jim@jagmac2.gsfc.nasa.gov (Jim Jagielski) (08/10/90)
I'm interested in upgrading my Daystar 50 MacII to the IIfx, but the only
real thing (besides bucks :) holding me back is this question of SCSI
termination. I know that the IIfx upgrade "makes" you remove the termination
from your internal disk and replace it with the IIfx terminator/filter.
But my hard disk (CMS -170, Imprimis) does NOT like having it's internal
terminators taken out. In fact, when I 1st got it, I tried it and the thing
wouldn't boot. I had to rearrange my disks and tape drives to put that 1st.
Of course, CMS says that "it should work without the internal terminatators
although WE DON'T RECOMMEND IT." Translation: if it don't work without 'em,
we don't know why, SO DON'T DO IT.
How do I find out if this drive will work if I upgrade to the IIfx? I can't
move it to some other point in the chain because one of my tape drives
doesn't allow daisy-chaining, so that has to go last. There is some
electrical difference between internal and external terminators, so I guess
the question is whether the IIfx terminator is a TRUE internal terminator
or if it's just a "modified" external one... if a drive needs an internal
T, using an external T won't cut it...
--
=======================================================================
#include <std/disclaimer.h>
=:^)
Jim Jagielski NASA/GSFC, Code 711.1
jim@jagmac2.gsfc.nasa.gov Greenbelt, MD 20771
"Kilimanjaro is a pretty tricky climb. Most of it's up, until you reach
the very, very top, and then it tends to slope away rather sharply."kaufman@Neon.Stanford.EDU (Marc T. Kaufman) (08/11/90)
In article <3086@dftsrv.gsfc.nasa.gov> jim@jagmac2.gsfc.nasa.gov (Jim Jagielski) writes: >I'm interested in upgrading my Daystar 50 MacII to the IIfx, but the only >real thing (besides bucks :) holding me back is this question of SCSI >termination. I know that the IIfx upgrade "makes" you remove the termination >from your internal disk and replace it with the IIfx terminator/filter. I have been confused by this from the beginning, and now that the terminator is documented (TN 273) I am even more confused. It appears that the 'fx' termination differs from a standard external termination in that it has extra capacitance from +5 to ground, to prevent induced crosstalk between the signal lines (by stiffening the power supply with the 2.2uf cap and bypassing high frequency spikes with the .01uf cap). Now an internal terminator should be relatively close to a LOW impedance source of +5, so the problem should never arise. Why, then, must one remove the internal terminators and replace them with external ones? By the way: has anyone else noticed that the capacitors in TN 273 are installed backwards? Marc Kaufman (kaufman@Neon.stanford.edu)
dplatt@coherent.com (Dave Platt) (08/12/90)
In article <1990Aug11.042741.22709@Neon.Stanford.EDU> kaufman@Neon.Stanford.EDU (Marc T. Kaufman) writes: > I have been confused by this from the beginning, and now that the terminator > is documented (TN 273) I am even more confused. > > It appears that the 'fx' termination differs from a standard external > termination in that it has extra capacitance from +5 to ground, to prevent > induced crosstalk between the signal lines (by stiffening the power supply > with the 2.2uf cap and bypassing high frequency spikes with the .01uf cap). > Now an internal terminator should be relatively close to a LOW impedance > source of +5, so the problem should never arise. Why, then, must one > remove the internal terminators and replace them with external ones? Well, as I read it, the IIfx SCSI interface is so very fast that a TERMPWR bounce can occur unless the decoupling capacitance is _very_ close to the common TERMPWR point. Most internal terminators I've observed are mounted to sockets on the PC board, and do not necessarily have decoupling caps located directly by the sockets. There is usually several inches of PC-board trace lying between the terminator sockets and the power-supply jack, and several more inches of wire cabling leading back to the power supply itself. This amount of trace and wire may introduce enough of a time-delay so that the power-supply filters can't prevent the TERMPWR line from glitching down significantly in a worst-case SCSI-bus state transition. At these high speeds, even the power-supply and ground lines must be considered as transmission lines! Adding capacitance at the common TERMPWR tie-point is a surer way of keeping the TERMPWR level stable. Now... as I read Tech Note 273, it is NOT necessary to remove the termination resistors on an INTERNAL hard disk in a Mac IIfx. The IIfx does come with an "internal terminator" block... one which consists of a set of termination resistors, and a separate capacitive filter block. If you don't have a hard disk in your Mac IIfx, you need both of these parts... they ensure that the SCSI bus is terminated properly at the Mac IIfx end. If you have an internal hard disk in your IIfx, you should continue to use the termination resistors already installed on the hard disk... and you should plug the capacitive filter block in between your hard disk and the Mac's internal SCSI port. This will add some extra capacitance to your hard disk's TERMPWR line, and prevent glitches from occurring within the Mac. This block "can and should" be installed even if your internal hard disk has capacitors buffering the TERMPWR line... it won't hurt, and might help. A similar amount of capacitive filtering must be added at the far end of the SCSI bus, to make sure that the far-end terminator doesn't suffer from surge nausea. The problem is probably most acute if you're using an external terminator (non-filtered) which is getting its power from the Mac II over a long cable, and isn't receiving power from other SCSI devices on the bus (not everybody provides terminator power!). The problem might also be apparent if your bus ends with a device that has an internal terminator, but which doesn't have an adequately stiff or fast power-supply connection to the terminator. In the latter case, you _could_ get around the problem by adding capacitance within the device at the end of the bus... but this would require soldering an extra capacitor or two onto the PC board, and would almost certainly violate the warranty, damage the device, or get Senators Helms or Proxmire upset. It's safer to simply remove the internal terminators, and install a filtered external terminator (graciously provided by Apple). It's a bit of a shame, though, that Apple decided to stick with the 220/330-ohm parallel termination scheme, and simply add a capacitor to it to provide some voltage stability. A more sophisticated termination scheme has been developed, using serial termination and a regulated voltage derived from the TERMPWR line. This newer circuit provides a better match between the cable impedence and the terminator, consumes less power, and is equally resistant to termination-voltage sag (perhaps moreso). It's also quite compatible with existing SCSI devices and drivers, and is recommended in the SCSI-2 standard. Of course, it's more expensive, as it requires several active components... so I can't blame Apple for sticking with a simpler solution. I might buy or build one of these, though.