Controlling a trigger system through fiber links. Query

Thread Starter


Joined Feb 28, 2019
Hi all,

I'm controlling a trigger system through fiber links. The optical transceiver for triggering the pulse that activate the channel is commanded by a port expander which I send commands through I2C to write it registers. The problem is that once I connect the system and I feed the board where the port expander and the transceiver are, the indeterminate states of the IO pins of the port expander gives 1.6V till I write the registers with 0, this 1.6V are enough for activate the LED of the transmitter, having a logical high state in the other side, although the system is 0V-5V. I don't want to write specifically 0 in the registers to have 0V at the output, and a pull down resistor makes no sense because I have 1.6V at the output of the port expander. How could I do to assure that I have 0 at the IO outputs although the board is feeded and still I don't wrote the registers for have 0 at the output ?

In the other hand at the receiver where I use this scheme:


I get RXVCC (5V) in RXD when there is no light in the fiber, and 0.325V when there is light. I want to get the opposite, 0V or 0.325V (low state) when there is no light and 5V when there is light in the fiber.

I based my design on that:

Thanks in advance

EDIT: Sorry about the subject, I made a mistake in the title and now I can't edit that.


Last edited:


Joined Oct 25, 2020
Re: title: Are you sure? I've been able to go back and edit the title after the fact.
There should be an icon with three dots near the top where you can select "Edit thread".

Thread Starter


Joined Feb 28, 2019
Finally putting a low value pull down resistor worked.

Now all that is left for me to do is achieve electrically HIGH state in the receiver side, when there is HIGH state in the fiber. What I get now is 5V when there is no pulse in the fiber and 0.48V when there is. I cant understand why the manufacturer decide to make the electrical signal as inverse function of the optical signal... I made a PCB and I can't figure out how to do this without adding components.

thanks again

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Joined Oct 2, 2009
Mark and Space

The concept of mark and space is used in telecommunications and dates back to the telegraph system.

A mark was made on the telegraph receiver paper when the pen was down. A space happened when the pen was up and no mark made. The original telegraph was a current based system. Hence a mark occurred when current was applied. A space occurred when there was no current.

When RS232 came along, a mark (or logical 1) was represented by a negative voltage. A space (logical 0) was represented with positive voltage. RS232 driver ICs invert the logic. Hence sending a logical high signal resulted in a mark signal or negative voltage being transmitted.

The fibre optic system using HFBR-14x2Z and HFBR-24x2Z pair of transmitter and receiver does the same thing. The data signal IN is inverted by the transmitter circuitry. The received data OUT is again inverted to recover the TTL signal in its original phase.