[Advice/Information] Maximum number units per BUS ( 485.... etc ) - Page 1
Forum section: electronics / projects

author: ElectricGuy
author_rank: Regular Contributor
timestamp: April 29, 2014, 12:22:33 am
content: Hi everyone;I'm curious about the design of systems that uses lots of units per BUS over large distances.What kind of bus should i have to implement to have more than 256 devices?I was searching and i realise that i can use, for example, the MAX3061E (1/8 load ) and i can get 256 units/rs485 bus, but if i have more devices?Is a solution using a Microcontroller with 2 uart ports, and use one rs485 bus with 256 units on each uart?How do that kind of systems work? There are some systems on the market, like, security systems, that can have thousands of devices in BUS ( Multiple Bus ).Thank you.

author: ignator
author_rank: Regular Contributor
timestamp: April 29, 2014, 01:51:52 am
content: I'm not aware of any RS485 specification addressing protocol, so unless you've invented one, or used an industry standard on top of the electrical RS485 standard, I don't see your 256 limit.The specification for the MAX3061E may support a minimum of 500KBPS, but your also wanting long transmission line wiring. If you dig into the details, your probably limited to 5-20 meters of cable at that bus rate.The limit to the number of transceivers will be the ability to drive the bit rate/edge speed against the system distributed transmission line parameters.What bit rate are you trying to run?How fast of an edge rate is required?What type of transmission line wire are you using?Your RS485 drivers need to have edge rate control, unless the bit rate is slow enough for the reflected pulses to settle out. Even then the noise produced could result in a BER that renders the system unusable.If you run the baud rate slow enough, and you have drivers that can charge/discharge all the bus wire capacitance, and you control the edge rate to limit the transmission line inductance charging to prevent 'reflected voltages', and the source drivers of every one of the bi-direction RS485 drivers can drive the sink resistance of every load (and previously mentioned cable parameters), there is no (theoretical) limit to how many transceivers can be on this multi-drop bus. (and yes that's a run-on sentence).I don't recall any class I had in transmission line theory that covered multiple transceivers on a transmission line.The industry I worked in was avionics, there the buses used were ARINC429/629/664.The competitor used a RS485 proprietary standard ASCB (Honeywell) that memory says ran at 600KBPS. I don't know the number of nodes, but I do know it was a PITA for the OEM airframer manufacture to get the aircraft wiring impedances right.

author: jerry507
author_rank: Regular Contributor
timestamp: April 29, 2014, 05:11:52 am
content: RS485 standard defines the number of devices on the bus in a strange way. They use the "unit load", of which there can be at most 32 on a segment. They are defined with strange current/voltage diagrams that I honestly never found worth looking at because they're stupidly over-complicated. See this link for details:http://www.ti.com.cn/cn/lit/an/slla166/slla166.pdfSo yes, with your 1/8 unit load transceivers you can support 8*32=256 devices on a segment. How do you get more? Simple, you use repeaters. Since RS485 is only a physical layer, you need some sort of higher level protocol on top of it. My experience is building automation systems, so BACNET, Modbus and N2. Realistically you never hit more than 100 devices on a single RS485 trunk because of other limitations in your protocols. Baud rates are typically screaming at 78.6kbaud or less. The typical solution to very large systems these days is using multiple layered topologies. Ethernet is great for long runs to relatively low numbers of devices and RS485 fills the need for local "last mile" (though not literally a mile, think hundreds of feet, <1000ft) on the very low cost devices.Otherwise you spend a lot of repeaters which really isn't cost effective.