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Termination bidirectional tracks

joser , 10-09-2015, 01:33 AM
I'm aware about proper way of termination with series resistor at te begining of the signal track (or parallel termination at the end of the track among others), but if you are developing a board with bidirectional ports, where do you have to put the resistor termination, at the beginning, at the end, both, neither, .... do somebody have any experience with this situation?. If you have very short track considering they are electrically short from the rising/falling edge timming consideration, someone with experience, is always a good practice put a termitation, even if at last you put a 0ohm resistor, with no signal integrity problem
robertferanec , 10-09-2015, 03:22 AM
This is an interesting question. If you are not sure, I would check which of the chips has faster edge and I would place the series termination resistor closer to it.

In reality, I have seen both (placing series termination on the beginning and at the end). For example:
- in many reference designs, series termination resistors for bidirectional pins are placed close to the CPU / FPGA
- however, if you have a look at SODIMM memory modules, you will notice, the data line series termination resistors are there - close to memory chips.

If the connection is more complex (more chips on one bus), I would probably simulate it and try to find the best position, for example have a look at this thread: http://www.fedevel.com/designhelp/fo...ecommendations

... is always a good practice put a termitation, even if at last you put a 0ohm resistor, with no signal integrity problem ...
If you are not sure, I would recommend to use the resistors.
joser , 10-10-2015, 09:02 AM
Thank you Robert for your response. The edges are around 1ns in both sides and the comunication is bus point to point, only two chips, most time the comunication is in one direction but have to work ok in both of course, I have seen in some forums that it is good to put series resistor in both sides since both are drivers, maybe the series resistor at the end of the line from the source point of view is simply added to the line impedance. You talk about simulation, do you know some good manual, tutorial related with simulation for signal integrity in altium designer ( i know hyperlinx is good but is far away for my in the budget project). Thanks.
robertferanec , 10-12-2015, 05:44 AM
You can try to simulate in Altium, however you may want to double check the results with a different simulator.

PS: I have never seen series termination resistors on both sides - I am not sure how effective that is. Also, if both devices are on the same PCB I normally do not use series termination - when you match the impedance and route the tracks properly, it should be fine.
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