INSTRUCTION MANUAL
MULTINET SERIAL TO ETHERNET CONVERTER – INSTRUCTION MANUAL
3
3.
RS485 Serial Port
RS485 data transmission and reception is accomplished over a single twisted pair with
transmit and receive data alternating over the same two wires. Using this port, continuous
monitoring and control to a PLC, protection relay or any serial Modbus IED is possible.
Connect the “+”, “–“ and common terminal of MultiNet to the “+”, “–“ and common terminals
of all the IEDs. Correct polarity must also be observed. For instance, the IEDs must be
connected with all RS485 “+” terminals connected together, and all RS485 “–” terminals
connected together.
To minimize errors from noise, the use of shielded twisted pair wire is recommended. The
COM terminal should be connected to the common wire inside the shield, when provided.
To avoid loop currents, the shield should be grounded at one point only. Each relay should
also be daisy-chained to the next relay in the link. A maximum of 32 relays can be
connected in this manner without exceeding driver capability.
Lightning strikes and ground surge currents can cause large momentary voltage
differences between remote ends of the communication link. For this reason, surge
protection devices are internally provided at the communication port. An isolated power
supply with an opto-coupled data interface also acts to reduce noise coupling. To ensure
maximum reliability, all equipment should have similar transient protection devices
installed.
Both ends of the RS485 circuit can be terminated with impedance as shown in
Figure 2:
Typical Configuration
.
Termination networks should only be applied if long cable runs
are used between RS485 connections
. A typical load value for a terminating network is a
120-ohm resistor in series with a 100-pF capacitor.
NOTE
Note
The termination capacitors are used for AC termination to reduce the power consumption
of the idle link, as well as to reduce the ringing voltage. The negative effect is a reduction in
cable length and bit rate. Termination capacitor values are dependent on various factors,
including the number of Unit Loads, network data rate, and cable length. The termination
capacitor value provided in the instruction manual should be taken as a guideline only. In
order to determine the optimum capacitor value factors, such as number of devices
connected on the network, network speed, physical media, and network cable length
should be considered.
