Q: How many nodes can MAX22501 support on a bus? But MAX22501 should be designed for point to point. Can it support more nodes?
A:
- If have 5V supply, then its more than 3.3V supply, because the driver is stronger (means larger differential driver output voltage) – check the TOCs in the data sheet
- RS-485 spec is that a driver needs to drive 54ohm (min) load and produce 1.5V output. 54ohm is the parallel of 60ohm (120ohm double termination) and all receivers on the line.
- Note that this is RS-485 spec. Your customer can decide to use a criterion lower than 1.5V, since lines are short and RS-485 receiver thresholds are below 200mV.
- All receivers must have larger than 459ohm equivalent resistance.
- MAX22501 has (12V/1.35mA) = 8.9kohm input resistance. So, 19 x MAX22501 in parallel is 459ohm. So, MAX22501 can support 19 nodes according to spec.
- This is with 3.3V driver supply. BUT if use 5V supply, then the output voltage is much larger:
- At 28mA load current, the 5V supply is about factor 1.6 higher larger output voltage. That is why I think that can have at least 1.6x 19 nodes = 30 nodes with 5V supply.