Post by thread:PIC & RS485

Hello all: For the next phase of my PIC project (home security), I want to have remote keypads communicating (only) with the master control unit. I originally wanted to use RS232, but was under the impression that RS232 is difficult to deal with when using long runs (i.e. through a house) and multiple interface nodes. For future expandability of the "network", I am leaning toward RS485, using a Maxim interface chip. However, I will not run it as a multi-drop configuration, but as a star-type configuration, with the keypads running (throughout the house) directly to the master. My question is: is my impression of RS232 unfounded? If so, why? Has anyone used RS485 in the way I want to use it? If so, did you use a terminating resistor as each node to minimize reflections? Any other information you could provide would be very helpful Regards, --John

PICLIST@MITVMA.MIT.EDU wrote: > Hello all: > > For the next phase of my PIC project (home security), I want to have remote > keypads communicating (only) with the master control unit. I originally > wanted to use RS232, but was under the impression that RS232 is difficult > to deal with when using long runs (i.e. through a house) and multiple > interface nodes. For future expandability of the "network", I am leaning > toward RS485, using a Maxim interface chip. However, I will not run it as > a multi-drop configuration, but as a star-type configuration, with the > keypads running (throughout the house) directly to the master. > > My question is: is my impression of RS232 unfounded? If so, why? Has > anyone used RS485 in the way I want to use it? If so, did you use a > terminating resistor as each node to minimize reflections? Any other > information you could provide would be very helpful > > Regards, > > --John You can use the RS232 for long distance but you have to make an amplifier to use it without problems. I saw one circit sometime, that could use the RS232 in a ddistance of 100m from the computer. Maybe, if you are intereted in this circuit, I can search it and send you. Carlos L.M.

Hi, I've not actually used RS485 in a star connection although I use it all the time as multi-drop. At baud rates <4800 i find the terminating resistors unnecessary, even when there are many nodes with the most cheapest of cable (including just wires!!). You cant use a terminating resistor for each connection simply because the total parallel resistance of each would be so small that the RS485 lines would effectively be short-circuited. You could however experiment and put one terminating resistor at the master end only but personally, i'd leave 'em out altogether. I almost guarantee you wont have any problems as long as you keep the baud rate to say 1200. Regards, Darren

Carlos: What about using RS232 in a multi-drop or star configuration? I though RS232 was point-to-point. --John > My question is: is my impression of RS232 unfounded? If so, why? Has > anyone used RS485 in the way I want to use it? If so, did you use a > terminating resistor as each node to minimize reflections? Any other > information you could provide would be very helpful > > Regards, > > --John You can use the RS232 for long distance but you have to make an amplifier to use it without problems. I saw one circit sometime, that could use the RS232 in a ddistance of 100m from the computer. Maybe, if you are intereted in this circuit, I can search it and send you. Carlos L.M.

Recommendation is if you just want a star configuration RS232 that will work reliably at long distance use RS422 (5-Wires, Full Duplex) This is same interface used on Macintosh serial ports. RS485 is an improvement on RS422 that allows multimaster, which you don't need in a star configuration. For RS422 you can basically just use CAT5 Cable, and terminate both ends with 100ohm resistors, see the national semiconductor application notes. On Fri, 28 May 1999, John Esposito wrote: {Quote hidden}> Hello all: > > For the next phase of my PIC project (home security), I want to have remote > keypads communicating (only) with the master control unit. I originally > wanted to use RS232, but was under the impression that RS232 is difficult > to deal with when using long runs (i.e. through a house) and multiple > interface nodes. For future expandability of the "network", I am leaning > toward RS485, using a Maxim interface chip. However, I will not run it as > a multi-drop configuration, but as a star-type configuration, with the > keypads running (throughout the house) directly to the master. > > My question is: is my impression of RS232 unfounded? If so, why? Has > anyone used RS485 in the way I want to use it? If so, did you use a > terminating resistor as each node to minimize reflections? Any other > information you could provide would be very helpful > > > Regards, > > --John >

