RS485 FAQ?

[simon]

Hello.

Can anyone point me to an Internet source for general questions on RS485
please? I need to know several points such as; maximum line length vs.
speed, calculating termination resistor wattage etc.

Just for the record, I've been asked to develop a half-duplex 60 node
system running at 10Kbps (max.) over a cable length of up to a 100 metre
limit. Just a quick question... given the aforementioned, is twisted
pair essential? The application is in the retail environment so the
worst-case noise is likely to be from fluorescent light fittings.

Thanks for any advice.
Simon.

 

[Frank Bemelman]

Hello.

Can anyone point me to an Internet source for general questions on RS485
please? I need to know several points such as; maximum line length vs.
speed, calculating termination resistor wattage etc.

Just for the record, I've been asked to develop a half-duplex 60 node
system running at 10Kbps (max.) over a cable length of up to a 100 metre
limit. Just a quick question... given the aforementioned, is twisted
pair essential? The application is in the retail environment so the
worst-case noise is likely to be from fluorescent light fittings.

That is roughly 9600 baud, and at that speed 100mtr is nothing for
RS485. I'd certainly use twisted pair, like cheap screened UTP cable.
The price of the cable is nothing compared to the labor (thanks
speff) costs involved, putting the cables in.

And google 'RS485' will give you tons of information. For drivers,
I'd suggest the more expensive varieties, that give better protection
against shortage/overvoltage etc, since your cables run into the
real world. If you can spend the money, use isolated drivers.

Termination at the start and end, typically at the host and the last
device. Use 1W resistors, 120 ohm.

 

[gagir]

B&B electronics does have good info.

Eric Girard

Hello.

Can anyone point me to an Internet source for general questions on RS485
please? I need to know several points such as; maximum line length vs.
speed, calculating termination resistor wattage etc.

Just for the record, I've been asked to develop a half-duplex 60 node
system running at 10Kbps (max.) over a cable length of up to a 100 metre
limit. Just a quick question... given the aforementioned, is twisted
pair essential? The application is in the retail environment so the
worst-case noise is likely to be from fluorescent light fittings.

That is roughly 9600 baud, and at that speed 100mtr is nothing for
RS485. I'd certainly use twisted pair, like cheap screened UTP cable.
The price of the cable is nothing compared to the labor (thanks
speff) costs involved, putting the cables in.

And google 'RS485' will give you tons of information. For drivers,
I'd suggest the more expensive varieties, that give better protection
against shortage/overvoltage etc, since your cables run into the
real world. If you can spend the money, use isolated drivers.

Termination at the start and end, typically at the host and the last
device. Use 1W resistors, 120 ohm.

 

[Spehro Pefhany]

Termination at the start and end, typically at the host and the last
device. Use 1W resistors, 120 ohm.

Hmm.. why would you use 1W resistors, Frank? Pd is more like 200mW
worst case, and it's not like a few percent drift is going to hurt
anything.

Best regards,
Spehro Pefhany

 

[simon]

Hello.

Can anyone point me to an Internet source for general questions on RS485
please? I need to know several points such as; maximum line length vs.
speed, calculating termination resistor wattage etc.

Just for the record, I've been asked to develop a half-duplex 60 node
system running at 10Kbps (max.) over a cable length of up to a 100 metre
limit. Just a quick question... given the aforementioned, is twisted
pair essential? The application is in the retail environment so the
worst-case noise is likely to be from fluorescent light fittings.

That is roughly 9600 baud, and at that speed 100mtr is nothing for
RS485. I'd certainly use twisted pair, like cheap screened UTP cable.
The price of the cable is nothing compared to the labor (thanks
speff) costs involved, putting the cables in.

And google 'RS485' will give you tons of information. For drivers,
I'd suggest the more expensive varieties, that give better protection
against shortage/overvoltage etc, since your cables run into the
real world. If you can spend the money, use isolated drivers.

Termination at the start and end, typically at the host and the last
device. Use 1W resistors, 120 ohm.

Thanks for the reply Frank.

I did 'Google' but there were so many returns I was hoping this post
would narrow-down the most informative links I should look at.

As for the transceiver, I was looking at the Maxim part MAX3340E.

The cable info is much appreciated.

Kind regards,
Simon.

 

[Frank Bemelman]

Hmm.. why would you use 1W resistors, Frank? Pd is more like 200mW
worst case, and it's not like a few percent drift is going to hurt
anything.

The're nice and big, I would even consider 5 Watt, as some jerk
may wire it up to the 24V= doorbell.

 

[Rene Tschaggelar]

Hello.

Can anyone point me to an Internet source for general questions on RS485
please? I need to know several points such as; maximum line length vs.
speed, calculating termination resistor wattage etc.

