Electronics > Projects, Designs, and Technical Stuff
RS422 terms at both ends?
(1/1)
bitbanger:
Under what circumstances would one want to parallel terminate both the TX and RX ends of a 422 network with 120ohm? I see this referenced every once and a while. Is the hope to diminish reflections at both ends? Does this not inherently conflict with matching cabling characteristic impedance of ~100-120ohm?

One example:
http://www.interfacebus.com/Design_Connector_RS422.html
rstofer:
See Figure 4

https://www.maximintegrated.com/en/app-notes/index.mvp/id/723

From the point of view of the master, the transmit pair are terminated on the far end and the receive pair on the near end.

David Hess:
For bidirectional operation, the transmission line is terminated at both ends.  The RS-422 transmitter only sees the near termination as an additional load.  The termination on the far end from the transmitter is the one which matters.

This is also why transmitters in the middle do not require any termination on their output at all.   They see two parallel transmission line loads and both end terminations absorb reflections.
T3sl4co1l:
422 is always-on so the transmitter impedance is already very low (typically less than Zo) and so you're only wasting power and range while doing nothing for reflections.

485 has an enable so there may be some value in terminating the transmitter -- namely if it's at the end of a line as mentioned above.

Tim
bitbanger:
Sorry perhaps I should have been a little more clear. In the included link, they provide calculations for fail-safe resistor network. The calculations seems to imply that on a single pair, from TX to RX, you would have 120ohm on each end of the pair. No matter multi-drop or not, each RS422 pair is still considered "one direction", there is no TX enable like 485, so I don't see the point in terminating both ends.
Navigation
Message Index
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod