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Compatibilities between RS-485 and RS-422

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xandimg

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half duplex transmission protocol for rs485

Is it possible to use a RS-485 interface with a device which uses RS-422?

In other words, is RS-485 fully compatible with RS-422?

Thanks in advance for your help.
 

freedelphi

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max 485 pinout

The electrical characteristic is the same. Many vendors sell chips that can be used as RS485/RS422 line drivers.

BUT :
- RS 485 is half duplex (2 wire)
- RS422 is full duplex (4 wire)

Off course you can't connect it directly
 

jodokus

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rs422 one way

RS422 is a one way bus: one transmitter and multiple receivers on the bus.
RS485 is two way: multiple transceivers on the bus.
Both busses use two-wire differential signals with identical voltage levels.
From a hardware point of view it is possible to connect RS422 receivers to an RS485 bus. You cannot place an RS422 transmitter on an RS485 bus. In other words: with a RS422 receiver you can only listen to an RS485 bus. RS485 is only the hardware standard and says nothing about transmission protocol, so keep that in mind.
 
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arturt134

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max485 transceiver connection in half duplex mode

Of course, you can connect RS422 to RS485. All you have to do is connect R+ and T+ (RS422) to D+ (RS485) and R- and T- (RS422) to D- (RS485).
Also you have to control signals: RE (receiver enable) and DE (driver enable) like in RS485.
You have to notice that RS485 is only half-duplex transmission, so your protocol should be half-duplex too.
Regards.
 

freedelphi

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rs422 to rs485

Of course, you can connect RS422 to RS485. All you have to do is connect R+ and T+ (RS422) to D+ (RS485) and R- and T- (RS422) to D- (RS485).
Also you have to control signals: RE (receiver enable) and DE (driver enable) like in RS485.
You have to notice that RS485 is only half-duplex transmission, so your protocol should be half-duplex too.
Regards.

Above is true if you can control both device...
If the device has RS422 interface and you want to control it using PC with RS485 interface. You will got problem...
 

arturt134

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rs422 multiple receiver

If you have device with RS422 and non-stop generating data, you can't connect this one to RS485.
But many PLCs and meters can work in half duplex mode, with PC working as master.
In this case you can connect RS422 to RS485
 

Sobakava

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I've a full duplex MAX485 transceiver mounted board. (two ICs)
Could I use a RS233-RS422 converter to connect this board to PC?
 

arturt134

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Sure. But you have to use only half-duplex mode. In other words, in one time only one of them (PC or your board) can transmit data. Many protocols working in question-answer mode. This is half-duplex.

Also, if you have 2 MAX485, you can make 2 transmission canals: first for transmitting data to PC (first MAX485 (DE=1, RE# = 1) -> MAX232 TX), and second for receiving data from PC (MAX232 RX -> second MAX485 (DE = 0, RE# = 0). This will work in full duplex mode.

DE - Driver enable
RE# - receiver enable (negated)
 

tlp71@hotmail.com

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Electrically they are the same levels.
If you need you can have also 3.3V or 5.0V devices.
485 are bidirectional, you can share the same bus for drivers and receivers, in 422 you have a driver and many receivers (max 32)
.Bye.
You can find all info at www.national.com
G.
 

Sobakava

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I've an RS232-RS422 converter with RJ45 jack on it... I used an motor driver with RS485 interface before... It has RJ45 also...

Is there standart pinout for RJ45 on RS485/422?
So I can place RJ45 to my own board according to this standart..

Regards
 

Sobakava

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I place RX+,RX-,TX+,TX- connections on the RS-4xx connector of my board. I also placed GND with 100ohm resistance to my board's GND.

But there is no GND connection on my RS232-RS4xx converter's terminal block. Only RX+/- and TX+/- exists...

In MAX485 part of my schematic, I added biasing resistors, terminating resistors , terminating jumper and unidirectional protection diodes...

If the PC and my board powered from different power supplies in the building, will this cause a problem?
 

arturt134

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RS485 has differential signals:
"1" is transmitted as D+ = VCC D- = GND
"0" is transmitted as D+ = GND D- = VCC
So, there is no need of GND connection.
 

Ale

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you must ground the shield of cable only in one extreme, never in the two extremes....


sorry, my english is bad

Saludos
 
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