Just for the record, I've been asked to develop a half-duplex 60 node
system running at 10Kbps (max.) over a cable length of up to a 100 metre
limit. Just a quick question... given the aforementioned, is twisted
pair essential? The application is in the retail environment so the
worst-case noise is likely to be from fluorescent light fittings.

Don't forget to have the GND in the cable as reference.
The common mode of the usual RS485 drivers/receivers is
in the range of 7V. So if you exceed it and later add
an isolation with some couplers, you need the GND.
Too bad, if the cable only has 4 wires then.

Rene

 

[Frank Bemelman]

In message <[email protected]>, Frank Bemelman
<[email protected]> writes

Thanks for the reply Frank.

I did 'Google' but there were so many returns I was hoping this post
would narrow-down the most informative links I should look at.

As for the transceiver, I was looking at the Maxim part MAX3340E.

I didn't read the datasheet, but a quick look at the MAX3340E
revealed that for new designs the improved MAX3450 is recommended.
The 15KV protection is only against short static discharges,
which is nice for (portable) devices that are plugged in/out very
often.

Also take a look at the LT1785 at www.linear.com
Withstands 60V abuse.

 

[simon]

Don't forget to have the GND in the cable as reference.
The common mode of the usual RS485 drivers/receivers is
in the range of 7V. So if you exceed it and later add
an isolation with some couplers, you need the GND.
Too bad, if the cable only has 4 wires then.

Rene

Hi Rene.

I am trying to source a suitable cable with a twisted pair and screen
(GND) with an additional heavy gauge pair of wires for driving
peripheral apparatus. So when it comes to cabling, I'm considering
various options.

Thanks for the reply,
Simon.

 

[Lizard Blizzard]

Hello.

Can anyone point me to an Internet source for general questions on RS485
please? I need to know several points such as; maximum line length vs.
speed, calculating termination resistor wattage etc.

Just for the record, I've been asked to develop a half-duplex 60 node
system running at 10Kbps (max.) over a cable length of up to a 100 metre
limit. Just a quick question... given the aforementioned, is twisted
pair essential? The application is in the retail environment so the
worst-case noise is likely to be from fluorescent light fittings.

If you use twisted pair, especially Cat5, you won't have any problems.
We run 9600 bps over several thousand feet (1000+ M). The terminating
resistor is just a regular 100 ohm 1/4 watt however if you're going to
fool around, a half watt will hold up better with less chance of
breaking the leads.

Thanks for any advice.
Simon.

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[Ben Pope]

Hello.

Can anyone point me to an Internet source for general questions on RS485
please? I need to know several points such as; maximum line length vs.
speed, calculating termination resistor wattage etc.

Just for the record, I've been asked to develop a half-duplex 60 node
system running at 10Kbps (max.) over a cable length of up to a 100 metre
limit. Just a quick question... given the aforementioned, is twisted
pair essential? The application is in the retail environment so the
worst-case noise is likely to be from fluorescent light fittings.

You may want to double check that the drivers you choose are of sufficiently
high impedence that they can do as many as 60 nodes. I've never used this
type of network, but I believe the original standard is closer to half that
many transceivers and you may come across an issue that only appears after
you've stuck a load of the drivers on the bus. It's probably not going to
be a problem, but worth checking now.

Ben

 

[John Jardine]

Hello.

Can anyone point me to an Internet source for general questions on RS485
please? I need to know several points such as; maximum line length vs.
speed, calculating termination resistor wattage etc.

Just for the record, I've been asked to develop a half-duplex 60 node
system running at 10Kbps (max.) over a cable length of up to a 100 metre
limit. Just a quick question... given the aforementioned, is twisted
pair essential? The application is in the retail environment so the
worst-case noise is likely to be from fluorescent light fittings.

Thanks for any advice.
Simon.

In this instance, twisted pair isn't -essential-, it just helps things
along. It's usual just to think in terms of using cheap Category #4 or #5
cable terminated with a 100ohm resistor at the farthest end.
There's a lot of rubbish out there re' RS485, mostly written by companies
wishing to sell their products and as such, only little descriptive info is
provided.
The best description I've come across, came originally from the 'Circuit
Cellar' magazine.
(a copy here ...).
www.gdscorp.com/pdf/rs485.pdf

Note: There's nothing "clever" about RS485, it's -exactly- the kind of thing
you'd come up with if you have the chance to think about it for a few
minutes. Chips such as the MAX485 are nice but the system will still work
fine using just 74HC logic and LM393 comparators. Software is entirely down
to yourself. Max line lengths depend on local noise but 100metres can be
regarded as a 'short' run.
regards
john

 

[simon]

Thanks to everyone on this thread for taking the time to reply.
Much appreciated.

Simon.