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FAQ#076 - Why it is necessary add a termination resistor to the last digitized sensor of each RS-485 chain?

All SISGEO digitized instruments  (IPIs, Tiltmeters, H-Level…) use the RS-485 serial communication protocol

The RS485 protocol contemplate a termination resistor.

The recommended chain connection is point-to-point (multidropped) nodes as a bus mode (line). Star, ring or multiply network connection are not recommended.

SISGEO dataloggers has already a termination resistor assembled (typically 120Ω). On the opposite side (sensors side) is necessary to mount a resistor on the last device of each chain.

Without termination resistors, reflections of fast driver edges can cause multiple data edges and consequently data corruption. Termination resistors also reduce electrical noise sensitivity. The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 Ω for twisted pairs and for only one chain).

SISGEO supply the correct termination resistor (120 Ω, 240 Ω, etc.), considering the number of the chains connected to the same master unit (i.e. OMNIAlog).

The termination resistor must be connected on the appropriate sensor connector.

At the following link an example scheme:

SCHEME