Involved in network communication protocol development, I have recently bought a DFRobot I/O expansion module having in mind to use it for RS485 network spying.
I have received the module, had a look to the schematics and was surprised by the RS485 implementation
- Termination line resistor R7 (120 ohm) could not be disconnected. RS485 based network shall have such termination line resistor at both ends only to adapt line impedance and avoid signal reflections, but not in each node.
- Biasing resistors R3 and R4 (2,2 Kohm) could not be disconnected. same remark as before, network shall have these biasing resistors but only at one place and not in each node.
OK, therefore I will use this shield for other subjects than networking. And maybe the I/O expansion shield V6.0 will comply network requirements