Terminal program for testing

Hello,
i made a couple of pcbs that conect over optical fibers for data transfer. The pcbs can then connect to a pc or another device with an usb-uart cable.

I want to test the response and measure the delays between the sent and recieved bits... I already did some tests with an osciloscope and a function generator, a simple square pulse wave, but the square wave isn't perfect because the output impedance is non negligible...the FG output at 1MHz is just terrible. So i tried testing with the bray terminal app and got some good results, surprising even...the problem is that the graph in that program makes no sense to me and there aren't many settings and so far i couldn't find any help on the internet.
 
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