I've set up the following simple, non-terminated RS-485 network:
The bus is 10m long CAT5e cable, carrying a 4800 baud signal. RS-485 transceiver (TRX) is an XY-k485 module (MAX485-based).
Transmission works fine, and the sensor reports correct readings. Curious about what kind of voltages develop in the RS-485 segment, I measured the voltage between:
- A and B of the sensor: 3.3 to 3.4 V
- A of the sensor and GND: 3.4 to 3.5 V
- B of the sensor and GND: 0.0 to 0.2 V
After obtaining the voltages, I've connected one 120 Ohm termination resistor at the far end of the bus like so:
The sensor was still reporting correct readings no problem. However, measuring the voltages between the same points again produced very different results with the termination resistor in place:
- A and B of the sensor(term-d): 0.0 - 0.7V
- A of the sensor and GND(term-d): 1.81 to 1.86 V
- B of the sensor and GND(term-d): 1.79 to 1.92 V
Why did the voltages in the bus change so dramatically after the resistor was connected, and what does this change mean in terms of signal strength/transmission reliability?
I.e. are lower voltages of the "terminated" version of the bus better? Or does it make little difference, since both terminated and non-terminated versions worked just fine?
P.S: I am aware of the necessity of the termination in RS-485 networks at the "it's real important trust me bro" level, but would like to get a more in-depth look at what goes on in the bus and what physical impact connecting the resistor makes.