Delta RMC101 - Manual (Page 255)

Digital I/O 5.1
Communications 5-5
Figure #5: Open Collector Outputs to RMC CPU Inputs
The RMC’s DI/O inputs are not optimized for use with open collector (sinking) outputs. The
difficulty arises from the fact that sinking outputs have a common ground, but the DI/O also needs
a common ground. Therefore, pullup resisters must be used, as shown in the following diagram
(resister values are described below):
Figure #6: Open Collector Outputs to RMC DI/O Inputs
For 24VDC power, R should be a 3.3kW, ½ watt resister. The output must be capable of
switching 7.5mA when closed. When open collector output is open, the DI/O input sees 7V @
5.1mA.
For 5VDC power, R should be a 560W, 1/8 watt resister. The output must be capable of switching
9.0mA when closed. When open collector output is open, the DI/O input sees 3.1V @ 3.4mA.
Dividing the Input Voltage
Because the inputs are designed for use with 5V outputs, the threshold is 2.75VDC. This is a
small percentage of the 24V outputs. As a result, it is important that the inputs have very little
noise. This section describes using a voltage divider to raise the apparent input threshold.
Note: Before resorting to dividing the input voltage, take all possible measures to eliminate noise
from the inputs. Also, you may want to employ the software input filters.