Micromod Micro-DCI: 53IT5100B Micro-DCI 4-Channel Indicator Totalizer - Manual (Page 53)

53IT5100B Indicator/Totalizer
INSTRUCTION MANUAL
49
Appendix A: Discrete Contact Output CCO’s
The discrete output CCOs are not mechanical contact closures but NPN Transistors that are analogous to
single pole, single throw switches with one terminal connected to power common. This circuit type layout is
commonly called an Open Collector Output. (See Figure A-1.)
Capability limits of each CCO are as follows:
 50 mA maximum current flow when closed.
 30 V dc maximum tolerance voltage when open.
A CCO will operate any external device that can be made to switch if it does not require more than 50 mA of
current to the (+) terminal.
Typical uses for a CCO are to actuate a small relay, activate an external alarm buzzer, provide
Contact Input (CCI) to another instrument, or provide output to an annunciator panel.
A CCO circuit is equivalent to an unpowered switch. If the 24 V dc supply of the instrument is not already fully
loaded (see note), it can be used as a source of power for a CCO; otherwise, a separate, external dc supply
must be used. In the upper circuit illustration of Figure A-2, the +24 V is obtained from screw lugs 1 or 4 of
TB1 located on the instrument rear terminal board. (See Figure 2-4.)
 NOTE: Power source - 80 mA maximum.
In Figure A-1, the switch is closed when the output logic bit (L24 for CCO0, L25 for CCO1) is set to 1,
provided that its respective invert bit (L288 for CCO0, L289 for CCO1) is 0. The relay in Figure A-2 is
energized when the output bit is set to 1 if the invert bit is 0. If the invert bit is set to 1, the output bit will cause
the relay to de-energize when it is set to 1.
Figure A-2 also shows how a dc electromechanical relay is operated using an internal or external power
source. There are many dc relays available with a coil resistance of 430 ohms or more. (Note: with the 24 V
dc supply, a 430 ohm coil resistance will pass 50 mA of current.) Figure A-3 shows how several CCOs can be
arranged in parallel using one power source so that any one CCO can actuate a single relay.
Figure A-4 shows a CCO used to control a solid state relay (see note). The resistor is added to the circuit
from the power source to limit the current flow, and also to establish the voltage across the relay when the
CCO switch is open.
 NOTE: Several manufacturers are Crydom Division, International Rectifier and OPTO 22.
In Figure A-5, the CCO of one instrument is applied directly to the CCI of another instrument.
When the CCO closes, the CCI circuit of the second instrument is complete. An appropriate resistance is
required in the second instrument, as it provides the power for its CCI operation.