Maker Pro
Maker Pro

485 multi-drop, multi-protocol advice

D

David Fussell

Jan 1, 1970
0
Hi,

I'm looking at an application that requires a few different devices to
share an RS485 connection. The problem (as I see it) is that each
device has it's own protocol, which I would regard as something of a
showstopper, since if it's going to work, I have to analyse each
devices protocols to ensure that (in spite of each device being
addressable) no inadvertent addressing of the wrong device can occur.

I'm tempted to say it can't be done without a common protocol, but I'd
be interested in any comments, either confirming contradicting my
reluctance.

Thanks!

David
 
T

Tilmann Reh

Jan 1, 1970
0
David said:
I'm looking at an application that requires a few different devices to
share an RS485 connection. The problem (as I see it) is that each
device has it's own protocol, which I would regard as something of a
showstopper, since if it's going to work, I have to analyse each
devices protocols to ensure that (in spite of each device being
addressable) no inadvertent addressing of the wrong device can occur.

I'm tempted to say it can't be done without a common protocol, but I'd
be interested in any comments, either confirming contradicting my
reluctance.

Without more details about the protocols in question, it just
can't be said.

Maybe several different protocol can "coexist" on the same bus,
especially if they use different start characters and do not
reply with NAK or the like when they receive something unexpected.
It all depends...

--
Dipl.-Ing. Tilmann Reh
Autometer GmbH Siegen - Elektronik nach Maß.
http://www.autometer.de

==================================================================
In a world without walls and fences, who needs Windows and Gates ?
(Sun Microsystems)
 
C

CFoley1064

Jan 1, 1970
0
I'm looking at an application that requires a few different devices to
share an RS485 connection. The problem (as I see it) is that each
device has it's own protocol, which I would regard as something of a
showstopper, since if it's going to work, I have to analyse each
devices protocols to ensure that (in spite of each device being
addressable) no inadvertent addressing of the wrong device can occur.

I'm tempted to say it can't be done without a common protocol, but I'd
be interested in any comments, either confirming contradicting my
reluctance.

Thanks!

David

That's the main problem with using RS-485. I once did an instrumentation
project of this type (multiple intruments of two different types all using
RS-485), and couldn't get them talking together on the same port. I ended up
purchasing another RS-485 card to have separate ports for the two different
types of instruments. Inelegant, but not too expensive, and it worked.

With this type of problem, it's sometimes better to remember the value of
engineering time. An inelegant, but workable solution is usually better than
blowing a deadline.

Good luck.
Chris
 
W

Wouter van Ooijen

Jan 1, 1970
0
That's the main problem with using RS-485. I once did an instrumentation
project of this type (multiple intruments of two different types all using
RS-485), and couldn't get them talking together on the same port. I ended up
purchasing another RS-485 card to have separate ports for the two different
types of instruments. Inelegant, but not too expensive, and it worked.

Actually it is a problem of any multi-drop network. RS-485 as such has
nothing to do with it, except that it allows multi-drop.


Wouter van Ooijen

-- ------------------------------------
http://www.voti.nl
PICmicro chips, programmers, consulting
 
L

Lizard Blizzard

Jan 1, 1970
0
David said:
Hi,

I'm looking at an application that requires a few different devices to
share an RS485 connection. The problem (as I see it) is that each
device has it's own protocol, which I would regard as something of a
showstopper, since if it's going to work, I have to analyse each
devices protocols to ensure that (in spite of each device being
addressable) no inadvertent addressing of the wrong device can occur.

I'm tempted to say it can't be done without a common protocol, but I'd
be interested in any comments, either confirming contradicting my
reluctance.

Thanks!
David

Since RS-485 is a single pair, it has to be half-duplex, and only one
device can talk at a time, so this generally means it's a single master,
many slaves. You have your PC poll each device, and talk so that only
it answers back to the PC.

--
----------------(from OED Mini-Dictionary)-----------------
PUNCTUATION - Apostrophe
Incorrect uses: (i) the apostrophe must not be used with a plural
where there is no possessive sense, as in ~tea's are served here~;
(ii) there is no such word as ~her's, our's, their's, your's~.

Confusions: it's = it is or it has (not 'belonging to it'); correct
uses are ~it's here~ (= it is here); ~it's gone~ (= it has gone);
but ~the dog wagged its tail~ (no apostrophe).
----------------(For the Apostrophe challenged)----------------
From a fully deputized officer of the Apostrophe Police!

<<Spammers use Weapons of Mass Distraction!>>

I bought some batteries, but they weren't included,
so I had to buy them again.
-- Steven Wright

FOR SALE: Nice parachute: never opened - used once.

(Problem) Evidence of leak on right main landing gear
(Solution) Evidence removed

F
o
d
d
e
r

f
o
r

s
t
u
p
i
d
"
n
o
t

e
n
o
u
g
h

i
n
c
l
d
u
d
e
d

t
e
x
t
"
e
r
r
o
r

m
s
g
..
 
T

Tim Shoppa

Jan 1, 1970
0
Actually it is a problem of any multi-drop network. RS-485 as such has
nothing to do with it, except that it allows multi-drop.

No, it's not a problem of any multi-drop network. Ethernet over thin-net
doesn't have this problem, I2C doesn't have this problem, HPIB doesn't
have this problem, etc. That's because the definitions of those
interfaces specify the protocol and collision dectection/backoff mechanisms.

The "problem" with RS-485 is that it is only an electrical interface
standard. I put "problem" in quotes because it was never intended to
define a protocol.

Tim.
 
W

Wouter van Ooijen

Jan 1, 1970
0
Actually it is a problem of any multi-drop network. RS-485 as such has
No, it's not a problem of any multi-drop network. Ethernet over thin-net
doesn't have this problem, I2C doesn't have this problem, HPIB doesn't
have this problem, etc. That's because the definitions of those
interfaces specify the protocol and collision dectection/backoff mechanisms.

I can follow your reasoning but I think it makes little sense. It is
not a problem of RS-485, it is a problem of what RS-485 is not (it is
not a higher layer protocol).


Wouter van Ooijen

-- ------------------------------------
http://www.voti.nl
PICmicro chips, programmers, consulting
 
Top