current needed for rs485 ??

[BABESH]

current to rs485

Hi,
I want to derive power from rs-232 signals and convert rs232 signals to rs-422 levels,

any idea on how much is current requred to transmit and receive RS-422/485 level signals for distance of say 100m???

--babesh

 

[dipnirvana]

rs485 current

This all depends on baud rate , duty cycle, bus topology (termination & fail-safe) and transceiver.

 

[GrandAlf]

If you use low power rs422 chips at lowish speed without termination, you may get away with charging a cap from the data stream, to provide just a bit of power. I can see a problem to start with, you would need to have the data running initially to provide power, before you actually transmit what you want. Maybe you could get round this, but I have a feeling that it may be a lost cause. Remember you have to level convert as well, at least on the transmitting side. How are you controlling direction, if you are using a pair of devices, you would be lucky to get enough power. Commercial devices I have seen, seem to use external power. Anyway, worth a try. Good luck

 

[BABESH]

Hi all,

Thanks for analysing my problem,
I am new to rs422 communication.
Actually power is derived from RS232 signals DTR & RTS, these are constant signals and dosen't change,
so i can have a constant voltage.

About my application:
i have to do point to point communication 100meters, baud is 9600bps.

IC chosen to convert to rs422 levels consume around 300uamp
So any estimation about the current consumed by my aplication.

thx
Babesh

 

[GrandAlf]

I did not realize that you were using a PC/Terminal. Yes this should work if you have constant power available. You may need to diode OR a few signal lines, so at least on of them is high at any given time. If you know one of them will always be high you will not need to do this of course. Remember also you will need to level shift the output, or at least clamp it, so that you do not have -v and 12v is limited to 5v, google for circuits. Given the above, you will really need to terminate across the data pair at each end, say around 120R. Termination is responsible for most power loss, but you should have enough current available. I am guessing without termination, it would probably not work reliably.

 

[noc_cti]

Hello Babesh,

Can you lift your schematic? I need a similar circuit that is fed from DB-9 terminal.

Thanks..

 

[BABESH]

Hi noc,

attached ckt from google, which i din't tried yet, seems it will work..

 

[GrandAlf]

This is for RS232. For RS485 you do not need the negative supply. Delete D2, R2, & C2. Pin 2 TXD directly to Transmit on RS485 Chip, likewise RXD Pin 3 to Receive. Unless you are using separate transmit and receive chips (4 wires) you will need to control the data direction, TXen, RXen. Can be tied together as one is active high, and the other active low. Check data sheets for operation.

Added after 2 minutes:

5 - 12v is for aux psu input. Hopefully you will not need this.