Is 300m of cable enough to connect MCU with RS232 ?

[Budiman]

300m RS232 ?

I'm planning to connect mcu (with RS232) in several location
with total cable length 300m. Can I make it ?

 

[SphinX]

300m RS232 ?

Hi,

No,

The maximum distance for rs232 is 15m.

You can use RS422 (distance up to 1200 m)

Bye

 

[xxtigerxx]

300m RS232 ?

yes you can make it...
but you will need a good cable and low speed...
if they do not communictae you can put some repeiters..

 

[shanren]

Re: 300m RS232 ?

hi,i think rs422 is better.if you do not need full duplex,RS485 is a good choice.if the cable is so long,you need to protect you device from voltage surge.
best wishes.

 

[raticus]

Re: 300m RS232 ?

Hi Budiman

You have to use rs485.
It is a very simple addition to your design.
Use SN75176 chip to do conversion.
Use 1 chip each side for half-duplex , or 2 chips each side for full-duplex.
**broken link removed**
http://focus.ti.com/docs/prod/folders/print/sn75176b.html
**broken link removed**

Cheers
Raticus

 

[shanren]

Re: 300m RS232 ?

rs485 is half duplex,not full duplex.that is ,when you sent data you can not receive data at the same time.
the driver is sn75176,that is right,but you should have a IO to control the direction of data.

 

[raticus]

Re: 300m RS232 ?

That is why I said for full-duplex,you use two pieces on each side!!!!

 

[Budiman]

Re: 300m RS232 ?

I need about 10kbit/s data stream only. Can I get it ?
BTW I'll use AVRTiny, so there is no much free I/O pin
for use RS485.

 

[papyaki]

Re: 300m RS232 ?

Hi,

If you planned to use RS232, you should have two pins, one for Rx and the other for Tx.

So why not use two 75176, one with the DE pin connected to +5v and D pin connected to your Tx pin. The second one with the RE/ pin connected to 0V and R pin connected to your Rx pin.

In this case you dont need extra port to drive direction pins. Remember to tied unused 75176 input pins to a logical level.

As everybody has written in the other answers, RS485 is best suitable for your line length.

 

[raticus]

Re: 300m RS232 ?

Papyaki is right. You only require 1 extra portpin. You have to use this method.Believe me,I have been there.We run our rs485 at 9600,but you can go faster.
19200 at 1000ft will not be a problem,even if you only use standard telecom wire,with two pairs.

ps
I just went back and read Budiman's last post. If it is just a datastream with no return,you would not require any extra pins and only one device?

 

[rob13572468]

Re: 300m RS232 ?

yes, i have built a number of projects with 500-1000 foot lengths qand not had a problem as long as the baud rate is low. in one application, there was quite a bit of noise and then i had to use a transciever to boost the voltage on both ends.

 

[Builder++]

Re: 300m RS232 ?

You need only:
1. High-current transmitters.
2. Terminators (GND) on the receiver side.

This is not simple, but it is very simple.

Couple years ago, my RS232 application works abt. 1000 meters long-range (9,6 kbps speed without any CRC-codes).

 

[cherrytart]

300m RS232 ?

It is not a good idea to use RS232 over long distances. Two systems that far apart can have vastly different ground voltages, sometimes just from equipment being turned on, sometimes from lightning strikes.

In days of yore you might have lost a serial card, now days you might find a sizeable chunk of your motherboard charred.

 

[bobcat1]

Re: 300m RS232 ?

Simply !

Assuming you are using only TX & RX signal's

Covert the signal to RS422 (2 phase ) at one end diploy RS422 signal's to your target and then convert again to RS232 .

All you need is MAX232 & 26LS31/32 or 2 max485 chips and a 4 wire cable

Enjoy

Bobi

 

[meqwerty]

Re: 300m RS232 ?

dun think it would be a problem if repeaters are used. ground voltage shouldnt have any problems as long as the signals are clear and undistorted.

 

[artem]

300m RS232 ?

bobcat1 is right , RS422 uses balanced wires with drivers . Drivers provide differential signal transfer over twisted pair cable . In this implementation cable impedance is tuned to drivers input . That means that cable type is also important parameter .

 

[HeiFelix]

Re: 300m RS232 ?

Simply use RS422. The other side is not a PC.

 

[ted]

Re: 300m RS232 ?

I have used RS-232 over about 100 m distance between two buildings quite long ago. It worked, BUT... see below:

I have to agree with cherrytart, the ground potential differences and transients are a real problem. Not only the equipment can be damaged/destroyed(and over longer period of time, most likely will!). Also there might be a safety hazard, likelyhood somewhat depending in what kind of electrical system those two ends operate. In some countries, such as Norway, there is unreasonably high propability of faulty neutral wire groundings and ground faults due to the local distribution system structure.

My so otherwise successful experiment ended in damaged equipment and a nasty shock when I grabbed the cable after unplugging it (and that was not in Norway or Albania, btw.) The ground potential difference was measured to be over 50 V when some heavy equipment were in use nearby!

Using RS-485 or RS-422, or other balanced, but galvanically connected transmitters and receivers does help only a little: The data connection is much more reliable and the maximum speed much higher for a given distance/cable. However, the ground potential risk remains in most cases because those chips can hardly handle much more than 10 V DC of common mode, let alone mains voltage transients or large ground potentials.

I would recommend using some more hardware. Devices such as RAD SRM-5A do the thing right; they have transformer isolation (see for example w*w.rad-direct.com/Product-Short-Haul-Modem-SRM-5A.htm for some idea of specifications)

Another OK way is the old good current loop by using optoisolators. Some interesting stuff might be found at w*w.epanorama.net/links/tele_interface.html

Good luck, and think safety!

Ted