[Coco] calling all MM/1 owners
gene.heskett at verizon.net
Fri Jan 25 00:26:35 EST 2008
On Thursday 24 January 2008, Joel Ewy wrote:
>Bob Devries wrote:
>> I just removed the I/O board from the MM/1 to more closely examine the
>> I found that one of the termination resistors for the SCSI bus is
>> oriented the opposite way to the other two. RN5, the middle one is
>> different. Now I'm wondering what effect this will have on the working
>> of the SCSI bus? Could this be my problem?
>> Joel, the termination resistor's 'notch' or 'dot' end should be in the
>> solder pad that's square, am I correct?
>Yeah, that's right. I think that's the common lead for the resistors.
>Resistors themselves aren't directional. Does it make a difference
>because of the way they're assembled in an array?
Yes, there are two common points in a resistor pack as used for scsi
terminations. There is a supply pin that is intended to be connected to a 5
volt supply, and a ground pin. In a 10 pin pack (they come on various
widths) you will have a power pin, a ground pin and 8 pins intended to be the
terminators. Each of the 8 pins has a 220 ohm internal resistor connected to
the supply pin, and a 330 ohm connected to the ground pin.
The scsi buss is a wired or buss, and these terminators furnish the pullup
resistance to hold a line that is not being driven low from anyplace on the
cable, at a guaranteed logic one plus about 600 mv for noise margin, or 3
volts. The active pins have a 220 ohm resistor connected to the supply pin,
with a 330 ohm connected to ground, which will set this 3 volts if the supply
is a full 5 volts. It is often not because of the use of an isolation diode
to keep the 5 volt busses isolated when one of them is tuned off. This
diode, if Si, will drop abut .65 volts and the voltages at the active pins
will drop accordingly.
The commonly used cable for scsi hookups, the flat ribbon cable, has if every
other wire is grounded, a characteristic impedance of about (its not a very
precisely made cable) 120 ohms. You need that resistance on each END of the
cable or a signal will bounce from end to end, echoing many times. Exactly
the same as when you are measuring the SWR of your antennas for your ham & cb
The parallel combination of a 220+330 ohm is about 132 ohms (and these are
often +- 20% parts people!) and generally does an adequate enough job of
absorbing these echo's that it will work if the supply is correct. With the
isolation diode, it gets borderline low and you have lost the noise margin
for the logic one state, which is about 600 mv if the lines are at 3 volts
idle. At 2.6 volts idle, the noise margin is down to 200mv, and it will be
much more sensitive to these echos, and at 2.4 volts the noise margin is gone
and its not going to work, period, full stop. This is where you start
sacrificing goats or whatever trying to make it work reliably but it won't.
The much lower forward drop of a power schotkey diode will help a lot, but if
the psu is also sagging with age, and is only putting out 4.82 volts or so,
then even the schotkey diode's lower voltage loss of about .15 volts might
not be enough.
A properly setup scsi bus is a work of art that you can run 40 meters at full
speed for scsi-2! Build it with 20% tolerance parts & a saggy psu 5 volt
line and you have a nightmare with only a 20" cable.
So called 'active terms' are in fact a voltage regulator set at 3 volts, with
a 120 ohm resistor between the output of the regulator and the data lines.
These are much less sensitive to the supply variations, and since the 330 ohm
to ground is done away with, draw considerably less power, about 5 watts
less. If one can find those in a suitable package, remove the resistors and
substitute the active chips output into the pcb holes, dependability will go
way, way up.
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-Ed Howdershelt (Author)
Many people write memos to tell you they have nothing to say.
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