I'm planning to control with Arduino some scientific instrument that have some I/O TTL trigger.
The instruments triggers have to be connected by 50 Ohm coaxial cables.
The first question is: Have I to adapt the impedance on the Arduino side to avoid signal reflections?
Other problem: when the signal travels through the cable, the "seen" impedance is 50 Ohm? In this case with 5V and 50 Ohm I could have 100 mA, and this is too much for Arduino, isn't it?
What does change if the instrument TTL trigger has an "high" or 50 Ohm input impedance?
I was thinking to use the same arduino digital pin as both an input or output for these signals. So I was thinking about putting a 47 Ohm in serie between the arduino pin and the central wire of the cable to prevent high current, and also to use a 4700 Ohm pull-down resistence.
Probably these are stupid questions, but I would appreciate if someone could help me. Any suggestion?
It is not clear if these TTL triggers are inputs or outputs.
I am assuming they are TTL inputs and you want to feed them from the arduino. In which case just connect them up.
The 50 ohm refers to the impedance of the cable not the resistance of it so don't worry about over current. Reflections are only a problem if you are switching a line fast, much faster than you can go with an arduino.
So simply connect it up with nothing to worry about.
I have to control both I/O TTL triggers:
I have to send some triggers to the instruments to operate and receive some triggers from the same instruments telling me that the instruments finished their operations.
I figured out the the cable wasn't a real problem for high impedance TTL triggers, but I think that I have to take care that the instruments TTL are not 50 Ohm adapted. Isn't it?
My idea was to create a "panel" with several BNC connector each of one could be used as a input as an output. So I was wondering if my proposed scheme could be useful for this and also to preserve my arduino from any mistake.
I mean the mistake that can be done using a switch from a pin to 5V and than erroneously setting the pin as output at 0V.
A series resistor will protect against excess current a value of about 200R should do if 5V is all you can possible have. Also transients outside the rails can be clamped with this:- http://www.thebox.myzen.co.uk/Tutorial/Protection.html