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thanks. i want it with description cause it is very useful in my project work so can u give it with working process and with neat and clean diagram.....
No the working is very different , specificaly for RS485 2 wire. RS232 to RS485 (full duplex to half duple or master slave) the master uses the RTS to switch from RX to TX
For RS485 to RS232 you will need a local microprocessor (to convert half duplex to full duplex)
Dear it works perfectly in half duplex mode without any need of local controller... I have tested it and used in my commercial product
I suggest you to make this circuit.....its not a complex one which take/waste your precious time...neither expensive as all components will cost under $2 (Rs200) max...it worth it.
The schematic in post #6 is better. I'm not sure the RS485 transmitter is wired properly in the first schematic but regardless of that, RS232 is a full duplex protocol and RS485 is not. It will probably work as a single point-to-point adapter but true RS485 allows for multiple receivers and transmitters on the same wires and hence needs some intelligence to connect and disconnect individual transmitters at the right times. It also needs some collision control detections which is not present in the schematics given.
here is the clear,modified and simple schematic....i have eliminated power side...don't forget to provide 5V and groundView attachment 232-485_schematic.jpg to all ICs
Hello,
I use your schematic, the clean last one on the post , but i do not get data from one port to other
All i get is echo on each side.
I made it eaxactly like in your schematic but i am lost
I tested the rs232, before i connect the rs485 and they send/receive to each other OK and no echo.
When i connect the rs485 chip , it is a texas sn75bc184, i just get a perfect echo on each side, but data is not going from one side to other
I have just two nodes, each having a 120 ohms resistor as terminator, and i installed your pull up/down resistors
Please help if you can
It is rdiculos because it is a such simple and elegant circuit, but i have no luck
Thanks
The schematics all show voltage level converters not protocol converters. There is a lot more to RS485 than simply using two wires. It has to have a method of controlling which direction the data is flowing or there is a risk of all stations listening at the same time or worse, more than one station sending at a time. If they all listen nothing happens and no damage is caused but there is a potentential for damage if more than one transmitter drives the same wire. It is highly likely that one driver will be making a wire high while another is making the same wire low, the supply current then runs out of one driver, in through the other and down to ground making a short across the supply. The driver outputs are built to withstand overloads like that but they may get hot and of course the current consumption may be high.
There are several solutions depending on the application:
1. If the reason for using RS485 is to send fast data over long distances, consider using RS422 instead. Electrically it is almost identical to RS485 but it uses a different pair of wires for transmitting and receiving which removes the need for switching data direction.
2. A timed approach can be used where there is a time window for data to be sent back. In this scenario, the transmitting device sends it's data then switches the direction to receive a reply. The responding station has a fixed time to make it's reply. This what IC4A does in post #6, it is a timer triggered by data being transmitted, when the timer period ends it changes the transmiter to off and receiver to on.
3. The transmitting station can include an address in the data which invites a specific remote station to reply, it then switches to receive and awaits the reply.
4. A byte with 9 bits is sent to indicate an address to select a specific remote station.
In all cases, it is advisable to use error protection such as a CRC byte. If the CRC is incorrect it could indicate a collision or other line problem and the last send/receive operation should be repeated.
Simple RS232 to RS485 converters generally work well with application protocols that provide a reasonable flow control. MODBUS e.g. does perfectly. RS485 doesn't offer collision control, strictly spoken. You might check, if the sent data can be re-read from the bus, buf if so, there's no guarantee of being collision-free.
Betwixt already metioned, that the post #6 circuit, involving monoflop based Tx-Enable is better than the post #1 (and post #8) circuit, which drives the bus in an open-drain manner, depending on pull-up/pul-down resistors. The difference matters when driving longer bus lines or even termination resistors. I also use a converter similar to post #1, but only for locally interfacing a single RS-485 device.
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