DaleP
Member
Hi,
I am trying to interface a 24 Vdc PLC output to a TTL input on an instrument. The 24 Vdc supply connected to the PLC output module is floating. The common on the instrument input is earth grounded.
I am running the DC output and DC common over a 10 ft. cable to 1 Kohm resistor (for current limiting) to the LED of an opto isolator (Motorola 4N26)on a breadboard. The open collector opto isolator output is connected to a 4.7 Kohm pullup resistor and the input of a 7414 Schmitt Trigger inverter. The 7414 output is driving my TTL input. (The Schmitt Trigger was necessary because my transition time was too slow for the TTL input). The opto isolator and 7414 are being supplied by the 5 Vdc supply of the instrument.
My problem is the noise on the PLC output. Every time a solenoid or motor that's being driven by a 24 Vdc PLC output turns on, there is a spike on the DC output that controls the instrument, and I get extra trigger signals. This seems to be true of all the DC modules connected to this supply.
Grounding the PLC DC supply doesn't seem to help.
Any ideas about how to solve this would be appreciated!
I am trying to interface a 24 Vdc PLC output to a TTL input on an instrument. The 24 Vdc supply connected to the PLC output module is floating. The common on the instrument input is earth grounded.
I am running the DC output and DC common over a 10 ft. cable to 1 Kohm resistor (for current limiting) to the LED of an opto isolator (Motorola 4N26)on a breadboard. The open collector opto isolator output is connected to a 4.7 Kohm pullup resistor and the input of a 7414 Schmitt Trigger inverter. The 7414 output is driving my TTL input. (The Schmitt Trigger was necessary because my transition time was too slow for the TTL input). The opto isolator and 7414 are being supplied by the 5 Vdc supply of the instrument.
My problem is the noise on the PLC output. Every time a solenoid or motor that's being driven by a 24 Vdc PLC output turns on, there is a spike on the DC output that controls the instrument, and I get extra trigger signals. This seems to be true of all the DC modules connected to this supply.
Grounding the PLC DC supply doesn't seem to help.
Any ideas about how to solve this would be appreciated!