RS233/485 Converter

[Nicolas]

Greetings.
My name is Nicolas and it's my first post here so I would like to introduce myself. I'm 25, student of electronic engineering and I live in Argentina.

For some class in college I have to build a project that involves data communication. The thing is for the sake of saving some time I decided to grab a project from the internet (really didn't have the time, and I was a little lazy too ). I decided to build a RS232/485 converter.
The price I'm paying for such a thing is that now the circuit doesn't want to work and I don't know what to do.
I copied the schamatics from here:

http://home.t-online.de/home/Vollmer.Ulrich/englisch/s_index.htm

The question is:
I tried to connect 2 computers via the serial ports and transfer a file with hyperterminal but it didn't work. I will be taking it to the college lab to be able to see what's happening with a scope. Meanwhile, do you think I should add a more sophisticated way to enable or disable (hi z) the 485 line?. I don't know for sure how it is done in that circuit because I couldn't found an application note of SN75176.
Any help will be welcome. Thanks.

 

[Sebi]

The RS485 originally work in half-duplex mode, with one master and many slave devices. The master send one address, and switch the transceiver IC to receive mode, wait for reply... switch to transmit... send the (another) address... etc. So need the specified software on RS232 for RX-TX switching function.
If You need the 485 only for distance increasing, apply two 176 on both side: one in RX mode, another in TX mode. In this case need two twisted pairs, (UTP cable have four) and Hyperterminal can work.

 

[Nicolas]

Hey Sebi, thanks for your answer.

So what you are basically saying is that the circuit I built could work only if I use special software to switch between Tx-Rx?. How about using a Modbus Master-Slave software?

 

[Sebi]

Unfortunately i don't know this software, but with master-slave configuration You cannot started file transfer from slave unit(s).

 

[mechie]

RS232 to RS485

As you are connecting only two devices together you can simplify things a bit by making a few alterations to your circuit.
>> see attached diagram <<
>> I picked this up on a website discussing design techniques - can't remember site name ! >>

The circuit I have (it works for me !) has a couple of features yours doesn't ...

... You seem to be missing the line resistors, one from 75176 pin 6 to +ve power and one from 75176 pin 5 to 0v (both 2k2), and you will require a resistor between pins 5 and 6 (about 100R) at one end of the line, possibly both.

... Your circuit is relying on the RS232 RTS line to control transmit/recieve.
This may or may not work as not all computers drive the RTS line correctly! M$ Windoze is reportedly very poor at timing, MSDOS is much better.
If you add the transistor the 75176 will be forced into transmit mode whenever data is sent on the RS232 line.
My circuit has recieve permanently enabled so the terminal program does not want local echo enabled, the interface (my version) performs an echo in hardware!

:? CAREFUL, you are using half-duplex, the two computers must not try to transmit at the same time, there is no detection of crashes.

I have the 75176 datasheet somewhere ... I just can't find it !

 

[mechie]

RS485 source

Found the website I copied the circuit from ... read their description!

**broken link removed**

I have also found the Texas Instruments 75176 datasheet but its on a website that requires an account!
pm me an email address and I can forward it if you wish (200k .PDF format).

ps. THIS IC IS STATIC - SENSITIVE !!!

 

[rrb011270]

Re: RS232 to RS485

As you are connecting only two devices together you can simplify things a bit by making a few alterations to your circuit.
>> see attached diagram <<
>> I picked this up on a website discussing design techniques - can't remember site name ! >>

The circuit I have (it works for me !) has a couple of features yours doesn't ...

... You seem to be missing the line resistors, one from 75176 pin 6 to +ve power and one from 75176 pin 5 to 0v (both 2k2), and you will require a resistor between pins 5 and 6 (about 100R) at one end of the line, possibly both.

... Your circuit is relying on the RS232 RTS line to control transmit/recieve.
This may or may not work as not all computers drive the RTS line correctly! M$ Windoze is reportedly very poor at timing, MSDOS is much better.
If you add the transistor the 75176 will be forced into transmit mode whenever data is sent on the RS232 line.
My circuit has recieve permanently enabled so the terminal program does not want local echo enabled, the interface (my version) performs an echo in hardware!

:? CAREFUL, you are using half-duplex, the two computers must not try to transmit at the same time, there is no detection of crashes.

I have the 75176 datasheet somewhere ... I just can't find it !

I fabricated a PCB from the circuit u mention (**broken link removed**) in this thread but I can'ge it to work? Can u help me on this? Maybe I miss something... I trace every connection and its find... I use two hyperterminal coz I fabricated two PCB but the circuit from this website I can't get a character?

Need your help.

Thanx


Thanx

 

[pebe]

Re: RS485 source

............I have also found the Texas Instruments 75176 datasheet but its on a website that requires an account!.....

Try here:
**broken link removed**

 

[Sebi]

Or You should use the low-power version...

 

[JimB]

Nicolas

The converter should work OK with hyperterminal, I have done things like this before.

You must use RTS from the PC RS232 port to switch the 485 driver to transmit mode, and send CTS back to the PC to let it know that the driver is in transmit mode.
Here is a schematic of a crude 232/485 converter I made some time ago for a demonstration.

JimB

 

[DanGroff]

The circuit I have (it works for me !) has a couple of features yours doesn't

Hey Mechie,
I have been trying to use that same design from airborn to run the stepper motor in a spectrometer and I can get it to allow the program to move the motor, but it doesn't let the motor tell the program where it is. Any idea? Maybe I wired something wrong. I dunno. Any help would be greatly appreciated. thx

 

[satishnagiri]

Hi i am new to RS-485
From my DTE I want to communicate data to a far away DCE and again
want to do the viceversa operation..For that purpose
can i use the same ckt given in airborn website to communicate data from rs-485 to rs-232...
Thanking U in advance..

 

[sirchokie]

than'k u for rs232 to 485 konverter