Author Topic: Long lines, voltage level translator  (Read 976 times)

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Offline fabiodlTopic starter

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Long lines, voltage level translator
« on: June 12, 2019, 09:08:25 am »
Hello, I need to interface a 3.3v bidirectional bus to a 5v system over long cables  (around 15cm - 6 inches).
I used a txs0108e.
I have an extreme level of ringing/reflections, which never stops once it triggers, and the system does not work at all.
I tried with a 100 ohm series resistor, but it makes no difference.
It seems there are other people having this problem
https://e2e.ti.com/support/logic/f/151/p/619173/2281846#2281846?jktype=e2e
however the part suggested there is for a unidirectional bus.
Do you know any parts that could serve the purpose (possibly without the need to have a pin for the direction, but as a last resort, that would be fine as well).
 

Offline radiolistener

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Re: Long lines, voltage level translator
« Reply #1 on: June 12, 2019, 11:11:14 am »
try to put odd wires (or even) in the ribbon cable to the ground. For example: GND, SIG1, GND, SIG2, GND, SIG3, GND.
Theoretically it probably will stabilize it's impedance, because in such way it will works as symmetrical transmission line.

If your circuit works with RF signals and doesn't depends on DC, you can use 1:2 impedance ratio RF transformer (1:1.5 voltage ratio).
« Last Edit: June 12, 2019, 11:23:05 am by radiolistener »
 

Offline SparkyFX

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Re: Long lines, voltage level translator
« Reply #2 on: June 12, 2019, 11:44:42 am »
Hello, I need to interface a 3.3v bidirectional bus to a 5v system over long cables  (around 15cm - 6 inches).
[...]I have an extreme level of ringing/reflections, which never stops once it triggers, and the system does not work at all.
Have you tried termination resistors or pull-up/pull-down for that matter?
Is twisted pair wiring an option?
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Offline T3sl4co1l

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Re: Long lines, voltage level translator
« Reply #3 on: June 12, 2019, 01:17:38 pm »
Don't use those things, they're a hack and a gimmick.  If you need a bidirectional bus, use control signals to set data direction (e.g. most old-school parallel buses), or double the number of lines and use fixed transmitters and receivers.

6 inches is not a "long line", I don't have any problem with running 3.3V logic over that, board-to-board, as long as, for example, ribbon cable with alternating grounds is used (as mentioned above).  Given some limitations of course, but that shouldn't be terrible for commercial purposes anyway.

If the risetimes are sharp (a few ns or less), use source or load termination resistors, or both; if both, consider a line receiver like 74HC7014, or anything comparable.  (You may need comparators to get a reliable logic input threshold, but in that case, consider LVDS receivers -- they are quite capable RRI comparators in their own right, and cheaper than the fully-spec'd alternative).

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
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Offline fabiodlTopic starter

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Re: Long lines, voltage level translator
« Reply #4 on: June 13, 2019, 01:08:25 am »
Thank you everybody, I tried individually sleeved cables (from a vga cable), but that was not sufficient.
It is indeed a parallel bus, at around 4Mhz. I am now thinking of trying a ‎SN74LVC8T245
 


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