RS485 trouble
in [Design]
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From: ScadaEng on 11 Jun 2010 19:39 "Joerg" <invalid(a)invalid.invalid> wrote in message news:87fvapFg10U1(a)mid.individual.net... > ScadaEng wrote: >> "Bit Farmer" <bit.farmer(a)yahoo.com> wrote in message >> news:V9-dnQM0RuB8Io_RnZ2dnUVZ_tGdnZ2d(a)giganews.com... >>> ScadaEng wrote: >>>> I have worked allot with RS232, and other serial protocols, but this is >>>> my first RS485 project. It seemed straight forward to me, but now I >>>> have >>>> a big problem. >>>> >>>> I am using the max490 chip. I'm doing full duplex, 4 wire. Each of my >>>> nodes have the max490 communicating with a 16F877A pic chip. The whole >>>> project will have 25 nodes. I just built two pcb's (nodes) and >>>> individually they work as expected. However when I put both of them on >>>> the rs485 bus, I can't talk to either of them! Probably some kind of >>>> collision, but as it turns out my scope just died and I'm on a tight >>>> deadline. Any help is greatly appreciated. >>> Check your output enables. You can't have both on at the same time. >>> The >>> system must be designed to allow only one transmitter to be on at a >>> time. >>> The easiest way to do this is to have the master poll each device in >>> turn. >>> When polled, each device can then send either a NoData packet or reply >>> with the data that it needs to send. This approach was used for several >>> generations of point of sale systems and works quite well. You will >>> need >>> to have unique addresses for each device and a means of setting them. >>> >>> b. Farmer >> That's the thing, there are no output enables with the max490 chip. ... > > > There are. It's called DE, pin 3 on the SO package. You need to use those. > > >> ... I do >> have unique addresses, and I can call each board by itself with it fully >> functioning. Its when I put more than one board on the network(485) that >> I >> can't commmunicate with any of them. Not that it should matter, but I am >> using a modified RTU Modbus protocol. >> > > If you don't use the DE pins I have no idea how you'd get this working. > In case you do use them, have a polling system set up like Bit Farmer > described and it still doesn't work I urgently suggest you borrow a > scope. Someone in the area has got to have one, it can be a fairly > simple and slow 2-channel scope. Best would be a digital scope but I > guess with the tight deadline you can't be picky. > > -- > Regards, Joerg > > http://www.analogconsultants.com/ > > "gmail" domain blocked because of excessive spam. > Use another domain or send PM. Thanks for your input, Joerg, however I must disagree. There are no enables on the max490 chip. They do exist on the 489, 491 however not on the 491. The 491 is apparently auto negotiable, however I can not find documentation on how it is implemented. Thanks.
From: Martin Riddle on 11 Jun 2010 19:40 "ScadaEng" <scadaeng(a)optonline.net> wrote in message news:4c12c635$0$4996$607ed4bc(a)cv.net... > > "Martin Riddle" <martin_rid(a)verizon.net> wrote in message > news:huugjt$qn0$1(a)news.eternal-september.org... >> >> >> "ScadaEng" <scadaeng(a)optonline.net> wrote in message >> news:4c12c363$0$5016$607ed4bc(a)cv.net... >>> >>> "Bit Farmer" <bit.farmer(a)yahoo.com> wrote in message >>> news:V9-dnQM0RuB8Io_RnZ2dnUVZ_tGdnZ2d(a)giganews.com... >>>> ScadaEng wrote: >>>>> I have worked allot with RS232, and other serial protocols, but >>>>> this is my first RS485 project. It seemed straight forward to me, >>>>> but now I have a big problem. >>>>> >>>>> I am using the max490 chip. I'm doing full duplex, 4 wire. Each of >>>>> my nodes have the max490 communicating with a 16F877A pic chip. >>>>> The whole project will have 25 nodes. I just built two pcb's >>>>> (nodes) and individually they work as expected. However when I put >>>>> both of them on the rs485 bus, I can't talk to either of them! >>>>> Probably some kind of collision, but as it turns out my scope just >>>>> died and I'm on a tight deadline. Any help is greatly appreciated. >>>> >>>> Check your output enables. You can't have both on at the same >>>> time. The system must be