Line Driver with 3-STATE Outputs

[Imanuel Shalti Ayele]

Hi guys,

I'm new here and want to be a pro in board design..(need alot of job for that)
I need something to understand here, did I need to put a pull-up/down resistors to this line driver inputs? and maybe 33R serial resistor for termination?

https://www.fairchildsemi.com/datasheets/74/74AC244.pdf

I need to work with RS-422 and want to split 1 input to 4 output with this driver.
Somebody have another idea for that?

Thx thx!!!

 

[aruipksni]

you need to have a rs422 transciver - this buffer is not suitable.

 

[D.A.(Tony)Stewart]

Read the design guidelines

https://focus.ti.com/lit/an/slla070d/slla070d.pdf

RS-485 compliant drivers can be used for Rs-422 multidrop with one driver.

THe termination resistors at each end of the bus match the differential impedance of the pair with active terminators ( ie. pull up/down or series to Vth

 

[Imanuel Shalti Ayele]

Hi guys and thanks!
I forget to mention that i've got a diff. Tx & Rx from outside world(host,PLC etc.) and this 4 wires get into a MAX3490 transceiver.
https://www.sos.sk/a_info/resource/c/max/MAX3485CPA+.pdf

The two lines regarding the transceiver output, one of them i need to split 1 to 4(Tx), and the other one i need to merge 4 to 1(Rx).
I did it with the line driver i mention yesterday.


Thank u again guys!!!!

 

[FvM]

To avoid talking on cross purposes, you better sketch the intended connection of RS-422 transceiver, 3-state driver and lines.

 

[dick_freebird]

If you pin one of the RS422 inputs to midrail then the receiver
will "work" with a single ended input. Not to expected specs.
And you'd then probably lose the input failsafe function if that
matters, unless the "live" output is also pulled to that center
point.

You could throw true and complement out the '244 outputs
using two of the available channels, plus a preceding inversion
somewhere for one. Keep the 100 ohms cross-output termination.
Then the only thing you lose is the reverse current protection
(cold sparing etc. is compromised).