I have a noise pb on a 4 wires Modbus network.
I would like to install a repeater, but i have no reference.
Here is an application note on using isolated RS-485 repeaters:
However, given all the RS-485 systems I've debugged the problem is rarely distance and usually grounding related. Are you are running a 4 or 5-wire system? 4-wire without a ground wire usually only works well if all (100%) of the devices are from the same vendor and powered the same way (all AC or all DC). This applies to the Master end also. A multi-drop of 10 RS-485 flow-computers from vendor X with an 11th RS-232/485 converter on your computer from vendor Y is still a *multi-vendor system*.
Assuming you have the most common design, I'd suggest this:
1) make sure the RS-232/485 converter (or RS-485 port) on the Master/HMI/Computer is full isolated. This means it has a floating earth
without reference to frame or true earth or the PC chassis.
2) Then run a 5th ground wire from this device to the first field device's RS-485 signal ground or DC power ground. This causes the floating output of converter to reference it self to the signal ground of the field device.
3) Assuming all the field devices have the same opinion of ground, you'll now have ALL devices sharing that opinion.
I'd give this a try before running off to buy/install repeaters - even if you do it by running an old cheap telephone line across the ground for a few hours just to try it.
Lynn August Linse
IA Firmware Specialist, Digi Int'l (www.digi.com)