At 12:49 PM 5/28/99 -0400, you wrote: >Carlos: > >What about using RS232 in a multi-drop or star configuration? I though >RS232 was point-to-point. > It usually is. For multi-drop, try RS485. There is an IEEE standard (1118 I think) that defines an address and packet system for talking to multiple black boxes. Basically, the first byte is 9-bits and contains the address, and the subsequent 8-bit bytes contain the data. As soon as a pause in the data stream occurs, or the packet is complete, the receivers must switch back to watching for the 9-bit address again. Supposedly the UARTS in the newer PICs support this. We are looking at doing a system with 100+ data loggers spread down an 18,000 foot long structure with a repeater every few thousand feet. The wire will end up costing more than the electronics :_p. If anyone has implemented this protocol in a 16C7xx or similar, I would sincerely appreciate a copy of the code or a point to it. Kelly **************************************************************************** ******** All legitimate attachments to this email will be clearly identified in the text. William K. Borsum, P.E. OEM Dataloggers and Instrumentation Systems <spam_OUTborsumTakeThisOuTdascor.com> & <http://www.dascor.com>

Carlos, I'd be interested in that circuit. Thanks. Vic Lopez -----Original Message----- From: Carlos L. M. <.....clmKILLspam@spam@PLANET.COM.BR> To: PICLISTKILLspamMITVMA.MIT.EDU <.....PICLISTKILLspam.....MITVMA.MIT.EDU> Date: Friday, May 28, 1999 9:09 AM Subject: Re: PIC & RS485 >EraseMEPICLISTspam_OUTTakeThisOuTMITVMA.MIT.EDU wrote: > >> Hello all: >> >> For the next phase of my PIC project (home security), I want to have remote >> keypads communicating (only) with the master control unit. I originally >> wanted to use RS232, but was under the impression that RS232 is difficult >> to deal with when using long runs (i.e. through a house) and multiple >> interface nodes. For future expandability of the "network", I am leaning >> toward RS485, using a Maxim interface chip. However, I will not run it as {Quote hidden}>> a multi-drop configuration, but as a star-type configuration, with the >> keypads running (throughout the house) directly to the master. >> >> My question is: is my impression of RS232 unfounded? If so, why? Has >> anyone used RS485 in the way I want to use it? If so, did you use a >> terminating resistor as each node to minimize reflections? Any other >> information you could provide would be very helpful >> >> Regards, >> >> --John > >You can use the RS232 for long distance but you have to make an amplifier to >use it without problems. I saw one circit sometime, that could use the RS232 in >a ddistance of 100m from the computer. Maybe, if you are intereted in this >circuit, I can search it and send you. > >Carlos L.M. > >

On Fri, May 28, 1999 at 08:39:44AM -0400, John Esposito wrote: ... > My question is: is my impression of RS232 unfounded? If so, why? Has > anyone used RS485 in the way I want to use it? If so, did you use a > terminating resistor as each node to minimize reflections? Any other > information you could provide would be very helpful FWIW, the latest issue of Circuit Cellar (June, 1999) contains an article by Jan Axelson entitled "Designing RS-485 Circuits". She does a pretty good job of explaining the issues surrounding signalling & slew rate, termination, and cable length, and how they affect attenuation, reflections, etc. Well worth the cover price. --Bob -- ============================================================ Bob Drzyzgula It's not a problem bobspam_OUTdrzyzgula.org until something bad happens ============================================================

On Fri, 28 May 1999, Bob Drzyzgula wrote: > FWIW, the latest issue of Circuit Cellar (June, 1999) > contains an article by Jan Axelson entitled "Designing > RS-485 Circuits". She does a pretty good job of explaining > the issues surrounding signalling & slew rate, termination, > and cable length, and how they affect attenuation, > reflections, etc. Well worth the cover price. Her articles were the sole reason to pick up The MicroComputer Journal -- always interesting, relvant, and useful. Check out her site: Lakeview Research -- http://www.lvr.com/ - j

