Combined RS232/485 Interface Chips

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Hi,

Does anyone know any nice chips that can be switched from an "RS232"
to "RS485" mode?

LTC do some:

<http://www.linear.com/prod/datasheet?datasheet46%&product_family=interface

But they seem a bit expensive ($5-6) compared to just using a "MAX232"
clone and 75176 combination. (~1$).

Thanks,

--

John Devereux

Re: Combined RS232/485 Interface Chips

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MAXIM has a couple of them, MAX3160-3162 IIRC, but they are not cheap either.
Your two chip solution seems to be the less expensive variant.

HTH,
Jens


Re: Combined RS232/485 Interface Chips
Remember that many RS-485 protocols require very timely switching of
the bus-enable line, and this is often hard to control in software, so
make sure you pick a solution that has hardware-timed bus-enable
control based on a time-out for the start pulse of the next serial
character.


-Robert Scott
 Ypsilanti, Michigan
(Reply through newsgroups, not by direct e-mail, as automatic reply address is
fake.)


Re: Combined RS232/485 Interface Chips

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Just curious, but just how much time do you have after you activate
the TX_EN line before you have to send the start pulse?  Are we
talking mS, uS, pS, nS or other?

Dennis.

Dennis Hawkins
n4mwd AT amsat DOT org (humans know what to do)

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Re: Combined RS232/485 Interface Chips

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The critical parameter is turning OFF the transmitter after the last
stop bit has been sent. This is often in order of microseconds, before
the opposite side starts to send the response.

Paul


Re: Combined RS232/485 Interface Chips

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 I had a 555, Enabling the Tx on the Start bit and originally
 timed to Disable the Tx about 20uS after the Stop (@ 9600).
 But the remote device would send its Start so quickly (they
 do an echo, in hardware) that my Disable had to be re-timed
 to be half way through the Stop.    Bummer. :(

--
Tony Williams. Change "nospam" to "ledelec" to email.

Re: Combined RS232/485 Interface Chips
On Sun, 16 Nov 2003 11:10:50 +0000 (GMT), Tony Williams

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How do you detect the start bit with a 555 ?

While the 555 can be retriggered by the start bit ("0"), it is also
retriggered by any 0-bit in the data or even by the parity bit, if it
happens to be 0.


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How do you detect the stop bit without actually counting the bits
after the _true_ start bit.
 
Paul


Re: Combined RS232/485 Interface Chips

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 Just some background.  This was a hack of a card doing
 RS485 half-duplex,  with the Enable/Disable of the line
 driver originally done in software.  But the high speed
 remote device could start transmitting the echo-byte
 before we Disabled and the first bits of the echo got
 mangled.

 So I strapped on a 555 monostable. It was triggered by
 the leading edge of  the Start bit at the input to our
 line driver, and in this state Enabled the output of the
 line driver.  1146uS later (see below) the 555 timed-out
 and Disabled the line driver.

 Crude, but when you are desperate...........

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 No, a 555 (in monostable mode) doesn't retrigger.

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 This was 9600 with a fixed 8E1 protocol, which is 1146uS
 from leading edge of Start to trailing edge of Stop. We
 have pullup/down resistors that pull the line into the
 same state as Stop as well,  so it is possible to Disable
 the line driver anywhere between the end of Parity and the
 end of Stop.... about 1042uS to 1146uS.

--
Tony Williams. Change "nospam" to "ledelec" to email.

Re: Combined RS232/485 Interface Chips

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 I've hacked a DS26LS32A+DS3691N pair from RS232 (or 423
 really) into RS485.

 Note though that I've had trouble with some recent, 5v-only,
 RS485 receivers (on the customer's equipment).  It seems that
 they don't really like input voltage swings that go much below 0v.

--
Tony Williams. Change "nospam" to "ledelec" to email.

Re: Combined RS232/485 Interface Chips

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Thanks, but the DS26LS32A seems to be ~$29 (National)! I didn't check
the other one... :)

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I did toy with the idea of connecting RS232 signal levels into an RS485
tranceiver. What with the RS232 current limiting and the RS485
supposed tolerance of voltages "outside the rails".

--

John Devereux

Re: Combined RS232/485 Interface Chips

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That's how everybody else does it.

