Questions about distributing power over RS485

[ehab_vlsi4]

Hi,

I'm working now on designing a control network over RS485 I have three questions

1-I'll use max3082E or max3060E (half duplex, 1/8unit load, slew rate limited, 15kv protection, 5v). any one here has a good or bad experience with this chip?

2-I want to distribute power over RS485, how can this be done ?

3-How RS485 repeater could be designed

thanks,

 

[IanP]

rs485 over power

Max3082 is an equivalent to 75176 (20 years old or more).
Although I haven't use this one I wouldn't worry about it. It will just work.

As far as powr distribution is concerned, RS-485 is not designed for that.
Even if you add bias resistors (probably the smallest values are around 1k) from V+ and 0V you still have to use termination resistor between Data+ and Data- (or A and B) lines. As I said in this configuration there is no room for power transmission.
You will be wasting time in trying to discover how to do it. As soon as you do it, it will not be RS-485 any more !!!

Repeater hardware depends on how are you going to transmit and receive data. If you transmit data only in one direction to all receivers the circuit is very simple. You will use 2 max485 (3082 or others) and connect them back to back (TTL to TTL). In receiving half you will connect pins RE/DE to 0V and thus activate reception. In transmitter half you will connect RE/DE to +V and thus activate tansmission.
If you want to transmit and receive (half duplex) then you will have to built a circuit that will have to take control over switching between transmission and reception. This can be done by hardware (may not work in some circumstancies) by connecting a timer to the RE/DE pins or you will have to use small microcontroller such as PIC or 8051, which will sense through it's INT(interrupt) inputs where is the signal comming from, read it and re-transmit it in the other direction.
If you are interseted I can put as an attachment a pdf file with an example of a repeater circuit based on 87C750/87C51A.

Also, if you are going to built an extencive network of RS-485 devices consider using somewhere optical isolation to prevent your system from ground loop problems.

 

[ehab_vlsi4]

rs485 power

thank you for your reply,

I'm going to use SN65HVD3082 from TI as a replacement of DS3082E since I didn't found a stock in digikey.

Regarding power transmission I was thinking in using coupling coils and choke coils, not just biasing resistor, and any way this topic is not so important to me now.

yes I'm interested in the repeater circuits, plz send me the PDF's



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[IanP]

power over rs-485

As promissed in my previous post attached is the RS-232-485_converter/repeater circuit.
The optical barrier between 232 (and 485x) and 485y is included into the circuit but if not needed can be removed.

 

[ME]

power over 485

As promissed in my previous post attached is the RS-232-485_converter/repeater circuit.
The optical barrier between 232 (and 485x) and 485y is included into the circuit but if not needed can be removed.

Where's the source code / hex file for the 87C51A microcontroller in your schematic?
Without this the circuit is rather useless.