I'm designing a system where we are communicating with a device (ultrasonic anemometer) which is at the end of a cable which could vary between 80-200m.
The sensor communicates via RS-485 half-duplex. There is mention of terminating bus resistors possibly being required. On the sensor, there is a 100R terminating resistor between TXD- and TXD+ which can be enabled or disabled.
Can this system be deployed without a terminating resistor and not have any problems? I read that 'long' cable lengths increase the requirement, however in the context of the guidance, long is relative.
I have used this calculator but there are some parameters which I do not know and I have made the assumption that the 'network cable' is similar to that of which is being used, such as: Bit Rate of Transmission Technique (bits per baud), UART Controller Sampling Rate (Clock cycles per baud).
My questions are these:
- Is it standard rule of thumb for terminating resitors to match cable impedance?
- If yes, how do I calculate cable impedance? (Example cable)
- How do I determine if terminating resistor is required?
- What is the tolerance of the terminating resistor for it to serve a purpose? (i.e. if the cable length varies, do I need to calculate a new resistance each time or is there +/- metres leeway)
- Is 200m considered long in this context
- Is 115k baud considered high-frequency in this context?
Thank you for your help in advance.