[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

 

[James Meyer]

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

 

[Markus Meng]

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

Jim

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

 

[James Meyer]

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

 

[Tony]

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

Any method that allows RS485 to be transformer-coupled will be
substantially different from RS485, and that one certainly qualifies.

I used a method circa 1990 that I believe was at least a little
cheaper / simpler, and still ended up looking like RS485 (at the time,
I called it "6+2M" encoding) - just encode every 3 message bytes into
a DC-balanced 4 byte sequence (there are more than 64 such sequences,
enough left over for preamble etc if you need it. It's all physical
layer stuff; it all depends whether you are limited at the hardware or
software level.

But failing that, as long as you're using a UART and the transformer
bandwidth is wide enough so all-ones bytes are still recognizable, you
can always add big caps between the chips and the transformers, and
clamp the chip side to around the rails; the periodic stop and start
bits should be enough to keep the DC under control.

Tony (remove the "_" to reply by email)