Listening to multiple ports under peri-trigger mode?

[tlan]

Hi all,

Under Peri Trigger mode, the sweep will start when the signal amplitude passes the threshold in the LAST ADC port in the sampled list. Is there a way to let it listen to multiple ports and trigger the sweep whichever port passes the threshold first?

For example, I have ADC 14 as left-hand EMG, and ADC 15 as right-hand EMG. Sweep will start whichever hand I twitch first. Is that possible?

Thanks in advance.

[Peter Rice]

You are correct in saying that Signal can only take a level trigger from an input channel that is the last ADC channel in the list.

However, a relatively simple solution to your problem would be to wire up an 8-pin dual comparator chip in a small box to take two inputs from T-pieces on the EMG channels that you want to scan for a trigger condition. You would send the common output of the box to one of the upper-byte digital input pins on the 1401 rear panel socket, and then select the chosen bit number as the (negative-going) trigger in the Signal configuration. The output from the box would go to logic low as the peristimulus trigger in Signal when either of the box inputs was more positive than the reference level.

To define the reference level in the range 0V - 5V, you could fit a potentiometer in the box so you could set it manually. Alternatively, you could set the trigger level from a 1401 DAC output that you program from a Signal script. A single BNC socket on the box could either output the voltage set on the potentiometer for logging on a separate 1401 ADC channel, or it could act as the trigger level input from the 1401 DAC (overriding the potentiometer setting).

If this scheme looks as though it could work for you, I can send you a sketch of the necessary circuit that would be straightforward to assemble in your lab. In this simple circuit, the reference voltage would have to be positive, but the channel inputs would be fully tolerant of negative voltages. They would not change the bipolar waveforms being logged on the ADC channels.

[tlan]

You are correct in saying that Signal can only take a level trigger from an input channel that is the last ADC channel in the list.

However, a relatively simple solution to your problem would be to wire up an 8-pin dual comparator chip in a small box to take two inputs from T-pieces on the EMG channels that you want to scan for a trigger condition. You would send the common output of the box to one of the upper-byte digital input pins on the 1401 rear panel socket, and then select the chosen bit number as the (negative-going) trigger in the Signal configuration. The output from the box would go to logic low as the peristimulus trigger in Signal when either of the box inputs was more positive than the reference level.

To define the reference level in the range 0V - 5V, you could fit a potentiometer in the box so you could set it manually. Alternatively, you could set the trigger level from a 1401 DAC output that you program from a Signal script. A single BNC socket on the box could either output the voltage set on the potentiometer for logging on a separate 1401 ADC channel, or it could act as the trigger level input from the 1401 DAC (overriding the potentiometer setting).

If this scheme looks as though it could work for you, I can send you a sketch of the necessary circuit that would be straightforward to assemble in your lab. In this simple circuit, the reference voltage would have to be positive, but the channel inputs would be fully tolerant of negative voltages. They would not change the bipolar waveforms being logged on the ADC channels.

Thanks, Peter! This sounds like a smart solution and could work. A sketch of the circuit would be very helpful and appreciated.

[Peter Rice]

Hi tlan,

I've scanned the sketch I have done, but can't find a way to attach the file to a private message from this forum.

Could you email me (peter[at]ced.co.uk) with the square brackets and enclosed text replaced with a single symbol? I will send a reply with the schematic attached.