1. Home
  2. Hobby Electronics Basics
  3. RS485 Why the limit of 32?

RS485 Why the limit of 32?

I've implemented a few different serial protocols, but never set up an RS485 network. I have several home automation do-dads that need to talk to each other from different parts of the house. I'm thinking of implementing a half-duplex 485 bus on an unused pair of the telephone cabling. Any non-obvious gotchas?

I've noticed that many transceivers specify a maximum of 32 nodes on the network. Since they are tri-stateable, why is there a limit? Is it really just a somewhat arbitrary number based upon bus-loading? Is there some overlying comm protocol that I'm supposed to implement that can only address

32 hosts? FWICT, it looks like I need only to fiddle with the enable lines and send/receive ordinary serial data (of my own format) to the transceiver. Am I missing something important here?
Reply to
Anthony Fremont
Loading thread data ...

Just guessing, but perhaps the address bus is only five bits wide?

--
Regards,
        Doug Miller (alphageek at milmac dot com)

It\'s time to throw all their damned tea in the harbor again.
Reply to
Doug Miller

"Anthony Fremont" schreef in bericht news: snipped-for-privacy@news.supernews.com...

Bus load is a part of the answer. As receivers should have an impedance >=12k, the 32 nodes bring the bus load 12/32~375 Ohm. The transmitters add no load to the bus as they are either sending or tri-stated. There's much more about RS485. Just Google for "RS485 specifications".

petrus bitbyter

Reply to
petrus bitbyter

It is due to the reciever bus loading. There are fraction load parts that allow more recievers on the bus. Maxim, National, Sipex, and others have 1/8 load recievers allowing 256 nodes.

Jim

Reply to
James Beck

RS485 is an electrical spec, not a network spec. There is no address there that is part of the receiver/transmitter hardware.

Jim

Reply to
James Beck

32 is just a round number derived from existing equipment that due to load on the pairs and driving abilities of the receivers pulling up the load..

In may devices you get that operate on 485 for computer interface normally use some form of ASCII or base line fixed length data streams that uses the first part as the device ID.

Modbus comes to mind.

The Device ID's is just part of the message/data that the device operates on. ALl device's actually see all data that is taking place on the bus.

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
Reply to
Jamie

Ok, thanks for the reply. :-) I've found some info on implementng MODBUS. Doesn't seem terribly hard, but I don't think I'll really need even that much sophistication. I'll likely have a master that simply polls the remote slaves. Collisions should be quite infrequent if slaves only speak when spoken to. Thanks again. :-)

Reply to
Anthony Fremont

I saw some transceivers out there that allowed for man more than 32 nodes. Do you have any favorite cheapy parts?

Thanks :-)

Reply to
Anthony Fremont

These page may help answer some of your questions;

formatting link

Yes you can increase the devices above 32.....

formatting link

Reply to
www.interfacebus.com

That's what I was figuring.

I probably won't have a need to connect anything that already implements a protocol, but it might be a good idea (and the experience wouldn't hurt) to go ahead and implement something like Modbus.

Yep, that certainly confirms my suspicions. Seems too easy actually. ;-)

Reply to
Anthony Fremont

My favorite (read the most rugged) is the Maxim stuff. We use a full duplex system, so the MAX3080 series is what I like. The down side is that Maxim is notoriously slow getting production parts in the channel. I have had 12 weeks lead times (many times). We just buy enough at one whack to make it to the next production run, or we use a Linear Tech part that works pretty good too. A lot of times you can find a distributor that carries the Maxim parts in stock, but you will pay their mark up too.

Jim

Reply to
James Beck

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.