How to connect terminating resistor to RS-485 Network?
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From: janii on 30 Apr 2010 09:00 I have a Master board and Two Slave Boards on a RS-485 network. The are connected to each other by 3 pins. A and B are differential signal and third pin is a ground. I want to add terminating resistor to one side. On which pins do I wire the terminating resistor? Also, on which side do I attach this resistor? The board side or the other side? Thanks! --------------------------------------- Posted through http://www.EmbeddedRelated.com
From: Stefan Brröring on 30 Apr 2010 10:24 > I have a Master board and Two Slave Boards on a RS-485 network. The are > connected to each other by 3 pins. A and B are differential signal and > third pin is a ground. > > I want to add terminating resistor to one side. On which pins do I wire > the terminating resistor? > Also, on which side do I attach this resistor? The board side or the other > side? > > Thanks! > The terminating resistor, normaly about 120 Ohms are connected from Signal A to B, or RXTX+ to RXDT-. Often, you see 2 additional resistors from VCC to A and from GND to B. They can be 390 Ohms, or 1k. This additional resistors are for the case, when the master and all slaves in receive mode to put a defined level on the Bus lines. They can be at one of the slaves, or at the master. The 2 termination resistors are located at the two ends of the bus-line, so it depends on the wiring of your system. best regards Stefan DF9BI
From: Mr. C on 10 May 2010 08:57 >I have a Master board and Two Slave Boards on a RS-485 network. The are >connected to each other by 3 pins. A and B are differential signal and >third pin is a ground. >I want to add terminating resistor to one side. On which pins do I wire >the terminating resistor? >Also, on which side do I attach this resistor? The board side or the other >side? Since EIA-485 is a differential bus, a terminator goes between A and B. They are "terminators" becausethey go at the end of a bus and prevent reflections when signals change state. Terminators are usually in pairs, one at each end of the bus. I recommend you read up on busses, line termination, and why and when it is needed. Also, if either your transmission rate is relatively slow, or your cable length is relatively short, you do not need ANY terminators on the bus. They simply aren't needed in those cases. You will have to calculate what "relatively" means above.
From: rickman on 13 May 2010 17:36 On May 10, 8:57 am, Mr. C <fakeem...(a)hotmail.com> wrote: > >I have a Master board and Two Slave Boards on a RS-485 network. The are > >connected to each other by 3 pins. A and B are differential signal and > >third pin is a ground. > >I want to add terminating resistor to one side. On which pins do I wire > >the terminating resistor? > >Also, on which side do I attach this resistor? The board side or the other > >side? > > Since EIA-485 is a differential bus, a terminator goes between A and > B. They are "terminators" becausethey go at the end of a bus and > prevent reflections when signals change state. Terminators are > usually in pairs, one at each end of the bus. I recommend you read up > on busses, line termination, and why and when it is needed. > > Also, if either your transmission rate is relatively slow, or your > cable length is relatively short, you do not need ANY terminators on > the bus. They simply aren't needed in those cases. You will have to > calculate what "relatively" means above. Just to clarify, the need for terminations is not related to clock or data speeds, but to edge rates. If the transition time between '0' and '1' is less than about six times your round trip delay, you will see distortion of the edges. For a clock signal this can cause double clocking on edges and mess up your signals. I have seen this in cables as short as 8 feet. The signals actually show a "plateau" mid point in the transition. Any receiver at that point on the run will be noise sensitive and will likely see multiple edges. For point to point cables, this is not an issue as long as the receiver is at the end of the cable. Rick
From: Oliver Betz on 14 May 2010 05:37
rickman wrote: [...] >Just to clarify, the need for terminations is not related to clock or >data speeds, but to edge rates. If the transition time between '0' >and '1' is less than about six times your round trip delay, you will >see distortion of the edges. For a clock signal this can cause double >clocking on edges and mess up your signals. I have seen this in >cables as short as 8 feet. The signals actually show a "plateau" mid >point in the transition. Any receiver at that point on the run will >be noise sensitive and will likely see multiple edges. For point to as we are speaking about async serial communication, multiple edges will not be harmful to most UARTs if the bit time is much longer than the ringing event. Valid also for CAN transceivers (sampling point/window). Oliver -- Oliver Betz, Muenchen (oliverbetz.de) |