The Multi RS485 board uses multiple SC16IS740 devices to provide I2C and SPI interfaced serial ports

The configuration is done by downloading and installing the device tree overlay files found in the zip file here

Device tree install and source files
[wget --no-check-certificate 'https://drive.google.com/uc?export=download&id=1pHVvHhVkmF1cfaAXzcOTrPe_S3C9HC1z' -O multiuart.tar.gz]

Install Notes :

Copy all the .dtbo files in the tar zip file to /boot/overlays

Add these 5 lines to the end of /boot/config.txt

dtparam=i2c_arm=on,i2c_arm_baudrate=400000
dtparam=spi=on
dtoverlay=i2cuarts
dtoverlay=spiuart1
dtoverlay=spiuart2

Reboot the system.

When next booted you can check the I2C ports have been detected by running the below :

root@raspberrypi:~# i2cdetect -y 1
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- UU UU -- -- UU UU -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- UU -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

The serial ports should now be running as :

root@raspberrypi:~# ls /dev/ttySC*
/dev/ttySC0  /dev/ttySC1  /dev/ttySC2  /dev/ttySC3  /dev/ttySC4  /dev/ttySC5

Baud rates that can be cleanly obtained on all ttySC* ports  (note the divisor number is for information only)

Baud	(Divisor)
50	2304
75	1536
150	768
300	384
600	192
1200	96
1800	64
2400	48
3600	32
4800	24
7200	16
9600	12
14400	8
19200	6
38400	3
57600	2
115200	1

If using ttyS0 then be sure to disable it from being a console port

The serial port assignment is as below, note that the ttyS0 and ttyAMA0 connector will depend on which pins the two UARTs are brought out on from the main CPU. The below is correct for the stock OS image setup.