RS232 Communication Port (optocoupler) to Contact Closure
Posted Jan 13, 2022, 10:41 PM UTC
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I have an Eaton UPS with a DB-9 communications port for RS232. The unused pins have configurable outputs, indicate contact characteristics (optocoupler) with voltage 48VDC max, Current 25mA max. I would like to connect a few of these outputs to an DSC alarm panel which is expecting a dry contact, but I'm confused by the diagram.
It's been 15 years since university and feel like I should know this :(
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So i read this the following: When you apply voltage to 4, Pin 1 and 8 will be pulled to gnd (5)
But i dont know if there is more logic behind Pin 1 and 8, because in my theory they are both driven together by Pin 4, but in the description it says that they have a different purpose
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DB9-1, 7 and 8 are configurable through software for various alarms. The default values are what are in the table.
In confused by the “sideways table with two arrows pointing left” symbol between say pin 1 and 5. I take it that pin 5 changes wrt ground (pin 5) on alarm, but I’m not sure what that change actually is? There is also one dry contact output elsewhere so I don’t think this is also a dry contact output?
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So i can resolve the symbols for you: Those are the optocouplers. A optocoupler works by using some kind of led and a photoresitor. Here is the wikipedia to that where also the symbol for it is shown.
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Thanks for this. I vaguely remember optocouplers - I know I used them in my capstone project. I believe then I would need a voltage source on the other side to drive a relay to connect to the security panel, or could the panel connect directly to the optocoupler?
Time to break out the Boylestad and Nashelsky textbook for a needed refresh - can remember the authors but not the content :p
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Using a resistor is a bad design because the RS232 output voltage can vary from device to device, because it depends on the RS232 voltage translator IC that varies from device to device. One RS232 port may output 5V, another might output 10V, another 12V, another 15V. https://en.wikipedia.org/wiki/RS-232#Voltage_levels
The UPS has the RS232 output, so I would expect the value provided reflects the voltage of the port? This is from the UPS manual. I actually think the DB-9 port is doing double-duty. RS232 and something else (I’m just not sure what).
The resistor is on an input pin, I’m more concerned about the output pins.