SmartHardware RS-485, Руководство пользователя, Страница: 9 / 12

SmartHardware
User Guide STR1000008X
9
3. RS-485 line
This controller manages the RS-485 line intelligently.
Controller can perform one command while receiving second command.
Unlike any other similar controllers, it checks the correct information,
which returns as result.
When controller transmits, simultaneously
read the information transmitted from RS-485 line. Each byte is
compared for validity. When an error is detected, transmission is
terminated. This helps to quickly release the line. In case of a short
circuit on the line, the transmitter is protected against overload.
Baud rate (300 – 115200)
At any time you can set new speed. The next command already is taken on
new speed. There is no need to reboot the controller.
R1, R2, R3 for RS485 line
By default, this controller comes with a transmitter SN75176 . This transmitter
has an input 1 unit load (12k ohms). By default, the line must be terminated
with two resistors of 120 ohms at both ends. You can connect up to 32
controllers.
R1 and R3 must be calculated according line. Into account, must take the
resistance of the wires.
By default, R1=R3=560 ohms. R2=120 ohms. When jumper set, resistor
connected to line.
How to calculate the values of R1 and R3 with 120 ohm resistors at both ends.
Example: 10 controllers and 5 V supply.
Each RS-485 node has a load impedance of 12K ohms. 10 nodes in parallel
give a load of 1200 ohms. Additionally, the two 120 ohms termination resistors
result in another 60 ohms load, for a total load of 57 ohms (1200 and 60 in
parallel). Clearly the termination resistors are responsible for a majority of the
loading. In order to maintain at least 200mV between the B and A line, we
need a bias current of 3.5 mA (200mV/57=3.5mA) to flow through the load. To
create this bias from a 5V supply a total series resistance of 1428 ohms or less
is required. Subtract the 57 ohms that is already a part of the load, and we are
left with 1371 ohms. Placing half of this value as a pullup to 5V and half as a
pulldown to ground gives a maximum bias resistor value of 685 ohms for each
of the two biasing resistors.
R1=R3=680 ohms.