RS485 with +-15V common mode

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Boy, I thought it was tough finding an I2C port expander chip that fit
all my requirements.  I am looking for an RS485 tranceiver that I can
use along with an RS232 chip to allow either protocol through the same
pins.  But I can't find an acceptable chip anywhere.  

This was discussed a few weeks ago, but I don't think anyone mentioned
any part numbers that were compatible.  I think I can find an RS232 chip
that will tristate the outputs, but I can not find a full duplex
RS422/RS485 chip that is +- 15 volt tolerant.  

Anyone actually done this before?

Re: RS485 with +-15V common mode
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Yes, Maxim. ( and I think Linear too ?)


Re: RS485 with +-15V common mode
Take a look at LTC1387.

BTW +-15V tolerant is not specifically within the standard RS485 spec,
which is -7V to +12V, so it may not be easy to find.

Mike Anton


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Re: RS485 with +-15V common mode
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Thanks for the info.  I am aware of the spec, but I have seen some
devices that have +-15 volt tolerance or even +25 -20 volts.  But they
are RS-485 either half duplex (so they won't do RS422) or no enables (so
they can't share outputs with the RS232 device).  

I have seen the LTC parts.  In fact the design was using an LTC1334.
But this part is $6 and they won't come down.  Sipex and Maxim have
similar parts at the same price.  I am trying to save a few bucks, but
not take up a lot of space.  

I may give up on the multiprotocol and use RS232 with an external
adapter.

Re: RS485 with +-15V common mode
Remember that full duplex RS485 will require 4 pins, plus a ground pin,
whereas RS232 requires 2 pins, plus a ground pin.

Mike Anton


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Re: RS485 with +-15V common mode
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Actually, I will be using a full 8 pins plus ground so that the RS-422
has Tx, Rx and clock either in or out.  The standard PC RS232 interface
uses 8 signals and ground although I will likely have 4 in and 4 out
while the PC uses 5 in and 3 out.  With 4 each way it will be short one
of each when talking to a PC (DCE) or acting like a PC (DTE) while with
5/3 it will be short two handshakes when doing one or the other.

Re: RS485 with +-15V common mode

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Isnt the RS423 spec a sort of combination of RS422 and RS232 ? So that
a RS423 buffer could be used in either a RS232 environmnet or RS422
environment ?

Regards
   Anton Erasmus


Re: RS485 with +-15V common mode
In comp.arch.embedded,
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Linear's LT1785 and LT1791 are +/- 60V tolerant. Both have driver enable
an receiver enable. The LT1785 is pin compatible with the 75176 and the
LT1791 can be used full-duplex.

--
Stef            (remove caps, dashes and .invalid from e-mail address to reply)

A boy gets to be a man when a man is needed.
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Re: RS485 with +-15V common mode
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Seems this is just not going to work.  I would need two of these along
with an RS-232 chip.  At $2.50+ each the total cost is more than the
single chip approach and also a lot more board space.  

Oh well...

Re: RS485 with +-15V common mode

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I just remember the MAX3160/3161. You can switch this part between  RS232 (
2Rx/2Tx) , Full-Duplex RS485 or Half-Duplex  RS485. RS232 and RS485 mode use the
same pins and according to the datasheet they allow +/-25V input voltage.

HTH,
Jens

Re: RS485 with +-15V common mode
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the
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Yes, Sipex and Linear Tech also make this sort of part.  But they all
charge a lot for them.  That is why I am looking for a two chip (three
actually) solution.

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