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Split from MODBUS RTU MASTER FOR CP340/341/441-X (NO DONGLE REQUIRED).

hi alaaproject,

please tell us which devices, master and slave, you have. 

Check the technical data if there are any hints for the maximum lenght of the cables between the devices and at all.

regards,

Hi,

Point-to-point (from point to point) with RS232 or RS485 multipoint case. With RS232, a master and a slave are maximum possible in RS485 there are max. one master and 31 slaves. The maximum cable length is 15 m with RS232 and 1.2 km with RS485.

Did you check the cable, is it allright?

Towome

At 15 m both ends were likely at the same ground potential or  at a low enough ground potential difference that the 485 worked OK>

At 100m distance, one device can be at one ground potential, the other at a different ground potential.

2 wire RS-485 is really 3 wire - there should be a signal ground wire connecting each device.  But, that fact is frequently lost in the shuffle.

Although RS-485 is a differential 'pair', each line references signal ground.  If the signal ground is earth ground, the difference in earth ground potential is the difference in signal ground potential. See the attachment.

When the signal ground common mode potential exceeds -12V to about +7Vdc (the EIA spec, not all drivers meet the spec) the drivers will saturate and comm fails.

A standard tool in my RS-485 toolkit is an RS-485 isolator module.   When I run into these problems, I install the isolator at one end (it needs 24Vdc power) and I run the signal lines through it.  9 times out of 10 the isolator solves the problem.

Siemens always assume that a plant site is 'equipotential' meaning all the grounds are at the same potential.   They are entitled to their fantasy, but I live the real world and the real world has ground potential differences everywhere.

thanx for all

it was my fault ,the data cable was in parallel with high voltage cable,so I made the distance between cables more than 1.5 meter and every thing became OK