SN74LVC541A: maximum length of limitation

Fred Cheng

Part Number: SN74LVC541A

Hi team,

my customer want to use this device to redrive eSPI,

there's a cable between buffer and UDC board,

they want to know how long can this cable be to fully redrive the 50MHz signal, (buffer is placed between MB and UDC cable)

or do you recommend we place the buffer close to TX or RX?

over 1 year ago

0 Clemens Ladisch over 1 year ago

Guru 242810 points

Bidirectional signals cannot be buffered.

0 Fred Cheng over 1 year ago in reply to Clemens Ladisch

TI__Expert 4580 points

Hi Ladisch,

forget about bi-directional . they have other specific requirement.

just want to know should I place buffer close to TX or RX?

ideally should be RX, but long trace from TX to buffer may induce reflection, please help comment

0 Clemens Ladisch over 1 year ago in reply to Fred Cheng

Guru 242810 points

To reduce reflections, terminate the line properly (source termination is simplest). If the trace and the cable have different characteristic impedances, then it makes sense to place the buffer at the beginning of the cable.

0 Emrys Maier over 1 year ago

TI__Guru* 96591 points

Hi Fred,

Any trace, wire, or cable is a transmission line when it becomes long enough to have transmission line effects. We have a detailed FAQ on this topic here that may be of help:

[FAQ] What happens when I connect a logic device's output to a 50 ohm transmission line?

0 Fred Cheng over 1 year ago in reply to Emrys Maier

TI__Expert 4580 points

Hi Maier,

thanks for your sharing,

but the materiel doesn't cover my question ,

where should I place the buffer close to?

Rx or TX?

0 Emrys Maier over 1 year ago in reply to Fred Cheng

TI__Guru* 96591 points

I'm not sure what you mean, but perhaps a schematic can help:

There is a buffer at either end of any communication line -- the one sending the signal, and the one receiving it. You can use the SN74LVC541A for either of these. Assuming your receiver cannot be modified and your goal is to increase the drive strength to send a signal over the cable, then the SN74LVC541A would have to be sending the signal into the cable - ie it would be the Tx in the above diagram.