Post by thread:Multi-Master RS-485

I am thinking of using RS-485 for hooking together some home automation bits which all can Tx & Rx. The Rx bit is easy but what if two or more devices try to Tx at the same time ? Collission detection is obviously required but am unsure of how to implement it on serial line. There will be no "master" in the system so a polling arangement is out. I did have the thought of when a collission is detected the unit could wait for the line to clear then try again. As each unit will have a unique address, I thought that I could use this as a variable for re-try timing to stop the units clashing more than once each time. Does this make any sense ? The serial data would be made up of 10-byte packets with header,crc & tail. I think there would be plenty of uses for such a multi-drop framework. Please tell me if I'm heading up that famous sh** creek ! :-) rEGARDS. Sorry... cOuLdN't hElP mYsElF !! (Yeah, I've been told already !) ______________________________________ Dave Duffy, Audio Visual Devices 8/9-11 Trade Street Cleveland Qld 4163, Australia Ph: +61 7 38210362 Fax: +61 7 38210281 E-mail: spam_OUTAVDTakeThisOuTmailbox.uq.edu.au ______________________________________

Oh, a little more thoughts. If every processor is actively watching the communications, it can detect when an "ack" has been sent to the message originator, and follow it immediately with it's own message, if it has something to send. Token ring methodologies also work extremely well for this, especially when you have a 9-bit capable USART such as is in a PIC or the MAX3100. Andy ================================================================== Andy Kunz - Montana Design - 409 S 6th St - Phillipsburg, NJ 08865 Hardware & Software for Industry & R/C Hobbies "Go fast, turn right, and keep the wet side down!" ==================================================================

At 10:50 PM 9/5/97 +1000, you wrote: >I am thinking of using RS-485 for hooking together some home automation Have the receiver, if it doesn't receive a valid packet (as determined by CRC, ChkSum, etc) reply with silence. If the transmitter doesn't get back an _addressed_ "ACK" signal, it times out and retries after a sorta-random delay (say, based upon TMR0). This would allow things to work reasonably well. Andy ================================================================== Andy Kunz - Montana Design - 409 S 6th St - Phillipsburg, NJ 08865 Hardware & Software for Industry & R/C Hobbies "Go fast, turn right, and keep the wet side down!" ==================================================================

On 5 Sep 97 at 22:50, David Duffy wrote: > I am thinking of using RS-485 for hooking together some home > automation bits which all can Tx & Rx. The Rx bit is easy but what > if two or more devices try to Tx at the same time ? Collission > detection is obviously required but am unsure of how to implement it > on serial line. There will be no "master" in the system so a polling > arangement is out. I did have the thought of when a collission is > detected the unit could wait for the line to clear then try again. > As each unit will have a unique address, I thought that I could use > this as a variable for re-try timing to stop the units clashing more > than once each time. Does this make any sense ? The serial data > would be made up of 10-byte packets with header,crc & tail. I think > there would be plenty of uses for such a multi-drop framework. > Please tell me if I'm heading up that famous sh** creek ! :-) Or token passing? Depending on the traffic CD or token would be better. One thing with token - you don't need electronics for detecting a collision, or delays for trying again - you just wait your turn... Negative side is you've always got traffic. > rEGARDS. Sorry... cOuLdN't hElP mYsElF !! (Yeah, I've been told > already !) > rEAlLY? WhO TOlD YoU?? MikeS <.....mikesmith_ozKILLspam@spam@relaymail.net>

KcW wrote: > > I remember an article by JaN AxElSoN in MicroComputer journal on doing > exactly this with basic stamps, so it should be no problem. I'll dig up a > scan for you if you would like. As I remember it was a pretty good article > (as most of hers are). > KcW Is she related to mIGEUL or his fRIEND? -- Friendly Regards Tjaart van der Walt tjaartKILLspamwasp.co.za ________________________________________________________ | WASP International http://wasp.co.za | | R&D Engineer : GSM peripheral services development | |Vehicle tracking | Telemetry systems | GSM data transfer| |Voice : +27-(0)11-622-8686 | Fax : +27-(0)11-622-8973 | | WGS-84 : 26010.52'S 28006.19'E | |________________________________________________________|

>I am thinking of using RS-485 for hooking together some home automation >bits which all can Tx & Rx. The Rx bit is easy but what if two or more >devices try to Tx at the same time ? Collission detection is obviously >required but am unsure of how to implement it on serial line. There will >be no "master" in the system so a polling arangement is out. I did have >the thought of when a collission is detected the unit could wait for the >line to clear then try again. As each unit will have a unique address, I >thought that I could use this as a variable for re-try timing to stop the >units clashing more than once each time. Does this make any sense ? >The serial data would be made up of 10-byte packets with header,crc & tail. >I think there would be plenty of uses for such a multi-drop framework. >Please tell me if I'm heading up that famous sh** creek ! :-) Your idea of data transmission is the same with that used in CSMA/CD systems (Carrier Sense Multyple Access/ Colosion Detection ) ( the Ethernet standard is the best example ). Speaking about RS485, I dont't know how you detect the colosion ( multyple active transmitters )> You can try for example to receive the transmitted data at the transmitting station and check it to see if it was transmitted correctly . Two years ago I built an RS485 communication system, but with a master device ( a PC ) which polls slave devices ( Philips 80c552 based ) for an interactive TV application. Dorin --------------------------------- Dorin Dogaroiu Research Institute for Computers Bucarest, Romania Email : .....dogaroiudKILLspam.....pcnet.pcnet.ro EraseMEddorinspam_OUTTakeThisOuTkappa.ro --------------------------------- {Quote hidden}>rEGARDS. Sorry... cOuLdN't hElP mYsElF !! (Yeah, I've been told already !) > >______________________________________ >Dave Duffy, Audio Visual Devices >8/9-11 Trade Street >Cleveland Qld 4163, Australia >Ph: +61 7 38210362 Fax: +61 7 38210281 >E-mail: AVDspam_OUTmailbox.uq.edu.au >______________________________________ > >

