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Converting TTL signals to voltage

https://forums.millisecond.com/Topic18066.aspx

By Dylonius - 1/12/2016

Hello, 

I'm starting a new study which combines Cognitive Tasks, Polysomnography and ERPs.   I would like to use Inquisit for cognitive tasks that participants will be completing after a night in the sleep lab.  Therefore, I will need to send TTL signals from Inquisit into the SomnoMedics Domino system, which is designed to measure sleep EEG.

I contacted SomnoMedics and apparently the only hardware solution to interface these two software packages is to use a DC box to convert the output triggers from Inquisit into voltages which will be recorded as pulses of various voltage amplitudes in Domino, which I will later convert to stimulus/response markers.  

How to Present TTL Signals Through the Parallel Port (Windows and Mac)

How to Present TTL Signals Through the Parallel Port (Windows and Mac)

My question is, do different TTL signals convert to different voltages when sent through a DC box? For example, does a signal of 10101010 yield a different voltage than a signal of 10001001?   This is the only way I understand that it will be possible to differentiate stimulus in a way that corresponds with our EEG recording; unfortunately I don't have a way to test this as I currently don't have the hardware. 
By Dave - 1/12/2016

> My question is, do different TTL signals convert to different voltages when sent through a DC box? For example, does a signal of
> 10101010 yield a different voltage than a signal of 10001001?

This would largely depend on the technical specifics of the respective DC box, but it is plausible that the voltage would differ. In theory, a TTL-level pin on a parallel port's data register outputs 0V when set to "low" (0) and +5V when set to "high" (1). The actual voltages will somewhat deviate from those theoretical values in the real world. In other words, the more pins are set to "high" (1), the higher the cumulative voltage.

10101010 has 4 pins set to "high", i.e., +5V * 4 voltage in total. 10001001 only has 3 pins set to "high", yielding a (theoretical) +15V in total.

For example http://computer.howstuffworks.com/parallel-port1.htm provides a nice overview of how parallel ports actually work.

Hope this helps.