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The TTL output levels are driven b y a 75452 dual peripheral
driver, with open collector outputs, and a 220 o hm pull-up
to +5 volts.
The TTL data inputs drive 1TTL input, load
and a IK pull-up to +5 volts.
W hen the TTL inputs are not being used, they should be
left open or h eld high so as not to affect data input
from other sources.
The TTL Data Input line must b e left open and not held high
when the current loop inputs are used.
input drives opto-isolators and will respond to either 20
o r 30 milliamperes.
reverse voltage may be experienced,
circuits
(i.e., relays) are coupled to the data line, a
protective diode should be put across the line such that
any reverse voltage spikes will cause the diode to conduct
and thus protect the LED in the opto-isolator from too
large a reverse voltage.
The current loop output is switched by an isolated trans
istor through an opto-isolator and is provided with a
transient-shunting diode across the output transistor so
that it m ay be used to drive relays without risk of damage
to the output circuit.
diagramed on another page, both with and without the cur
rent source being used.
Setting the baud rate for serial I/O channels A and B is
done on the jumper select socket RJ in position Bll.
b aud rates designated o n the detailed sheet for rate select
are correct when the 8251 is programmed for a 16X asyn
chronous clock rate and a IX synchronous clock rate.
The details of selecting the desired baud rates are located
on the schematic.
In applications where a significant
such as when inductive
Typical wiring connections are
SIO 2 Board Rev. 3
Users Guide
Edition 2
The current loop
The
9 - 2 7
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