RS232 to RS485 Converter: Making the Right Connections

[ionlylooklazy]

I've got a rs485 device and an adapter for rs232<->rs485 and no rs485
experience, so before I solder this wire harness, I want to make sure
I'm making the right connections.


Assuming no flow control,

From the rs485 side, I need TX+,TX-,RX+,& RX-. On the 25pin D
connecter, these are pins 5,22,20,& 7 respectively.

For the rs232 side, I just need RD & TD.

Is there anything I'm missing?

Also, I tore open an old 25 pin rs485 connecter and found a few pins
that were soldered together. I can't find any pin-outs on the
connecter so I have no idea as to there function.

Thanks,
ionlylooklazy

 

[berkeman]

What adapter are you using? AFAIK, there is no standard pinout for RS-485 connections (unlike RS-232). What is the RS-485 connector defined as? Also keep in mind that the RS-485 network cable must be wired as a doubly-terminated bus, with zero-length stubs to any multidrop connections, and a Zo termination at each end of the bus. What is the Zo of the wire you are using for the RS-485 connections?

 

[ionlylooklazy]

yes, you are correct. I was referring to wikipedia.org for the pin-out, but I found the data sheet for the converter I'm using and it uses a different pin layout.

http://www.bb-elec.com/bb-elec/literature/485con.pdf

I don't understand what you mean by doubly terminated bus, and zero-length stubs, but the impedance of the wire I'm using is about 7 ohms. Aslo, right now I am just connecting to one system, so I don't believe I need to worry about multidrop connections.

 

[berkeman]

yes, you are correct. I was referring to wikipedia.org for the pin-out, but I found the data sheet for the converter I'm using and it uses a different pin layout.

http://www.bb-elec.com/bb-elec/literature/485con.pdf

I don't understand what you mean by doubly terminated bus, and zero-length stubs, but the impedance of the wire I'm using is about 7 ohms. Aslo, right now I am just connecting to one system, so I don't believe I need to worry about multidrop connections.

The Zo characteristic impedance of your twisted pair transmission line is most certainly not 7 Ohms. That may be the total resistance of the wire end-to-end or something, but not the Zo. The Zo of typical twisted pair cable is going to be something like 100 Ohms or 75 Ohms or 50 Ohms. Here is some basic info about transmission lines:

http://en.wikipedia.org/wiki/Transmission_line

Even for a point-to-point transmission line with only two devices attached, you need to terminate the two ends of that bus with a Zo resistor to keep from having reflection and ringing issues. A doubly-terminated bus just refers to that -- the wire cable has a termination at each end. And RS-485 has a requirement that any multidrop devices that connect to the bus along its length (including the two end devices at the terminations) has to use a zero-length stub. That is, it has to connect directly to the bus with a very short connection tap. Some multidrop networks (like the TP/FT-10 network that I work with a lot in control networks) allow stubs up to 3 meters long, to help make wiring devices up to the network easier. RS-485 does not allow this.

 

[ionlylooklazy]

OK, thanks for the help. Please remember, I know nothing about rs485.

Zo is 120 ohms, so I'll need a 120 ohm terminating resistor at both sides.

As for the stubs, those would be the Data Out connections from the twisted wire pair? If so, why would I not just wire the Tx+ and Rx+ together? If not, can you please clarify?


Thanks,
iolly

 

[berkeman]

No, the stub would be the short piece of wire or PCB trace that connects the actual twisted pair cable to the RS-485 transceiver IC. When the connection is just point-to-point between two transceivers as you have shown, it's usually easy to keep that connection short.

BTW, keep in mind that the two RS-485 transceivers at the cable ends should share a common ground. RS-485 is a DC-connected (no transformer) network, so the transceivers have to all have the same ground reference. RS-485 transceivers can tolerate a common mode voltage offset of a couple of volts, but not very many. The RS-485 standard mentions running a 3rd wire along with the twisted pair if necessary, in order to connect the 485 transceivers' grounds.

 

[serialeasy]

try to use the HXSP-2108C Industrial Level Optical Isolation RS-232 To RS-485/RS422 Converter produce by hexin-tech.com.cn, whcih with optical isolation,lightningproof and surge protection .The RS-422/485 side and the RS232 side is powered by an external power .

 

[FisseHans]

Is it true that if using a RS485 connection that is short, just a few feet, with any standard RS232 to RS485 converter, then I don't need to use a 100ohms terminal resistor? What is the difference of using a resistor or not using one? what does it do exactly?

 

[FisseHans]

Well after some research I guess I can answer my own question A 120ohms resistor is generally only recommended if the transmission lines are more than 1000 feet, this is also why a rs232 to rs485 converter with a built-in terminal resistor which cannot be disabled is a bad thing.
The reason for the terminal resistor: the resistor matches the resistance of the wires connected to the converter thereby minimizing the noise in the wire. However, the terminal resistor also induce additional current flow which puts load on the converter, which is why the resistor isn't recommended on wire lengths less than 1000 feet.
After a lot of research I finally found a RS232 to RS485 converter that suites my requirements.