Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
RS485 can easily work for 80 meters
Use minimum bud rate Speed and select the proper IC (Slew rate)
Use Shielded twisted Cable (Check the cable capacitance)
Grounding the Shield is important and Selecting the proper type of Grounding
All cable has capacitance between cores and cores/shielding. This affects the risetime of any fast pulse sent down it, it has in effect to charge the cable first. This is also an issue between traces on a pcb or i.c. This and inductive effects largely govern the speed of data transmission, hence the drive for ever smaller circuits. I have had problems with 9600B over 50 meters, you may need to go slower. Try it and see.
Yes it should be fine, there are better drivers than 176, it is now quite an old chip. However it is not this that is the problem, just line properties that affect the possible transmission speed. If it is not successful, try a lower speed.
I have tried the communication with two 200 meters & very thin cables (one of the cables are shorter than 200 meters), the communication didn't work properly. Rare messages come. Then I used 60 m. from the same cables for serial connection. Baud rate is 1200 for now. It seems working, I will increase the baud rate step by step.
I want to ask something about the resistor connections to the sn75176(I am using this ic for now, I will also try max487e). I have a master and one slave. In my circuit, between the A and B pins, there is a 1k of resistor, a 4,7k from A to +5V and 4,7k from B to ground. Do you think is that good, or should I make any corrections?
One more question. I want to apply some noise to the cables. How can I do it, do you have any suggestions?
The 4k7s are to pull the lines into a known state if floating. If you have a permanent connection between rs484 chips they should not be needed. Termination resistors are usually around 120R, 1K seems a bit high. There are lots of discussions weather to terminate or not, personally I have had more success leaving them out. Not sure how you inject noise onto the cable though for testing. I would remove all the resistors and try again.
I want to ask one more thing which is about the cable type. What kind of cable should I use. As far as I know, I should use a twisted pair cable for a and b, and one ground. CAT5 includes more cables than I need. Is there a type of cable which includes only the number of cables that I need(which is 3 cables).