Framegrebber Triggers
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Framegrebber Triggers
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FOR IMMEDIATE RELEASE
Some frame grabbers such as the PCVision and ICAsync use TTL inputs to trigger the storage of the appropriate frame of interest. These two Imaging Technology frame grabbers have camera configuration files, which allow the user to specify active-high or active-low triggering. In the real world, the sensors used to sense the presence or absence of objects run on 12 to 30 volts DC. Besides the fact that the voltages running the sensors are incompatible with TTL they are often noisy in terms of ringing. In fact, the faster the sensor, the more likely it is to have ringing. A small amount of ringing on a sensor that operates at 15 volts DC can represent a large amount of ringing when interfaced to a 5 volt DC TTL input on the frame grabber. These sensors are made by myriad of manufacturers and not all sensors are created equal and there is no telling what sensor the customer may specify. We found that since the input trigger is TTL, sensitivity to noise and ringing is greater with active low signals than the sensitivity to active high signals. This is due to the fact that a guaranteed TTL low is .8 volts or lower. While a guaranteed active high is 2.0 volts or greater. Any voltages that are between .8 and 2.0 volts are undefined and can be either high or low depending on the particular sensitivity of the actual TTL device on the frame grabber. It is easier for electrical noise to climb above .8 volts than climb above 2.0 volts. This is what makes active low more sensitive. A Schmitt trigger is an electronic circuit that produces an output when the input exceeds a predetermined turn-on or threshold level. The output is maintained until the input falls below a minimum threshold level. This feature or capability is known as hysteresis, it works in both the rising and falling edges and reduces noise and ringing significantly. With this knowledge at hand, we at InoSys, chose to build an interface circuit, which will capitalize on this. First, the circuit must provide optical isolation between the sensor, the sensor power supply and the operating machinery. We isolate the power supply for the sensor from the power supply of the TTL input. To suppress noise due to ringing we added a TTL Schmitt trigger to the output of the optical isolator and then drive the input of the frame grabber from the output of the Schmitt trigger. This approach works equally as well on active-high or active-low modes of operation. Since a number of our customers have experienced problems with various sensors, including noise and double triggering (triggering on the falling and then the rising edge of the sensor signal). We determined the necessity to build an interface circuit, which would allow the use of any sensor made by any manufacturer and operate properly at any speed. InoSys is making this interface circuit available to all integrators and end users who may be experiencing or who want to avoid experiencing similar triggering problems. The interface circuit utilizes screw terminals and will power up to four sensors; the sensor power supply is included. In addition, we include a wired 26 pin connector for the PCVision and ICAsync frame grabbers. If you are using another model frame grabber you can order the interface circuit without the 26 pin connector and supply your own. If you have the need for such a circuit or need to discuss a particular triggering problem feel free to contact us at InoSys. You can call us at 630-293-3474 or Email us at solutions@inosys.com. For More Information Contact: |
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