Dyacon UA-1, Руководство пользователя, Страница: 19 / 29

DYACON UA-1 Manual Modbus
5.3 Modbus Electrical Connection
5.3.1 Bus Connection
Dyacon devices use a two-wire (half-duplex) electrical connection.
The Modbus port is a slave port that can be connected to a Modbus Host or Master.
Pin 1 – RS-485 RX/TX+ (A)
Pin 2 – RS485 RX/TX- (B)
Connecting to 4-wire Master
When connecting the sensor to a 4-wire master, install a jumper between the RX+ and TX+ as
well as between RX- and TX-. The Master device may have to be configured to operate in half-
duplex mode. Refer to the instructions for your equipment for half-duplex and full-duplex
settings.
5.3.2 Termination
RS-485 (EIA-485) networks require a termination resistor of 120Ω. A termination resistor is not
used on Dyacon sensors. Depending on the bus, a termination resistor may be needed.
Additional bias resistors may also be needed.
Modbus is based on a bi-directional RS-485 data bus. Since the bus is bi-directional, anytime
either the master or slave is not driving the bus, the bus floats. If the bus has termination
resistors on it (as it should) the two lines will be at the same voltage if no other device is
transmitting. While Dyacon equipment utilizes full fail-safe (Open, Short, Terminated/Floating)
RS-485 transceivers, this is not the case with all equipment. Some equipment needs a bias on the
RS-485 lines to prevent the bus from being interpreted as being active while it floats. This can
be even more important in a noisy environment.
The following would be a common biasing and termination circuit. While this circuit works well
in most situations, adjusting the resistors may be needed depending on the bus configuration,
other termination resistors on the bus, and the operating voltage. Modbus Commands
Page 19/29
RX+
TX+
RX-
TX-
RX/TX+
RX/TX-
Modbus Master CM-1
+V
RX/TX+
RX/TX-
R1 Ω
120 Ω
680 Ω
GND
+V R1 Value
5 V 680 Ω
12 V 1.3 kΩ