I wouldn't recommend RS232 for long runs if it can be avoided and it is definitely outside the formal specification BUT at 300 baud it will run with essentially 100% reliability over very large distances on wet string (well, maybe the water needs to be salty :-)). Think about it - you have a nominal +/- volt swing - say +/- 5 in practice. At 300 baud you have 3ms per bit. With 1 mile/1600 metres of wire you probably have say 500 ohms max resistance and 100 nF capacitance. Time constant of this is about 50 microsecond or about 1/60th of a bit time. Faster baud rates will give narrower margins but I'd expect reliable operation around a house at baud rates significantly above 300 baud. That said, it's outside the spec - look at the results on a 'scope and decide for yourself. Summary: For around a house and SLOW data rates it will work. If you can use differential drivers (RS422) then you'll get better reliability. (Being picky AFAIR RS422 is mono-drop and RS485 has the spec extensions to cover multi-drop) regards Russell McMahon From: John Esposito < > >For the next phase of my PIC project (home security), I want to have remote >keypads communicating (only) with the master control unit. I originally >wanted to use RS232, but was under the impression that RS232 is difficult >to deal with when using long runs (i.e. through a house) and multiple >interface nodes. For future expandability of the "network", I am leaning >toward RS485, using a Maxim interface chip. However, I will not run it as >a multi-drop configuration, but as a star-type configuration, with the >keypads running (throughout the house) directly to the master. > >My question is: is my impression of RS232 unfounded? If so, why? Has >anyone used RS485 in the way I want to use it? If so, did you use a >terminating resistor as each node to minimize reflections? Any other >information you could provide would be very helpful

Russell McMahon wrote: > BUT > at 300 baud it will run with essentially 100% reliability over very > large distances on wet string (well, maybe the water needs to be > salty :-)). Sounds fine to me. RS-232 *is* after all differential - it operates on the difference between the TXD and GND wires. If you use *real* RS/ EIA-232 with *real* +/- 10V drive, it should work quite a long distance at quite high baudrates. Accordingly, I see just three problems with it. 1} The single-ended protocol is quite susceptible to bias from ground returns, so it may well need an isolated receiver (just like that recommended for RS-422/ 485!). 2} You must *use* RS-232, not "imaginary RS-232" with 5V CMOS logic levels, which is really "asking" for threshold problems. 3} Many/ most receivers include termination because - that is specified in the standard. Paralleling these for "multi-drop" may well result in out-of-spec termination, in which case again it would be deceitful to say "RS-232 is not good enough". -- Cheers, Paul B.

Hi ! John Esposito wrote: > > > For the next phase of my PIC project (home security), I want to have remote > keypads communicating (only) with the master control unit. I originally > wanted to use RS232, but was under the impression that RS232 is difficult > to deal with when using long runs (i.e. through a house) and multiple > interface nodes. For both RS485 and RS232 you can increase the maximum distance by decreasing the speed (boudt rate). I expect for interfacing a keyboard or card reader you wouldn't need more then 1200 bps. For future expandability of the "network", I am leaning > toward RS485, using a Maxim interface chip. However, I will not run it as > a multi-drop configuration, but as a star-type configuration, with the > keypads running (throughout the house) directly to the master. If you want to do the termination right, you can use at your control unit one transmitter and receiver driver for each line and make the connection of all signals on the TTL side of the drivers. So physically every line will have a driver circuit (and a terminator for RS485) on each end. It costs more, but imho it makes sense for a security system. If someone attacks one of the lines and blows the driver circuit away with a high voltage applaied to the cable you can manage it to keep the other keypads working. > > My question is: is my impression of RS232 unfounded? If so, why? Has > anyone used RS485 in the way I want to use it? > Haven't tried it myself. There are some ideas : - Terminate only one side of the line (at the transmitter). Then a reflected wave travels back from the end to the transmitter and is absorbed by the terminator and doesn't affects the next transmission. - You can terminate only the AC part of the signal by putting a C in series with the terminator. So the voltage wouldn't drop if you connect parallel more terminated cables. - If not terminating at all you can use diodes to clamp the positive and negative overshoot of the reflected wave. Such diodes are build in most ICs. If cost is a problem you can try it first by reducing the speed and by using a check sum in the transmission protocol to make it immune to errors. (That's the way the one-wire bus by Dallas works: no termination, slow speed and a CRC check.) Just my 2 cents ... St.