There are 2 types of terminators, Active and Passive. The active one
has some electronics included whereas the passive one has a resistor
network that is wired from the +5 Volt line to the data line and then
from this data line to ground (0V) The termination impedance is
effectively the resistors in parallel . I can not remember the actual
values But I seem to think it is 330 ohms to +5v and 220 ohms to
ground.
+5v ------ 330 ohms---data line--- 220 ohms ------- 0V (ground)
The impedance is 330 ohms in parallel with 200 ohms 330 x220 /330 +220
=330 x 220 /550=72600/550=132 ohms
The nett effect of either type of terminator is as follows.
With SCSI if you do not have any termination then the various data
lines will not have any voltage on them and when the data line is
dragged down to 0V by the card (active low) there will be NO CHANGE in
the voltage to ground. If you then plug the terminator in all the
data lines will be able to move between 2/5 of the 5 Volt line and 0v
as the terminator can be thought of as a voltage divider as well.
Using the values I have suggested you will have 5Volts across the 330
and 220 ohm series and the mid point will have 3 volts across the 330
ohms and 2 volts across the 220 ohms.
This may sound complex but is really quite simple. Most computer data
lines are what is called "active low: because most of the integrated
circuits use what is called "open collector" mode where all the
transistor can do is pull the data line low if the transistor is
turned on (data asserted) or if the bit is not selected then the
voltage there will depend on the termination (in our case here 2
volts)
Jim Mccardle (
[email protected])
