Micromod Micro-DCI: 53SL5100A Single Loop Controller - Manual (Page 129)

Single Loop Process Controller Instruction Manual
Discrete Contact Output CCO 121
Appendix A: Discrete Contact Output CCO
The discrete output CCO is not a mechanical contact closure but an NPN Transistor that is analogous to a
single pole, single throw switch with one terminal connected to power common. This circuit layout is com-
monly called an Open Collector Output. (See
Figure A-1 .)
Capability limits of a 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 to provide output to an annunciator panel.
A CCO circuit is equivalent to an unpowered switch. If the 24 V de supply of the instrument is not already
fully loaded
1
, it can be used as a source of power for the 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 pins 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) is set to 1, provided that its
respective invert bit (L288 for CCO0) 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
2
. 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 resistance is required in
the second instrument, as it provides the power for its CCI operation.
1. Power source - 80 mA maximum.
2. Several manufacturers are Crydom Division, International Rectifier, and OPTO 22.