OT - RS485 full duplex question
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Hi all,
I want to design a full-duplex multi-point RS485 system using for example the
LTC490 (or MAX490) RS485 transceiver chips on each node.However, I just
cannot understand how you multi-drop these devices ??
The chips have no transmit enable/disable pin so you cannot put the outputs
into a high impedance state, therefore, every device will have it's outputs
connected to the bus at the same time - surely this will cause bus contention
?(Before anyone says it, I know you can get devices with enable/disable pins
on-chip but I really must know how these devices are supposed to be connected
together first).
Thanks,
Darren
I want to design a full-duplex multi-point RS485 system using for example the
LTC490 (or MAX490) RS485 transceiver chips on each node.However, I just
cannot understand how you multi-drop these devices ??
The chips have no transmit enable/disable pin so you cannot put the outputs
into a high impedance state, therefore, every device will have it's outputs
connected to the bus at the same time - surely this will cause bus contention
?(Before anyone says it, I know you can get devices with enable/disable pins
on-chip but I really must know how these devices are supposed to be connected
together first).
Thanks,
Darren
Comments
Thanks for the pointer to the data sheet. You are correct, the device far
left is the master BUT... this still does not satisfy my question because it
shows devices with driver enable/disable pins. In this situation I think the
output drivers are all disabled apart from the device which is sending data -
that makes sense but WHAT ABOUT THE 490's which don't have the output driver
enable/disable feature ??????
Does this diagram mean the 490's are connected like this as well ? (it
suggests so but no-one dare draw it, it seems !!!).
Vbr,
Darren
[noparse][[/noparse]Non-text portions of this message have been removed]
Take a look at http://pdfserv.maxim-ic.com/arpdf/MAX1487E-MAX491E.pdf
Page 15 figure 25 (half duplex) and figure 26 (full duplex). The MAX490 is
equal to LTC490.
What strikes me is that for full duplex, all nodes have outputs tied
together
except for one (the node most left in figure 26), so only one node can
read. If another node is to read the data, the data must be transmitted
onto the other twisted pair also. Apparently the node at the left is
responsible
to deliver data to all other nodes (master?)
Regards peter
Oorspronkelijk bericht
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Verzonden: woensdag 13 november 2002 13:19
Aan: basicstamps@yahoogroups.com
Onderwerp: [noparse][[/noparse]basicstamps] OT - RS485 full duplex question
Hi all,
I want to design a full-duplex multi-point RS485 system using for example
the
LTC490 (or MAX490) RS485 transceiver chips on each node.However, I just
cannot understand how you multi-drop these devices ??
The chips have no transmit enable/disable pin so you cannot put the outputs
into a high impedance state, therefore, every device will have it's outputs
connected to the bus at the same time - surely this will cause bus
contention
?(Before anyone says it, I know you can get devices with enable/disable pins
on-chip but I really must know how these devices are supposed to be
connected
together first).
Thanks,
Darren
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Perhaps the chips use open collector outputs, though I haven;t noticed that.
Why not use two half duplex channels, where a master (any node) would sent
using
the A/B wires and the addressed slave (any node) would use the Y/Z wires to
transmit.
That way any master/slave pair can have a full duplex connection.
Regards peter
Oorspronkelijk bericht
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Verzonden: woensdag 13 november 2002 17:42
Aan: basicstamps@yahoogroups.com
Onderwerp: Re: [noparse][[/noparse]basicstamps] OT - RS485 full duplex question
Hi Peter,
Thanks for the pointer to the data sheet. You are correct, the device far
left is the master BUT... this still does not satisfy my question because it
shows devices with driver enable/disable pins. In this situation I think the
output drivers are all disabled apart from the device which is sending
data -
that makes sense but WHAT ABOUT THE 490's which don't have the output driver
enable/disable feature ??????
Does this diagram mean the 490's are connected like this as well ? (it
suggests so but no-one dare draw it, it seems !!!).
Vbr,
Darren
[noparse][[/noparse]Non-text portions of this message have been removed]
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used for full or half duplex RS-485 networks. It's
just a pain in half duplex/1-pair mode to be seeing
your own signal all the time! Better leave these
devices as level shifters for datacomm interfaces
(RS-530, RS-449) where you are only working with point
to point connections.
RS-485 is very flexible in that you can define your
own protocols. Typically, in a full duplex mode, you
will have only one master and many slave/listener
devices. The slaves in turn will only respond back to
the master. In half duplex mode, unless a unit is
transmitting, it is in listen/receive mode. Therefore,
it is possible to have many message originators but
only one can talk at a time.
There are no default procedures for handling data
collisions from multiple devices speaking together.
You, as well as commercial vendors, are on your own in
the handling of message collisions. That's part of the
"fun" with RS-485 - there is no standard way of doing
it.
When I was doing firmware for embedded communications
processors, we used a lot of 8051 cores as they made
RS-485 easy to work with. Intel supported a 9th bit
protocol (actually by setting the parity bit) in which
you can tag the nodes address in your protocol to
indicate that you were wanting to speak to a certain
node. If the address received by the internal UART was
yours, then you continued to read in the message.
Otherwise, you disregarded the traffic until the next
node address was sent on the bus.
Defining full duplex communications for more than 2
RS-485 nodes is not worth the headache of defining the
network configuration/mesh required to pull this one
off. One will have to have multiple transceivers or
switching logic to pull it off. Keep it simple!
kevin k asato
--- peter verkaik <peterverkaik@b...> wrote:
> Hi Darren,
>
> Perhaps the chips use open collector outputs, though
> I haven;t noticed that.
>
> Why not use two half duplex channels, where a master
> (any node) would sent
> using
> the A/B wires and the addressed slave (any node)
> would use the Y/Z wires to
> transmit.
> That way any master/slave pair can have a full
> duplex connection.
>
> Regards peter
>
>
>
Oorspronkelijk bericht
> Van: DAZLOGAN@a... [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=kogP3txdvwsaUzCzcBMGb4apercenhWpbavNROvbKCilfL9nyJ-G9_U3clKm1eGC6Fs86Xo_vvB_]DAZLOGAN@a...[/url
> Verzonden: woensdag 13 november 2002 17:42
> Aan: basicstamps@yahoogroups.com
> Onderwerp: Re: [noparse][[/noparse]basicstamps] OT - RS485 full duplex
> question
>
> Hi Peter,
>
> Thanks for the pointer to the data sheet. You are
> correct, the device far
> left is the master BUT... this still does not
> satisfy my question because it
> shows devices with driver enable/disable pins. In
> this situation I think the
> output drivers are all disabled apart from the
> device which is sending
> data -
> that makes sense but WHAT ABOUT THE 490's which
> don't have the output driver
> enable/disable feature ??????
> Does this diagram mean the 490's are connected like
> this as well ? (it
> suggests so but no-one dare draw it, it seems !!!).
>
> Vbr,
> Darren
>
>
> [noparse][[/noparse]Non-text portions of this message have been
> removed]
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed.
> Text in the Subject and
> Body of the message will be ignored.
>
>
> Your use of Yahoo! Groups is subject to
> http://docs.yahoo.com/info/terms/
>
>
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed.
> Text in the Subject and Body of the message will be
> ignored.
>
>
> Your use of Yahoo! Groups is subject to
> http://docs.yahoo.com/info/terms/
>
>
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