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Full duplex RS485 Comm?

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abicash

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Hello

I intend to use SN75176 as the 485 driver to communicate over RS485 to other slaves.
As i understand ,i wont be able to use a Full duplex communication.Then i saw **broken link removed**
I tried this circuit with a standard 485 port to communicate with a PC RS232 serial port.
But i am not able to send data from the PC side.Although i can receive data.

My question is , How does a standard system of RS485 comm link look with using only a single transceiver (2-wire) ?

I saw some Maxim parts MAX1481-6 but they recommend 4 wire comm.

Please someone get back.

Thanks and regards
 
RS485 is a half-duplex communication standard, using two wires.
Tx/Rx switching is done by the RTS and CTS signals from the RS232 port.
An example of this is shown in figure 3 of the Maxim datasheet.

If you want full-duplex, ie the ability to transmit in both diections simultaneoulsy, you need a four wire system, usually known as RS422.
Examples of this are figures 1 and 2 of the Maxim datasheet.

Does this help?

JimB
 
Hello JimB
Thanks for reading and replying.

Yes i understood that from the datasheet.I need to know ,how is full duplex comm achieved in 2-wire 485 link?
Did you see this link **broken link removed**
I tried this ckt and i see that voltage at DE and RE pins is dynamically altered upon PC serial port requests.But when i connect this to a 485 port ,i cannot write any data.I can read data though

Thanks and regards
 
Hello again

After reading a while i have realised that i need a Bi-Directional-Half-duplex communication & not a Full duplex one.
Can someone help?
 
I need to know ,how is full duplex comm achieved in 2-wire 485 link?
It isn't!
Half-duplex - no problem
Full-duplex - not possible using two wire RS485

Did you see this link **broken link removed**
I had a quick look, but the schematic needed some software to view that I dont have and I could not be bothered to download it!

I tried this ckt and i see that voltage at DE and RE pins is dynamically altered upon PC serial port requests.
Yes, the DE will turn on the Driver when transmitting, and RE will turn on the Receiver when in receive mode.
This will have to be driven by CTS from the RS232 port on the PC.

But when i connect this to a 485 port ,i cannot write any data.I can read data though
I am not sure what you are saying here.
Do you mean that you have some device which is sending data from an RS485 port and you can receive that data on a PC, but that you cannot send ant data back to the device from the PC?
Please clarify.

JimB
 
Hi JimB
I have attached the schematic here

I am connecting the PC serial port to MAX232 and connecting RS485 A & B to a device which can read and write data.

I send a command from the PC (on hyperterm) to access a single byte of data from that device,which unfortunately isn't serviced. But when i set the device to send data continuously , i can see the data on my hyperterminal window.
 

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  • Schematic.jpg
    Schematic.jpg
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That circuit seems to want to do things the hard way , most bizarre!

Have a look at my attachment, it is a thing which I put together a few years ago to use as a demonstration and worked OK.
I used the RTS signal to switch the MAX485 tx/rx sections rather than the convoluted connection driven from the tx data as per the circuit which you have used.
I suggest that you try my circuit, it should be easy to modify from the one you have already.

When using hyperterminal on the PC, you will have to set the PC serial port for hardware handshaking.

JimB
 

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  • Basic RS232 RS485 Converter.PNG
    Basic RS232 RS485 Converter.PNG
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Sorry for bothering you guys, but i have a question about RS485 (or Max485) connector. I'm supposed to connect a MAX485 to DB9, but i can't find any schematic which can be used..... How to connect a DB9 with just 2 outputs A and B like that ?

Best regards
 
^
Errr.... i don't really understand...

What bothering me is the Max485 have 4 pins as input (RO, DE, DI, RE), and 2 pins as output (A and B). How's that suppose to connect with DB9 ?
 
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Hi july3d
The MAX485 is a half duplex converter.The DE and RE pins determine the direction control for the device.The actual data is on pins DI and RO (microcontroller side or TTL).
Either you have to build an intelligent circuit which controls the status of DE,RE on the go (similar to an RTS) if RE=low then it is in receive mode and when DE=high,it is in transmit mode so that it is bi-directional or you have to use it as uni directional .The DB9 pins 2 & 3 can be used for A & B for a RS485 communication whereas you can put some intelligence on your board to switch DE & RE.
 
Last edited:
Varunme, this thread is 5 years old. But to answer your question.

Full duplex means a communication link that goes in both directions at the same time, which requires two pairs of wires. The max485 is a transciever that alternately talks, then listens, on one pair of wires.

Yes, you could use four max485 chips, and have one of two at each hardwired as a transmitter and as a receiver.

But the max488 has a hardwired transmitter and receiver in one part, so it would do the same link with two parts instead of four.

Either way, you still need two wire pairs for full duplex.
 
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