MyPi RS485 Card Configuration

The ISO-485 adapter board uses a MAX13487 RS485 line driver, this has transparent automatic hardware flow control and so does not require any additional code to supervise flow control.

The following line needs to be added to /boot/config.txt to enable the hardware UART0 to appear on the correct pins for the adapter card :

 dtoverlay=uart0,txd0_pin=32,rxd0_pin=33

This enables /dev/ttyAMA0

Note that for correct RS485 operation it is vital that the pull up/down bias resistors be fitted, as the values for the pull up/down resistances can vary depending on the network arrangement the ISO-485 card has multiple options here.

LK4/6 & LK5/3 links can be used to enable the on board 640R resistors, PTH resistors can alternately be fitted to R4/R5 positions if different pull resistor values are required.

The ISO-485 card also features and on board 120R termination resistor which can be used if the unit is to be placed at the end of the bus.

A single 5-40V opto isolated digital input is also provided on this card and connected to GPIO38

When connecting a RS485 network we strongly recommend using appropriate cabling e.g. Belden 9481 120 Ohm impedance twisted pair wire (or equivalent)

 

Belden 9481 RS485/CAN-BUS Data Cable Datasheet

MAXIM Application Note on RS485

Modbus RS485 FAQ Site

Low Cost UK Supplier of Belden 9841 Equivalent

 

 

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