[Q] Fast Logic To Reset Core of a Communication Transformer

[Markus Meng]

Hi all,

I have the following situation. In a design I use the MAX3088
as a failsafe half-duplex RS485 driver, driving a communication
transformer in order to isolate the external lines from the
internal logic. The bitrate is ~3MBit. The link layer telegram
may not be completly dc-balanced. At the end of the telegram
the core of the communication trafo may still contain dc-biased
energy. I have a third yet unused winding on the transformer
I would like to use in order to reset core by "short-cutting"
this third winding for 1..2 usec. Telegram interfame spacing
would allow for this. Now I am looking for a hint or for some
good ideas how it can be done.

Any suggestions or ideas hints would be appreciated.

Best Regards
Markus

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[James Meyer]

On Thu, 17 Feb 2005 16:02:18 +0100, Markus Meng <meng.engineering@bluewin.ch>
wroth:

Hi all,

I have the following situation. In a design I use the MAX3088
as a failsafe half-duplex RS485 driver, driving a communication
transformer in order to isolate the external lines from the
internal logic. The bitrate is ~3MBit. The link layer telegram
may not be completly dc-balanced. At the end of the telegram
the core of the communication trafo may still contain dc-biased
energy.

The core will NOT retain any energy. You need to do nothing special.

Jim

 

[James Meyer]

On Fri, 18 Feb 2005 07:58:31 +0100, Markus Meng <meng.engineering@bluewin.ch>
wroth:

Using a core, and an RS485 half duplex communication bus will result
into a so called trafo-kickback, being dangerous for proper
end-delimiter recognition and yealds to unwanted noise at the end
of a telegram. This will happen if the protocol is not dc-balanced,
and the half-duplex driver will be switched off after xmition of
frame.

Markus

Please look at page 14 of the pdf file at
http://www.ccontrols.com/pdf/Tutorial.pdf to see a way to use transformers in an
RS-485 network without problems due to DC currents.

Jim