for devices run at standard speeds on short lines.

    Active termination uses a 2.85V LDO voltage regulator off the +5 line with output to 110 ohm series resistor to each signal line. This presents a "stiff" termination matching the characteristic SCSI line impedance and owing to the "active" voltage regulator is quite "stiff"--and uses less power. It's pretty much a must on long signal lines running at full bus speed.
    Hmm sounds like passive termination is just garbage and should not be used. Maybe I can find a ribbon cable with an active terminator built in.

    Am I thinking correctly that mixing the passive on one end with the active on the other would defeat the purpose of the active's regulator by pulling up the voltage?

  9. #9
    Join Date
    Jan 2007
    Location
    Pacific Northwest, USA
    Posts
    34,147
    Blog Entries
    18

    Default

    In practice, no. As I said, passive is fine for short runs; it's included on just about every SCSI-2 controller I've ever seen. In your application, you're only talking about two devices on the SCSI bus (other than the controller). Stick seven on the bus and passive termination will have issues.

    The thing to understand is that SCSI-2 SE is basically old-school open-collector bus implementation. That works fine for floppy disks and tape drives and their bus (SA400 and Pertec); it should work fine for SCSI-2 single-ended devices.

    If you need higher performance, then there are differential implementations (e.g. LVD) that are common on later SCSI topologies.

  10. #10

    Default

    I actually have far more than 2 SCSI devices that may be connected at one time. I purposely kept the example scenario simple to avoid having to field questions about how my cables are chained and people telling me not to terminate devices in the middle of the chain.

    I hear you that passive is OK for simple setups, but I was really trying to focus on the specifics of mixing active at one end and passive at the other

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •