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Need clarification regarding max485 communication

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Full Member level 5
Nov 23, 2012
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Please help me for the following queries..

I want to have a max 485 communication for a length of about 100 meters. What would be perfect choice of wire (size in AWG & cable type) which does not have any voltage drop or data loss.

If i pull up (A) and pull down (B) for all the slaves including master, will it be good or not ??


ALL cable produces loss but for 100m most people, including myself use CAT-5 network cable. It's cheap, easily available and good to several Mb/s.

If you have a system where it is possible nothing is driving the wires, it's a good idea to pull one line high and the other low so they assume a default logic state. If you don't do this there is a risk of the receivers picking up random noise.
You should also terminate the cable at each end. This is explained in the data sheet.

Thanks brian.

I am using 40 slaves and one master. Should i pull one line high and other line low for all the 40 slaves (41 pull up for A and 41 pull down for B) including master.

or is it enough to do that for master only.

Which is better of these two?

The normal wiring is to simply use two resistors, across the wires, one located at each end of the cable. The idea is that an open cable end will reflect signals back from it and corrupt the data and matching it's impedance with a resistor will prevent this happening. The values normally used are 120 Ohms when CAT-5 cable is used. So really, only two resistors are needed but it's important they are wired at the extreme end of the cable, regardless of where the masters and slaves are connected to it.

If you are sure there will always be a device driving the cable you don't need pull-up or pull-down resistors. A problem can arise where one or more devices is listening but nothing is driving, the wires will go high imedance relative to ground and this can cause random pick-up to show as a logic 'noise' coming out of the receivers. If any device is driving the wires it will ensure they are at one logic level or the other so the problem will not show. For example, if your master is always transmitting and the slaves are always listening you don't need pull-up or pull-down resistors. If there is a risk of the wires going high impedance because nothing is driving, adding the resistors will keep the lines in a known state while not loading them so much that a device can't override the pull up/down current. I normally add 1.8K resistors, one pulling one wire high and the other pulling the other wire low. You only need one pull-up and one pull-down for the whole cable.

So, using a system here as an example, the master is at one end of my cable and has ' ground - 1.8K - 120 Ohms - 1.8K - VCC ' with the IC driving across the 120 Ohm. At the far end of the cable (~100m away) there is just a 120 Ohm resistor across the wires.

Thanks much brian for your valuable comments.

Also is it possible to have a communication using max 485 IC through the 12V DC line.

I mean, Is it possible to use A,B line as the power line itself (power line communication)? If possible please tell me how to do this?


It would be difficult because there are two data lines which are always in opposite logic states. You still need a ground in the system so it would technically be possible for both lines to be raised by 12V then subtract 12V from them before using them as logic signals again but the capacitive loading on the lines would restrict you to very slow data rates. A better solution if you use CAT-5 cable would be to use another of the four pairs to carry power.


Thanks again Brian.

Can you please give me the circuit for that. I am going to use 9.6 kbps data rate only. So if it works, i can go for that design which have some line for power and data.


I'm afraid I haven't got a circuit and haven't got time to develop one at the moment. I don't think you would manage 9.6Kbps though because of the high capacitive loading on the lines, it's difficult to predict but I would guess 1.2 Kbps maximum if you were lucky.

By carrying power you would also lose most of the advantages of differential signalling too so your system would be more prone to interference. To be honest, you need a ground connection anyway so it would be far more economical to add an extra power wire, especially if you are using CAT-5 which has 8 wires available and you are only utilizing 3. Also doing that wouldn't sacrifice any performance.


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