--
Grant Edwards                   grante             Yow!  Where's th' DAFFY
                                  at               DUCK EXHIBIT??
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Re: Combined RS232/485 Interface Chips
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I am not sure how you would use RS-232 to receive RS-485 since one is
single ended and the other is differential.  The main point to using
RS-485 or RS-422 is the added noise immunity and high drive currents
which results from using low impedance drivers and rejecting common mode
noise.  How would you do this with RS-232 devices regardless of voltage
levels?  I guess an RS-485 receiver could work with an RS-232 xmitter,
but how would RS-485 drive into RS-232 properly?  

As to the original question, I have found multiprotocol parts from
Sipex, Maxim and Linear Tech.  All cost upwards of $4 compared to about
$1 for either RS-232 or RS-485/422.  I use them because of space issues,
although they are starting to put RS-232 in MLF packages which start to
errode the size advantage of the single chip.  

--

Rick "rickman" Collins

snipped-for-privacy@XYarius.com
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Re: Combined RS232/485 Interface Chips

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It wouldn't. The RS232 chip, or portion of the chip, would be used
when the connection is RS232.

The application is a piece of industrial test equipment. At this stage
it looks like it will be used with RS232 (connected to a PC). But in
the future we might want to network them with RS485, for example. If
there is a low cost, simple way to make the interface software
selectable between RS485 and RS232 that would be nice.

It looks like the "combined chip" solutions are all quite a bit more
than the cost of two separate chips. The cost alone isn't really that
much of an issue, but it also seems that the "combined" solutions
would involve relatively obscure devices (compared to MAX232 and
75176, say).

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Thanks, rickman, Grant, and everyone else too :)


--

John Devereux

Re: Combined RS232/485 Interface Chips


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My experience is that the industrial customers want both RS-232 and RS-485
offered, but nobody buys the RS-485 version or is willing to obey a
half-duplex protocol (which is an absolute necessity with RS-485). This
experience is gathered during nearly 20 years of process instrumentation.

In the newest design, there are pads for both RS-232 and RS-485 buffers but
only RS-232 chip is in. Nobody has requested an instrument stocked with the
485 chip yet.

Tauno Voipio
tauno voipio @ iki fi





Re: Combined RS232/485 Interface Chips

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Hi Tauno,

Yes, you are right (although this is early days yet for the product),
this is already starting to sound familiar!




--

John Devereux

Re: Combined RS232/485 Interface Chips
On 17 Nov 2003 19:00:48 +0000, the renowned John Devereux

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My solution is to put two sockets (MAX232 or equivalent, 75176 or
equivalent), 4 capacitors and two connectors in. Then either of the
two chips is plugged in, or both could be soldered in using tiny
packages and the receiver input selected using some method (a resistor
and the tristate output on the RS-485 chip might be okay). Crude, but
it does the trick. If you're adding RS-485 to an existing device that
has only RS-232, watch the worst-case current draw on the drivers, it
can be quite significant.

Best regards,
Spehro Pefhany
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Re: Combined RS232/485 Interface Chips

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Perhpas I misunderstood. What I was refering to was haveing both RS-232 and
RS-485 drivers and receivers hooked to the same pins.  Then you just enable
wichever set you want to use.

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What do protocols have to do with it?

--
Grant Edwards                   grante             Yow!  Where's th' DAFFY
                                  at               DUCK EXHIBIT??
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Re: Combined RS232/485 Interface Chips
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Yes, I misread the posts.  I have considered using both types of parts
since the newer RS-485 parts will tolerate a wider voltage range than
before.  Size is also an issue, but I am finding new MLF packaged RS-232
parts which helps a lot.  I think I have not found a good RS-485/422
part yet.  Most already have the driver and receiver connected on the
bus side and so will not support full-duplex operation.  


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That is the term they use for a chip supporting mulitple interface
standards.  

--

Rick "rickman" Collins

snipped-for-privacy@XYarius.com
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Re: Combined RS232/485 Interface Chips
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SP334 from Sipex. IMO pretty nice chip. Here, in Poland, it costs
about $6.80. The problem is that Sipex distributors can sell you a
lorry of those chips off-hand but if you want to buy 5-10 pieces for a
prototype, you have to wait 2 months for samples. :-( It is rumored
that Sipex has such a sales policy.

Regards,
/J.D.
--
Jan Dubiec, snipped-for-privacy@slackware.pl, mobile: +48 602 101787

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