Rs 485 Communication - Hanna Instruments HI 24 Series Instruction Manual

RS 485 COMMUNICATION

HI 23xy2 and HI 24xy2 are provided with an RS485 port.
RS485 standard is a digital transmission method that allows
long lines connections. Its differential transmission system
makes this standard suitable for data transmission in noisy
environments.
SPECIFICATIONS
The RS485 standard is implemented in EC process meter
controllers with the following characteristics:
Data rate:
Communication:
Line length:
Loads:
Internal termination:
CONNECTIONS
The connections for the 4-pin RS485 terminal
provided (#1 on page 8) are as follows:
The instrument has no internal line termination. To terminate
the line, an external resistor equal to the characteristic line
impedance (typically 120 ohm must be added at both ends
of the line.
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1200, 2400, 4800 and 9600 bps
Bidirectional Half-Duplex
up to 1.2 Km typical with 24AWG cable
up to 32 typical.
none
Up to 32 units can be connected to the same RS485 line,
with a total line length of up to 1.2 Km using 24AWG cable.
To minimize electromagnetic interference, use shielded or
twisted pair cable to connect the units.
Each EC controller unit is identified by its process ID number
(setup item "01").
The EC controller acts as a "slave" device: it only answers to
commands received from a "master" device (e.g. an indus-
trial PC) connected to the line.
As additional feature, the controller is also
provided with two pins (5V and 0V) in or-
der to apply the Fail Safe Open Line
protection method. To avoid erroneous
readings in Open-Line conditions, pull-
up and pull-down resistors should be
connected as shown.
The Fail-Safe resistors are connected only to one unit in the
line, and their value depends on the application and char-
acteristic impedance of the connection cable (typical between
15K and 50K).
The RS485 port is galvanic separated from measuring circuit
and power line. If both analog output and RS485 port are
present, they have the same ground.
SETTING THE SERIAL COMMUNICATION
The external connections for RS 485 communications are
accessible to the left panel (number 1) and the pins are as-
signed as shown in figure.
Pin 1 is ground, pin 2 is signal line, pin 3 is the inverted
signal line and pin 4 is connected to +5V.
The network will be connected at pin 2 and 3 and cable
shield, if present, will be connected at pin 1
To set the communication parameters the following steps must
be performed:
• Enter the setup procedure by pressing the
SETUP key. After password procedure, enter
the parameter code 01.
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