Dorin Dogaroiu wrote: > >I am thinking of using RS-485 for hooking together some home automation > >bits which all can Tx & Rx. The Rx bit is easy but what if two or more > >devices try to Tx at the same time ? Collission detection is obviously > >required but am unsure of how to implement it on serial line. There will > >be no "master" in the system so a polling arangement is out. I did have <snip> > Speaking about RS485, I dont't know how you detect the colosion ( multyple > active transmitters )> You can try for example to receive the transmitted > data at the transmitting station and check it to see if it was transmitted > correctly . If you're communication is half-duplex (which I guess is almost always the case when there are more than two nodes present), then you can use the Rx/Tx line for both communications and for arbitration. I can't go into details, but the gist of the idea is: 1) Talking node transmits its message. 2) At the end of the message the talking node switches control from the UART to a general purpose I/O. 3) After keeping the comm line at the same level as the stop bit for some predefined time, the talker can initiate an arbitration state by driving the line low. Then the talker can tri-state the comm driver and allow passive termination resistances to maintain the state of the comm line. 4) The listeners catch the transition marking the beginning of the arbitration. 5) Now the tricky part: Fight for the bus. There are a number of ways to do this. Probably the simplest is to pre-assign a time slot in which a device may drive the comm line. If a node has a message to transmit, then it will drive the comm bus during its time slot. Higher priority nodes can be given time slots closer to the start of the arbitration edge. The first node to drive the bus wins. If no node wishes to talk, then the last talking node can win the bus by default and send some sort of sustaining message to keep the bus alive. If you make the inactive period of the arbitration interval greater than one byte time, then it is very easy for unsynchronized nodes to hop on the bus. This whole scheme only works if your 485 driver is a bidirectional one like the LTC485, DS3695 or the infinite other equivalent pin outs. There is a control pin on this device that can disable the transmitter. Suffice it to say that I have proof that this scheme works for RS485 (or is EIA485 now?) when the baud rate is 38.4k and the distances are a half of a kilometer (I didn't have enough wire to test longer distances - although a fiber optic link into the network did increase the lenght to over a kilometer). Scott -- __o I buy pizza instead of gas. \< (*)/(*)

At 10:50 PM 9/5/97 +1000, you wrote: >I am thinking of using RS-485 for hooking together some home automation >bits which all can Tx & Rx. The Rx bit is easy but what if two or more ...... I've done rs485 half duplex, bus topologi, command-response/polling protocol. please look at my page, packets for master to slave are ; SYN STX DEST 'P' ETX ; SYN STX DEST 'S' FROM CNT DATA..DATA ETX and for slave to master: ; SYN STX MASTER ACK ETX ; SYN STX MASTER 'D' TO CNT DATA..DATA ETX I know this is not perfect, no checking for data and maybe too many overhead. But, it is enough, it works :) and I've tested it using hc11(master), PC and 8031. goodluck. http://www.poboxes.com/f f

Hello, < home automation & collision detection > > If you're communication is half-duplex (which I guess is almost > always the case when there are more than two nodes present), then > you can use the Rx/Tx line for both communications and for > arbitration. I can't go into details, but the gist of the idea is: > < lot's of idea's deleted > Funny, I am working on a 485 bus system + protocol myself. I figured out that I could use a half duplex bus ( 2 wires ) using 'switchable' transceivers ( Maxim has got some nice ones --> 1480 B ) The system works with one real master and 'psuedo' masters. The master sends a broadcast or a direct request to one of the slaves. If it is a broadcast the first slave will react and send everything that might be important and will direct the second slave to react. So basicly every point listens until it's adressed Greetings, Gerard Gerritsen ESD Electronics Zwaansprengweg 20 7300 AE Apeldoorn, The Netherlands email : @spam@gerritseKILLspamesd1.esd.nl tel : ++31-55-5499499 fax : ++31-55-5427288

Hi, <snip> > Speaking about RS485, I dont't know how you detect the colosion ( multyple > active transmitters )> You can try for example to receive the transmitted > data at the transmitting station and check it to see if it was transmitted > correctly . I recently designed a 485 bus topology network with 8031 nodes and an embedded 188 and 386. I also tried detecting collisions by receiving what I transmit. I found, though, that this only works if you use a seperate receiver (not the receiver in the same package as your transmitter, like LTC485). Else you always receive what you transmit - irrespective of a collision condition. You need to look at the stuff that is actually ON the two wires. I used a simple checksum and acknowledge scheme. It worked quite well. Niki