I successfully soldered the board and tested it with the demo software provided on the CD. But now I’m encountering some difficulties with the correct usage of the open collector outputs of the card. I want to use it for the following scenario:
I have an external control box for a piece of hardware that has digital input lines and provides a 24V supply as well. If you connect a input line to +24V you get a HIGH signal on the control box to work with. So I just have to realize a simple switch with the K8055 board. I followed the examples provided in the manual and did the following things:
connected GND line of the control box supply to GND on the digital output panel
connected +24V line of the control box supply to CLAMP on the digital output panel
for each digital output added a 10K resistor, connected between CLAMP and the digital output itself
connected input line directly to the output line of the K8055, so the connection order is output connector <-> input line <-> resistor <-> CLAMP <-> +24V
With the card operating indepently I get +24V at each digital output line if the signal is set to LOW by the software. If the signal is HIGH, I get a reading of app. +0.6V. Each one measured to GND. The behavior is inverted, but that should be alright.
If I now connect the inputs of the control box, I only get about +3V on each line instead of +24V.
Can you tell me where my mistake could be located? I have basic understanding of electronics but whenever it comes to transistors and stuff I get totally confused
As an addition - the manual of the control box quotes these parameters for correct detection of the signals:
MIN TYP MAX UNIT
Input voltage -30 - 30 V
Input guaranteed OFF if -30 - 7 V
Input guaranteed ON if 10 - 30 V
Guaranteed OFF if 0 - 5 mA
ON Current (10-30V) 6 - 10 mA
Are the resistors I used dimensioned too large with 10K? And if, what would be a good value?
Yes, the resistors have too high value (10K).
The control box needs 6 - 10 mA current and 10 - 30 V to its input to set the input ON.
According to your measured voltage (3V) at the box input the input resistance of the box is about 1.4K.
By using 1K pull up resistors you should get about 14V to the box input.
This should be detected as ON state for the box input.
[quote]With the card operating indepently I get +24V at each digital output line if the signal is set to LOW by the software. If the signal is HIGH, I get a reading of app. +0.6V. Each one measured to GND. The behavior is inverted, but that should be alright.[/quote]Yes, the K8055 outputs are “active low”.
Please also see this thread for a very good explanation of the open-collector: viewtopic.php?f=4&t=7227
Thank you very much for your help
I tried it yesterday again with 2K2 resistors and it finally worked out! I didn’t measure the voltage yet, but at least it’s enough for the logic to detect the signal. If I encounter any problems, I will try the 1K alternative.
Just out of curiosity: How did you calculate the internak resistance of the input box? Or to put it in a better way, where did you take the information of the about 2mA current from?
It’s nice that you got the K8055 connection to the control box working!
Here you gave enough information to calculate the unknown internal resistance of the control box input:
[quote]for each digital output added a 10K resistor, connected between CLAMP and the digital output itself
……
I only get about +3V on each line instead of +24V.[/quote]
Your 10K resistor together with the input resistor (Rx) compose a voltage divider.
The voltage ratio is the same as the resistor ratio.
The equation is:
Rx/(Rx+10K) = 3V/24V
