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RS485 Qustions

A

Anti-Spam

Jan 1, 1970
0
Not used RS485 before, so I need some advice please.

I am designing a control system where there is one master and 80
slaves on a RS485 system. The area I need to cover is some 200+ metres
square. The slaves will be scattered over the location in no
particular order. I was going to use standard twisted pair Cat5 cable.
My questions are

I have chosen the MAX487 chip for the driver/receivers, is this a good
choice?

I cannot daisy-chain the wiring because of the way the conduits run
under the roads etc. My only option is to run 4 "backbone" runs of 200
metres and take taps off these backbones, off to the slave modules in
a tree like fashion. Is this going to work?

Where do I place the terminating resistor?

Is it better to use shielded twisted pair cable or unshielded?

If shielded is the better option, do I connect the shield to mains
earth or to my 0 Volt supply?

Thanking you in anticipation.
Mark

[email protected] (remove the X to reply)
 
P

Paul Burke

Jan 1, 1970
0
Anti-Spam said:
Not used RS485 before, so I need some advice please.

I am designing a control system where there is one master and 80
slaves on a RS485 system. The area I need to cover is some 200+ metres
square....My only option is to run 4 "backbone" runs of 200
metres and take taps off these backbones, off to the slave modules in
a tree like fashion. Is this going to work?

You can get almost anything to work if it runs slow enough. I'd drive
the 4 runs using 4 drivers, and keep the branches as short as possible.
A lot depends on the way you want to work the system. If you need a fast
fixed- time poll rate it's a lot harder than if you are checking every
few minutes.
Where do I place the terminating resistor?
Is it better to use shielded twisted pair cable or unshielded?
If shielded is the better option, do I connect the shield to mains
earth or to my 0 Volt supply?

Useful article:

http://www.bb-elec.com/tech_articles/rs422_485_app_note/system_configuration.asp#termination

How are yoou getting power to the remote units? How much do they take?
Is this the caravan site it sounds like?

Paul Burke
 
A

Anti-Spam

Jan 1, 1970
0
You can get almost anything to work if it runs slow enough. I'd drive
the 4 runs using 4 drivers, and keep the branches as short as possible.
A lot depends on the way you want to work the system. If you need a fast
fixed- time poll rate it's a lot harder than if you are checking every
few minutes.

I go pretty much go as slow as I like, refresh interval time for each
slave, can be as much as five minutes or so.

Can't do 4 drivers, not possible with the cabling routes available to
us.
Useful article:

http://www.bb-elec.com/tech_articles/rs422_485_app_note/system_configuration.asp#termination

How are yoou getting power to the remote units? How much do they take?
Is this the caravan site it sounds like?

Decided to power each slave individually, as mains power is available
at each point.
You have a good memory.
 
M

mpm

Jan 1, 1970
0
Not used RS485 before, so I need some advice please.

Mark,
Definitely take a look at:

http://bb-elec.com/RS485.asp

I haven't worked with 485 since a "nightmare" remote camera
installation project many years ago (pan/tilt/zoom, etc..)

But the above link has lots and lots of ideas, hardware, and the like.
-mpm
 
J

James Beck

Jan 1, 1970
0
Not used RS485 before, so I need some advice please.

I am designing a control system where there is one master and 80
slaves on a RS485 system. The area I need to cover is some 200+ metres
square. The slaves will be scattered over the location in no
particular order. I was going to use standard twisted pair Cat5 cable.
My questions are

I have chosen the MAX487 chip for the driver/receivers, is this a good
choice?

I cannot daisy-chain the wiring because of the way the conduits run
under the roads etc. My only option is to run 4 "backbone" runs of 200
metres and take taps off these backbones, off to the slave modules in
a tree like fashion. Is this going to work?

Where do I place the terminating resistor?

Is it better to use shielded twisted pair cable or unshielded?

If shielded is the better option, do I connect the shield to mains
earth or to my 0 Volt supply?

Thanking you in anticipation.
Mark

[email protected] (remove the X to reply)
Is there a reason why you are going half duplex?
With a single master arrangement there is no need for everyone to listen
to everything.
If you are going to use off the shelf CAT5 cables you have plenty of
conductors and if you go full duplex you can make hubs along the
backbone to run off to different areas. Just take advantage of the
failsafe of the Maxim receivers and it is pretty easy to make active
hubs with a pass through for the backbone. Then each "leg" can handle
its' own termination and so on.