designed to allow only one transmitter to >>>> be on at a time. The easiest way to do this is to have the master >>>> poll each device in turn. When polled, each device can then send >>>> either a NoData packet or reply with the data that it needs to >>>> send. This approach was used for several generations of point of >>>> sale systems and works quite well. You will need to have unique >>>> addresses for each device and a means of setting them. >>>> >>>> b. Farmer >>> That's the thing, there are no output enables with the max490 chip. >>> I do have unique addresses, and I can call each board by itself with >>> it fully functioning. Its when I put more than one board on the >>> network(485) that I can't commmunicate with any of them. Not that it >>> should matter, but I am using a modified RTU Modbus protocol. >>> >> >> You need the 489 - 491 with transmitter Enables and implement that in >> your protocols. >> Looks like you need to hack the board too, since the it's a 14 pin >> device. >> >> Cheers >> > I understand the transmit enables, but how does the 490 chip do it? It > has no transmit enables, it must negotiate the transmit in some > fashion. > No it does not. If you look at the application info, its used for repeaters. What you can do, is use one chip for RX and one for TX. And power down the TX chip when not transmitting. When the 490 is powered down its output might be in a high impedance state. Look at the LTC490. Or use a transceiver with enables. Cheers
From: Joerg on 11 Jun 2010 19:49 ScadaEng wrote: > "Joerg" <invalid(a)invalid.invalid> wrote in message > news:87fvapFg10U1(a)mid.individual.net... >> ScadaEng wrote: >>> "Bit Farmer" <bit.farmer(a)yahoo.com> wrote in message >>> news:V9-dnQM0RuB8Io_RnZ2dnUVZ_tGdnZ2d(a)giganews.com... >>>> ScadaEng wrote: >>>>> I have worked allot with RS232, and other serial protocols, but this is >>>>> my first RS485 project. It seemed straight forward to me, but now I >>>>> have >>>>> a big problem. >>>>> >>>>> I am using the max490 chip. I'm doing full duplex, 4 wire. Each of my >>>>> nodes have the max490 communicating with a 16F877A pic chip. The whole >>>>> project will have 25 nodes. I just built two pcb's (nodes) and >>>>> individually they work as expected. However when I put both of them on >>>>> the rs485 bus, I can't talk to either of them! Probably some kind of >>>>> collision, but as it turns out my scope just died and I'm on a tight >>>>> deadline. Any help is greatly appreciated. >>>> Check your output enables. You can't have both on at the same time. >>>> The >>>> system must be designed to allow only one transmitter to be on at a >>>> time. >>>> The easiest way to do this is to have the master poll each device in >>>> turn. >>>> When polled, each device can then send either a NoData packet or reply >>>> with the data that it needs to send. This approach was used for several >>>> generations of point of sale systems and works quite well. You will >>>> need >>>> to have unique addresses for each device and a means of setting them. >>>> >>>> b. Farmer >>> That's the thing, there are no output enables with the max490 chip. ... >> >> There are. It's called DE, pin 3 on the SO package. You need to use those. >> >> >>> ... I do >>> have unique addresses, and I can call each board by itself with it fully >>> functioning. Its when I put more than one board on the network(485) that >>> I >>> can't commmunicate with any of them. Not that it should matter, but I am >>> using a modified RTU Modbus protocol. >>> >> If you don't use the DE pins I have no idea how you'd get this working. >> In case you do use them, have a polling system set up like Bit Farmer >> described and it still doesn't work I urgently suggest you borrow a >> scope. Someone in the area has got to have one, it can be a fairly >> simple and slow 2-channel scope. Best would be a digital scope but I >> guess with the tight deadline you can't be picky. >> >> -- >> Regards, Joerg >> >> http://www.analogconsultants.com/ >> >> "gmail" domain blocked because of excessive spam. >> Use another domain or send PM. > > Thanks for your input, Joerg, however I must disagree. There are