RS485 expand beyond 254 nodes? - Page 1
Forum section: electronics / projects

author: onesixright
author_rank: Frequent Contributor
timestamp: September 05, 2018, 08:35:34 am
content: Hi All,This is a pure theoretical questionAssuming I have used all my 254 nodes in a RS485 network. Anyway to expand that?For example, if would use 2 USB/RS485 adapters on my RPI, would that mean I basically double the number of possible nodes?Thanks!

author: guenthert
author_rank: Frequent Contributor
timestamp: September 05, 2018, 09:20:02 am
content: Where does the number '254' come from? RS485 allows for 32 'loads' (like e.g. MAX3485), some chips use higher impedance, lighter (quarter) load and allow up to 128 participants on one 'bus'. That 8 bit limit seems to be due to the addressing scheme of a given protocol (not part of the RS485 standard) and not bound to the physical layer. So without extending the protocol, I do not see, how a second bus will solve the addressing issue. You'd need some kind of routing, if it is truly any-to-any communication (and not some RS422 like master-slave communication).Dunno, but I think RS485 was an interesting idea, which time has come and gone. Ethernet is cheap these days and offers IMHO distinct advantages (not just speed and number of stations, but also galvanic separation in case of e.g. 100baseTX).

author: onesixright
author_rank: Frequent Contributor
timestamp: September 05, 2018, 09:32:26 am
content: Quote from: guenthert on September 05, 2018, 09:20:02 amFrom:http://www.ti.com/lit/an/slla166/slla166.pdfandhttps://e2e.ti.com/blogs_/b/analogwire/archive/2016/03/08/rs-485-basics-how-to-calculate-unit-loads-and-the-maximum-number-of-nodes-on-your-networkQuoteI know about the 32 devices. But apparently you can address 256 devices (with ⅛ load). My question would be same: if you would go ¼ load (128 nodes), how would you extend beyond that? Just curiosity, since I can imagine that big factory have a multitude of nodes.To your TCP/IP solution: its seems enormous overkill for small sensor based devices.

author: JPortici
author_rank: Super Contributor
timestamp: September 05, 2018, 09:34:05 am
content: I think RS-485 evolved to become CANBUS

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 05, 2018, 09:40:17 am
content: Quote from: guenthert on September 05, 2018, 09:20:02 amRS485 has significant advantages in many cases - super easy and cheap to implement, debug and analyse, and does not require active circuitry ( switches) as part of the cabling infrastructure. Unless you need the bandwidth, it's often a much simpler, cheaper and more reliable slution than ethernet.RS485 isn't going anywhere - in some cases it is getting replaced by ethernet, but it's just fine for a huge range of applications.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 05, 2018, 09:41:44 am
content: Quote from: JPortici on September 05, 2018, 09:34:05 amCAN is specifically aimed at automotive, not a replacement for RS485

author: digsys
author_rank: Unknown
timestamp: September 05, 2018, 10:06:22 am
content: First .. 1000% agree with Mike. RS485 is NOT going anywhere, and it is the bus of choice for me. Simple, flexible, adaptable. For long run / nodes I runRS485 > opto < RS485 , which also gives me my isolation. I had a 6Km run of ~ 50 devices at a race course in Malaysia a few years ago. I mention the locationspecifically, because it was built over an old tin mine, and suffered severe electrical strikes every day. The combination was flawless.Note: The differential pair / slope control migrated to USB and SATA, so it a long way fromdead"The easiest approach is to run 2+ comms ports, if that is an option, or set up a settable address decoder, plus other methods. All work FINE

author: onesixright
author_rank: Frequent Contributor
timestamp: September 05, 2018, 10:22:31 am
content: Quote from: digsys on September 05, 2018, 10:06:22 amThank you. So the easiest way is just adding com ports. In this case, adding an extra USB COM port would do the trick?

author: digsys
author_rank: Unknown
timestamp: September 05, 2018, 10:31:33 am
content: Quote from: onesixrightIn software, you can run as many ports as you like. Also, one big advantage of multiple ports is redundancy ... you can split up your network, so if one portfails / HV spike / short etc, you're still running. RS485 is just a hardware layer and can be attached to literally any type of port.

author: JPortici
author_rank: Super Contributor
timestamp: September 05, 2018, 02:02:01 pm
content: Quote from: mikeselectricstuff on September 05, 2018, 09:41:44 amI didn't mean it as a replacement, but as an evolution. canbus solves the problem of addressing, there are network protocols built on it and i think it's using the same twisted pair as a medium?Please, correct me where i'm wrong..

author: langwadt
author_rank: Super Contributor
timestamp: September 05, 2018, 02:22:23 pm
content: Quote from: JPortici on September 05, 2018, 02:02:01 pmCAN bus was first done with RS485 transceivers so close enough

author: ajb
author_rank: Super Contributor
timestamp: September 05, 2018, 02:53:03 pm
content: You can extend to an arbitrary number of nodes in any topology by using active buffers/splitters. The entertainment industry, which uses the RS485-based DMX protocol extensively, has a bunch of devices designed to do exactly this, although in that application they're generally used to provide fault isolation between network segments rather than to expand the number of nodes--with only 512 channels per universe, there's almost never any need for more than ~100 devices on a single network. One trick with half-duplex systems (like the RDM extension to DMX), though, is that unless you have dedicated flow control lines, which no one does, all of the splitters and buffers have to be aware of the protocol so that they can turn their ports around to match the data flow at the right times, and the protocol has to be built with this turn-around time in mind. This isn't so bad in a one master to many slaves arrangement, but in a multi-master or peer-to-peer system things can get messy very quickly.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 05, 2018, 03:47:56 pm
content: Quote from: digsys on September 05, 2018, 10:31:33 amEasysync do some nice metal USB2-RS485 boxes with 1,2,4,8 and 16 ports. Unfortunately they don't do isolated versions, and they don't have protection above that provided by the drivers.

author: Kjelt
author_rank: Super Contributor
timestamp: September 05, 2018, 03:51:16 pm
content: Quote from: mikeselectricstuff on September 05, 2018, 03:47:56 pmYou could use an €29 usb isolator or is it the devices themselves that need to be isolated from eachother?https://www.olimex.com/Products/USB-Modules/USB-ISO/

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 05, 2018, 04:01:10 pm
content: Quote from: Kjelt on September 05, 2018, 03:51:16 pmMost USB isolators don't do USB2 Hi-speed. I typically run at 1-6Mbaud so this matters.