RS485 - 4-Wire or 2 Wire? What is standard in industry?

mc-electronic

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RS485 - 4-Wire or 2 Wire? What is standard in industry?

· Posted: 07.05.2012 - 11:39 · #1

Hi all,

for my data logging system I have developed a ModBus attachment to read in serial data from other ModBus slaves. I have used for testing so far a little RS232/RS485 converter from e-lab. This little converter has just one RS485 driver chip and is switched by the ATMega from sending to receiving by a separate control line. So, it can only be used for the 2-wire RS485 interface running in half duplex mode.

Now I am preparing to develop my own hardware to get away from the testboards, where all is running fine. So I have to decide now, whether I will support 2-Wire only (with one RS485 driver chip(SN75 176)) or 4 Wire (2 RS485 driver chips).

What would you recommend to build? I am aware, that building 2 driver in the hardware still allows to bridge the lines in the cable, thus getting a 2-Wire interface.

I would be grateful for any advice from the more experienced users here.

Thanx, Michael

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Re: RS485 - 4-Wire or 2 Wire? What is standard in industry?

· Posted: 07.05.2012 - 11:50 · #2

The 4 wire version of RS485 is known as RS422 (equally a balanced version of RS232). RS485 can only ever run half duplex. RS422 chips exist but in my experience RS485 chips are more resilient. Just shorting the wires together to get RS485 from RS422 is not recommended - if you are going to allow both, disable one chip and use the other in true RS485 mode.

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Re: RS485 - 4-Wire or 2 Wire? What is standard in industry?

· Posted: 07.05.2012 - 12:18 · #3

Hello Merlin,

to explain my ideas about bridging:
I had used a Mitsubishi FR-F700 Inverter (frequency converting device) for my ModBus testing, because that was the only ModBus RTU Device available to me. That F700 has 2 Rx Clamps (+ and -) and 2 Tx clamps (+ and -). The manual refers as a RS485 interface to it. To use it in 2-Wire Mode, Mitsubishi suggests to bridge Tx- to Rx- and Tx+ to Rx+. I suppose, it has one transceiver chip in Send-Mode and one in Receive-Mode permanentely. Of course, the F700 is listening to its own outputs when bridged.

I was just wondering, if this F700 interface type is "standard" or if the 2-Wire is enough to provide. So I take your statement, that for RS485, just one Driver chip in half duplex is the standard..

My question came from the 4-Wire interface of the Mitsubishi device..

Michael

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Re: RS485 - 4-Wire or 2 Wire? What is standard in industry?

· Posted: 07.05.2012 - 15:32 · #4

Quote

to explain my ideas about bridging:
I had used a Mitsubishi FR-F700 Inverter (frequency converting device) for my ModBus testing, because that was the only ModBus RTU Device available to me. That F700 has 2 Rx Clamps (+ and -) and 2 Tx clamps (+ and -). The manual refers as a RS485 interface to it. To use it in 2-Wire Mode, Mitsubishi suggests to bridge Tx- to Rx- and Tx+ to Rx+. I suppose, it has one transceiver chip in Send-Mode and one in Receive-Mode permanentely. Of course, the F700 is listening to its own outputs when bridged.

I was just wondering, if this F700 interface type is "standard" or if the 2-Wire is enough to provide. So I take your statement, that for RS485, just one Driver chip in half duplex is the standard..

The RS485 chips have a line (usually 2 lines) to turn around transmissions. It is possible on some (most?) chips to have them permanently receiving so they can evesdrop on their own transmissions and just turn transmissions off when not in use. The two lines are inverted wrt each other and can usually just be tied together.

This is a fairly long winded way of saying that everything you need is in one chip for 485 transmission and although shorting the pins out may be fine for the Mitsubishi FR-F700 Inverter I wouldn't recommend it as a general practice.

That is not to say that you cannot use 2 485 chips to provide a 4 wire asynchronous interface - clearly you can.

I don't know whether MODBUS devices normally use 2 or 4 wire interfaces, or even if MODBUS actually cares. All I can do is share my experiences of serial comms. You could always design the board for 2 chips and only fit one, planning to fit a second if sales demand it. It makes virtually no difference to the cost unless you actually fit the second device.

As an aside, some people think that leaving a 485 chip in permanent receive is a good way of detecting faults on a 485 line, the idea being that you compare what you receive with what you transmit. I can tell you from bitter experience that it doesn't work unless the short is on the pin of the chip.

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Re: RS485 - 4-Wire or 2 Wire? What is standard in industry?

· Posted: 07.05.2012 - 19:06 · #5

Hallo mc-electronic,

also meine Erfahrungen waren die, das zu über 95% 2-Draht genutzt wird (Halbduplex).

Die meisten 4 Draht-Geräte kann man einfach mit 2Brücken zu 2 Draht umstellen:
RX+ nach TX+ und RX- nach TX-, in den Software der Geräte auf Halbduplex umstellen und fertig.

Wir haben viele Anbindungen über Modem-Strecken, diese können über die Leitung im Standleitungsbetrieb sowieso entweder Senden oder Empfangen! Aber nicht gleichzeitig!
Da wir bei den meisten Anlagen NUR 2Drähte erhalten (4 sind zu teuer

)

Die einzigen Punkte wo ich 4Draht benutze ist bei Touch- und Anzeigegeräten, sprich bei einer 1zu1-Verbindung und das bringt bei Modbus eigentlich nichts. Man spart evtl. die Umschaltzeit beim Master von senden auf empfangen und beim Slave vom empfang auf senden.

Aber im Modbus bringt das eigentlich eh nicht viel!

Master sendet die Anforderung, Slave beantwortet sie. Aber da er erst am Ende der Master-Nachricht
den kompletten Inhalt und die Checksumme überprüfen kann, wird das NIE gleichzeitig passieren.

Also 2 Draht ist vollkommen ok im Modbus.

Ich bei meinen MC-Projekten immer die 2Draht-Version

Gruss
Thorsten

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Re: RS485 - 4-Wire or 2 Wire? What is standard in industry?

· Posted: 10.05.2012 - 20:37 · #6

Hallo Thorsten,

das bringt doch Licht ins Dunkel, danke. Werde dann die 2-Draht Variante bauen.

Die Mitsubishi F740 hat übrigens keinen Parameter für den Halbduplex-Betrieb, habe das ganze Handbuch in den relevanten Teilen durchsucht. Wenn man die Brücken setzt (wie von Mitsubishi beschrieben!), dann meckert das Gerät die ganze Zeit über den eigenen Output, den es dann als (logischerweise) fehlerhaften Input interpretiert.

Wahrscheinlich habe ich zum Testen das einzige Gerät erwischt, welches da irgendwie anders ist.

Gruß, Michael

1234

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Re: RS485 - 4-Wire or 2 Wire? What is standard in industry?

· Posted: 15.05.2012 - 23:07 · #7

As to the proposed 75176, dump them, they are outdated. Their main disadvantage is their power consumption. They draw in the order of 35mA, just standby. I use the ADM483EAR (250kBit, 350uA, 485), ADM489AR(250kBit, 35uA, 422), ADM3070EARZ(250kBit, 800uA, 3.3V, 422), ADM3491ARZ (1100uA, 10MBit, 3.3V, 422), ADM3491ARUZ(840uA, 20MBit, 3.3V, 422).

As to the subject, I prefer the RS422. I also don't have to care about the wiring. My applications use a couple of meters, not hundreds. RS422 gives simpler protocols that are easier to debug.

Rene

RS485 - 4-Wire or 2 Wire? What is standard in industry?

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