RS485 project help.

[j3Riks]

guys i'm monitoring something using some sensors connected to an 89C51 MCU.

what i want to do is send the collected data to my computer located 100 to 200 meters away.

how many RS232 to RS485 devices do i need? is it two? one for the 89C51 and another for my computers serial port?

how about the wire? im planning to use a CAT5e UTP cable.

here's the link of the converter circuit that i will use.

**broken link removed**

do you think this circuit is enough? considering it is already opto-isolated to protect my serial port.

what else should i need for my project? how do i terminate the cables?

 

[ivancho]

I have made a couple of RS232 to RS485 converters, but I have never daisy chain them, but you will only need to address each device and that is it. I believe you will also need a resistor end. I am not an expert on that by no means, I just built something that worked and went on with my life

But the attached article might be of great advice to you, one thing I did notice was that the circuit you show has the DTR and the RTS lines controlling the flow. There are RS485 circuits that use 4 pairs of twisted wires, one for receiving and one for sending, that is called Full comunications I belive. Then there is the half comunication, which uses only one pair, for both TX and Rx.

With that last configuration, you will need that data flow control, but PC (under windows) will not control the DTR and RTS lines good enough, so at higher baudrates it might not work to control the flow. A good way ("more complicated") way to control the flow is by using the start bit of the RS232 transmition to enable the Tx line.

Good Luck

Ivancho

 

[mechie]

RS232 to RS485 and native RS485 ?

See https://www.electro-tech-online.com/threads/ac-volt-and-amp-meter-with-a-0-5vdc-linear-output.7899/

Also, as you appear to be using a homebrew at one end (89C51 MCU) why have RS232 there at all ? produce the RS485 directly with a 75176 and then just one converter need be built for the computer end of the link.

Opto isolation is nice but probably not essential, are the comms lines likely to take a lightning strike?

 

[j3Riks]

@mechie

yes i already figured out that i only need a converter at the PC side.

how do i terminate the cable(CAT5 UTP)? i saw that the converter circuit already has a 120 ohm terminator resistor. so i will just need to terminate the cable at the 89C51.

its my first time to do this kind of stuff so i really don't know the right things to do.

do i only need to add the 120 ohm termination and it will already function?[/quote]

 

[j3Riks]

RS485 question

yes i already figured out that i only need a convertor at the PC.

how do i terminate the cable? CAT5 UTP

i think the converter already has a 120 ohm resistor terminator so i just need to put another one at the other end of the cable.

is this what i just need to do to make it work?

this is my first time to do this. please help me.

BTW im using Java to control the serial port.

what i just want to do is retrieve the data collected by the 89C51 to my PC.