Micromod Micro-DCI: 53IT5100A Indicator/Totalizer - Manual (Page 64)

APPENDIX A: DISCRETE CONTACT
OUTPUT CCOs
The discrete output CCOs are not mechanical contact closures but NPN Transistors that are analo-
gous to single pole, single throw switches with one terminal connected to power common. This cir-
cuit 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*, it can be used as a source of power for a CCO; otherwise, a separate, exter-
nal 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.)
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 to1.
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**. 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.
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 resis-
tance is required in the second instrument, as it provides the power for its CCI operation.
*Note: Power source - 80 mA maximum.
**Note: Several manufacturers are Crydom Division, International Rectifier and OPTO 22.
Appendix A. Discrete Contact Output CCO
A-1