Jim
 
H

Hawker

Jan 1, 1970
0
I'm suspecting you will need a few repeaters or drivers since you can't
daisy chain.

The best app note I know of for understanding RS-485 wiring is TI's
SLLA036. Unfortunately TI does not have this anymore. I googled for it
and found it here
http://www.grantronics.com.au/docs/TIA-EIA-485.pdf
TI seems to have another document SLLA173 now. I have not read it so
can't vouch for it. It appears to deal with High Speed issues only and
not basics.
http://focus.ti.com/lit/an/slla173/slla173.pdf
also might want to look at
http://focus.ti.com/lit/an/slla169/slla169.pdf


Hawker
 
J

john jardine

Jan 1, 1970
0
Anti-Spam said:
Not used RS485 before, so I need some advice please.

I am designing a control system where there is one master and 80
slaves on a RS485 system. The area I need to cover is some 200+ metres
square. The slaves will be scattered over the location in no
particular order. I was going to use standard twisted pair Cat5 cable.
My questions are

I have chosen the MAX487 chip for the driver/receivers, is this a good
choice?

I cannot daisy-chain the wiring because of the way the conduits run
under the roads etc. My only option is to run 4 "backbone" runs of 200
metres and take taps off these backbones, off to the slave modules in
a tree like fashion. Is this going to work?

Where do I place the terminating resistor?

Is it better to use shielded twisted pair cable or unshielded?

If shielded is the better option, do I connect the shield to mains
earth or to my 0 Volt supply?

Thanking you in anticipation.
Mark

[email protected] (remove the X to reply)

Seems you've nothing untowards there.
I've done RS485 a few times and the "Art and science of RS-485" PDF at ...
http://archive.chipcenter.com/circuitcellar/july99/c79bp11.htm
is a really good write up.
In a kind, sensible world, we'd be using RS485 instead of that USB
nightmare.
 
T

TT_Man

Jan 1, 1970
0
">
I have chosen the MAX487 chip for the driver/receivers, is this a good
choice?

I cannot daisy-chain the wiring because of the way the conduits run
under the roads etc. My only option is to run 4 "backbone" runs of 200
metres and take taps off these backbones, off to the slave modules in
a tree like fashion. Is this going to work?

Where do I place the terminating resistor?

Is it better to use shielded twisted pair cable or unshielded?

If shielded is the better option, do I connect the shield to mains
earth or to my 0 Volt supply?

Thanking you in anticipation.
Mark

[email protected] (remove the X to reply)
Up to about 56K you could probably get away with no terminators and run in a
Star....
 
A

Anti-Spam

Jan 1, 1970
0
Thanks All appreciated, now onwards and upwards.
 
J

jasen

Jan 1, 1970
0
Not used RS485 before, so I need some advice please.
I cannot daisy-chain the wiring because of the way the conduits run
under the roads etc. My only option is to run 4 "backbone" runs of 200
metres and take taps off these backbones, off to the slave modules in
a tree like fashion. Is this going to work?

you could do a folded bus

(view with fixed pitch font)

T O
R | detail 1 |
U O |
N | branch |~~~~~~| -----------o +
K \=-=-=-=-=-=-=-\|DEVICE| -------. /|
/=-=-=-\/--=-=-/|______| \/ |
| |O(2) (1) /\ |
O O| -------' \|
| |O .... sub branch -----------o -
O O| |
| |O
O .--\/--. detail 2
| | | ------------------
O |device| ------------------
| `------'
O ------+ +-------
| ----+ | | +-----
O | | | |
.-|--. | | | |
|[R] | last device has
| | terminating
`----' resistor
O
| and -=-=- represent a twisted pair
0
|
---- and | etc in the detail diagrams
| represent individual conductors

power wires are not shown, but if you use 4-pair cat5 cable there's
three unused pairs on the trunk and two on the branches that can be
used for power.

Bye.
Jasen
 
M

Mr. C

Jan 1, 1970
0
Up to about 56K you could probably get away with no terminators and run in a

I agree with TT_Man. The only reason to have line termination is to
eliminate reflections that can distort the waveform. If the bit rate
is low enough, you can show that reflections will not interfere with
the signal significantly. A lot of our RS-485 applications ran at
19200 and 38400 and they never needed termination. Also, without
terminators, there is less load on the drivers which means you can
have longer runs or more slave devices. What is your bit rate?

Lou
 
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