Please let me know what is the maximum distance for which we can use modbus.
There are 2 answers:
1) If you are PROMISING someone, then say 1000 meters if (and only if) they supply cable rated for RS-485 by the cable maker.
2) If you are trying to do the best you can, then there are EIGHT tradeoffs that determine how far you can CHEAT and push RS-485 beyond 1000 meter. They are clearly shown in this application note: http://www.robustdc.com/library/PDFs/AN032_Incr_485_Robust_screen.pdf
In a nut-shell, if you use good cable, isolated circuits, and are willing to drop the baud rate to 4800 or even 2400, then you should be able to run 3000 meter or more. But I would never PROMISE anyone this distance - especially if they plan to use 115kbps.
- LynnL www.digi.com, Snr Prin Engineer
for email address see: http://www.iatips.com/contact.html
Maximum distance is dependent of physical media used.
I suppose you are using RS-485 physical media, then normally the maximum distance (regarding standard RS-485) is about 1000 meters.
If you are using RS-232 max distance is 15 mts.
If you are more than 1000 mts you can use RS-232 or RS-485 transceivers to Optical Fiber (Phoenix Contact has these transceivers).
You can also use radio modems for great distances.
If you convert from RS-232 to RS-484, then your limitation is given by RS-485, as far as I know.
I used once that and worked ok within 50 mts
Hope this helps.
Speed is determined by the communication media that is used.
Designing a multidrop transmission line is tough, especially with voltage protection circuitry and various equipment. This makes fiber optics a better choice for longer runs.
Also, if the system may be exposed to lightning, etc, you may want to consider a fiber optic solution anyway.
I used to design SCADA for water treatment plants. I have seen older systems fry (computers too) everytime there was a storm. They had all kinds of TVS, GDT, MOV, etc designed to prevent damage (yet it degrades communication anyway). I suggested fiber optics. The expense was well worth it because it reduced down-time, "protected" the equipment, and allowed the possibility of longer runs with higher speed communication.
I quite agree. The only problem is how to show that the equipment would have blown up if you hadn't gone with FO :^). It solves a vast number of issues going between buildings and wiring systems. And the cost is becoming quite reasonable if you do the work yourself and avoid the spendy name brands.