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 once 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 compared to the direct mode in which the narrow 50 µs pulses might be missed if they are not sampled quickly enough by the receiving hardware.

Our interface units will work reliably with the TTL outputs from the Siemens trigger adapters in the direct mode because the TTL trigger input on them is edge triggered. Our interfaces will also work reliably if you select the toggle mode, but the result will be that you only get trigger outputs from the interface for every other trigger from the scanner, and this is probably not what you want.

It is not necessary to use the Siemens trigger adapter with our interface units at all and we recommend using the optical trigger directly.

Siemens Prisma Fit (3T)

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".

Siemens Terra (7T)

The optical signal for the trigger originates in the EPC1 cabinet on Port U3 of the LWL_INT D6 board.


Topic revision: r3 - 01 Sep 2021, BenDugan
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