no enables > on the max490 chip. They do exist on the 489, 491 however not on the 491. > The 491 is apparently auto negotiable, however I can not find documentation > on how it is implemented. Thanks. > Yep, sorry, mixed it up the the 481. Never heard of auto-negotiation though. How would they know which one is allowed to transmit? -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: hamilton on 11 Jun 2010 19:55 On 6/11/2010 4:42 PM, ScadaEng wrote: > I have worked allot with RS232, and other serial protocols, but this is my > first RS485 project. It seemed straight forward to me, but now I have a big > problem. > > I am using the max490 chip. I'm doing full duplex, 4 wire. Each of my nodes Looking at the data sheet: http://datasheets.maxim-ic.com/en/ds/MAX1487-MAX491.pdf The MAX490 does not have an enablse on either the TX or RX, see table on the first page. The MAX490 is not for multi-master applications. Its for a one-to-one or one to many applications. Check out the data sheet for the correct usage. sorry hamilton
From: Spehro Pefhany on 11 Jun 2010 19:55
On Fri, 11 Jun 2010 19:39:21 -0400, the renowned "ScadaEng" <scadaeng(a)optonline.net> wrote: > >"Joerg" <invalid(a)invalid.invalid> wrote in message >news:87fvapFg10U1(a)mid.individual.net... >> ScadaEng wrote: >>> "Bit Farmer" <bit.farmer(a)yahoo.com> wrote in message >>> news:V9-dnQM0RuB8Io_RnZ2dnUVZ_tGdnZ2d(a)giganews.com... >>>> ScadaEng wrote: >>>>> I have worked allot with RS232, and other serial protocols, but this is >>>>> my first RS485 project. It seemed straight forward to me, but now I >>>>> have >>>>> a big problem. >>>>> >>>>> I am using the max490 chip. I'm doing full duplex, 4 wire. Each of my >>>>> nodes have the max490 communicating with a 16F877A pic chip. The whole >>>>> project will have 25 nodes. I just built two pcb's (nodes) and >>>>> individually they work as expected. However when I put both of them on >>>>> the rs485 bus, I can't talk to either of them! Probably some kind of >>>>> collision, but as it turns out my scope just died and I'm on a tight >>>>> deadline. Any help is greatly appreciated. >>>> Check your output enables. You can't have both on at the same time. >>>> The >>>> system must be designed to allow only one transmitter to be on at a >>>> time. >>>> The easiest way to do this is to have the master poll each device in >>>> turn. >>>> When polled, each device can then send either a NoData packet or reply >>>> with the data that it needs to send. This approach was used for several >>>> generations of point of sale systems and works quite well. You will >>>> need >>>> to have unique addresses for each device and a means of setting them. >>>> >>>> b. Farmer >>> That's the thing, there are no output enables with the max490 chip. ... >> >> >> There are. It's called DE, pin 3 on the SO package. You need to use those. >> >> >>> ... I do >>> have unique addresses, and I can call each board by itself with it fully >>> functioning. Its when I put more than one board on the network(485) that >>> I >>> can't commmunicate with any of them. Not that it should matter, but I am >>> using a modified RTU Modbus protocol. >>> >> >> If you don't use the DE pins I have no idea how you'd get this working. >> In case you do use them, have a polling system set up like Bit Farmer >> described and it still doesn't work I urgently suggest you borrow a >> scope. Someone in the area has got to have one, it can be a fairly >> simple and slow 2-channel scope. Best would be a digital scope but I >> guess with the tight deadline you can't be picky. >> >> -- >> Regards, Joerg >> >> http://www.analogconsultants.com/ >> >> "gmail" domain blocked because of excessive spam. >> Use another domain or send PM. > >Thanks for your input, Joerg, however I must disagree. There are no enables >on the max490 chip. They do exist on the 489, 491 however not on the 491. >The 491 is apparently auto negotiable, however I can not find documentation >on how it is implemented. Thanks. THere's a good reason for that... it isn't. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff(a)interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com |