Triggering Information

Our optoelectronic interface units offer TTL and optical trigger inputs. Most MRI scanners offer one of these as outputs.

The information here is intended to be helpful but we can't be certain it is up to date or complete. We would be grateful to accumulate more and better information here to help customers using our devices so please let us know if you can help us fill in gaps.


Siemens scanners emit optical pulses using Agilent optical transmitters, on HFBR optical cables. Our interface units can receive these pulses directly and reliably convert them into USB keypress outputs.

These pulses are narrow (~ 50 µs). The inputs on our devices are edge-triggered in hardware and no pulses will be missed because there is no sampling going on.

Siemens trigger adapter

This is a small electronic box that takes the optical pulse from a Siemens scanner and converts it into a TTL output. There is a jumper to choose between a "toggling" mode (in which the TTL signal changes state one for every optical pulse and a "direct" mode (in which the TTL signal is asserted when the optical signal is asserted). Use of the toggling mode can produce the surprising result that the signal is high for one pulse and then low when the next one comes, but this may be advantageous because the narrow 50 µs pulses can be missed if they are not sampled quickly enough by the receiving hardware.

Siemens Prisma Fit

One of our users provided this information:

The optical signal for the trigger originates in the GCA cabinet on Port U2 of the D90 interface. The cable for this signal is labeled "W13470" and also "Trigger Out".

Topic revision: r1 - 03 Aug 2020, BenDugan
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