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implementation of discrete RS485 to interface it to AT89C51

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eng_ahmed22

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Salam All
i want to implement RS485 protocole using discrete components and interface it with multiple AT89c/s51 microcontrollers
i dont know much about the RS485 protocole. but the IC is not avialble in my place.
could anyone suggest schematic, tutorial, software (*.ASM or *.C) to implement this cuircuit.
thanks alot for ur time
Ahmed
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silvio

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Re: implementation of discrete RS485 to interface it to AT8

eng_ahmed22 said:
Salam All
i want to implement RS485 protocole using discrete components and interface it with multiple AT89c/s51 microcontrollers
RS485 it's not a protocol but a harware interface. You can't implemented with discrete components. Well, at least not without headache if you still insist.
There are a lot of cheap chips doing for you the job.

Read more about RS485 here: http://www.bb-elec.com/tech_articles/rs422_485_app_note/table_of_contents.asp
 

eng_ahmed22

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Re: implementation of discrete RS485 to interface it to AT8

silvio said:
There are a lot of cheap chips doing for you the job.
Hi silvio :)
thanks for ur interrest
could u give me some ic's numbers to try to find them here??
have a nice day
Ahmed
www.i-g.org
 

silvio

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Re: implementation of discrete RS485 to interface it to AT8

Well, depends on your needs.
Choose one among 142 listed here : http://para.maxim-ic.com/Default_RS422485LineDriverReceivers.htm
and if you're lucky you can receive a sample.

Or you can ask a seller in your appropiate shop for RS485 transceiver.
If he is not selling fruits and vegetables will know what to give you.
 

IanP

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Re: implementation of discrete RS485 to interface it to AT8

If distances among microcontrollers in a network are below, say 50m, you can consider Open Collector (or Open Drain) configuration ..
What is an Open-Collector/Open-Drain Circuit?

Open-collector/open-drain is a circuit technique which allows multiple devices to communicate bi-directionally on a single wire.

Open-collector/open-drain devices sink (flow) current in their low voltage active (logic 0) state, or are high impedance (no current flows) in their high voltage non-active (logic 1) state. These devices usually operate with an external pull-up resistor that holds the signal line high until a device on the wire sinks enough current to pull the line low. Many devices can be attached to the signal wire. If all devices attached to the wire are in their non-active state, the pull-up will hold the wire at a high voltage. If one or more devices are in the active state, the signal wire voltage will be low.

An open-collector/open-drain signal wire can also be bi-directional. Bi-directional means that a device can both output and input a signal on the wire at the same time. In addition to controlling the state of its pin that is connected to the signal wire (active, or non-active), a device can also sense the voltage level of the signal wire. Although the output of a open-collecter/open-drain device may be in the non-active (high) state, the wire attached to the device may be in the active (low) state, due to activity of another device attached to the wire.

The bi-directional nature of an open-collector/open-drain device is what makes this circuit so important in interconnecting many devices on a common line. The I2C Bus and SMBus uses this technique for connecting up to 127 devices.

Open-drain refers to the drain terminal of a MOS FET transistor. Open-collector is the same concept on a bipolar device.
To implement this option you will need 2 NPN transistors per microcontroller and 1 pullup resistor for Rx and 1 for Tx "BUS" ..

Regards,
IanP
 

eng_ahmed22

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Re: implementation of discrete RS485 to interface it to AT8

hi IanP
thanx for th information. but i didn't understand how to use these good information to make a link between several microcontroller boards.
could you sir drow a simple diagram shows how to make this.
thanks alot
Ahmed
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IanP

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Re: implementation of discrete RS485 to interface it to AT8

The concept of OR-wired bus is not new; for example, it is commonly used by I2C protocol ..
Have a look at the picture below .. it shows how devices (microcontrollers) are connected togther using only 2 wires (3-rd wire is the common ground): in this case one wire is clock and the other one is data (you can simply follow the I2C protocol),
or
only one wire used for both: Data Transmission and Data Reception (Half-duplex, similar to 2-wire RS-485, only one device can use bus at a time) ..

Regards,
IanP
 
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