Rs485 Connections - GE MM200 Instruction Manual

ELECTRICAL INSTALLATION

RS485 connections

CAUTION:
2–12
Figure 2-14: Typical RS485 connection
Z T (*)
OPTOCOUPLER
DATA
COM
SCADA, PLC, OR
PERSONAL COMPUTER
GROUND THE SHIELD AT THE
SCADA/PLC/COMPUTER ONLY
OR THE MM200 ONLY
(*) TERMINATING IMPEDANCE AT EACH END
(typically 120 ohms and 1 nF)
One two-wire RS485 port is provided. Up to 32 MM200 IEDs can be daisy-chained together
on a communication channel without exceeding the driver capability. For larger systems,
additional serial channels must be added. Commercially available repeaters can also be
used to add more than 32 relays on a single channel. Suitable cable should have a
characteristic impedance of 120 ohms and total wire length should not exceed 1200
meters (4000 ft.). Commercially available repeaters will allow for transmission distances
greater than 1200 meters.
Voltage differences between remote ends of the communication link are not uncommon.
For this reason, surge protection devices are internally installed across all RS485 terminals.
Internally, an isolated power supply with an optocoupled data interface is used to prevent
noise coupling.
To ensure that all devices in a daisy-chain are at the same potential, it is imperative
that the common terminals of each RS485 port are tied together and grounded only
once, at the master or at the MM200. Failure to do so may result in intermittent or
failed communications.
The source computer/PLC/SCADA system should have similar transient protection devices
installed, either internally or externally. Ground the shield at one point only, as shown in
the figure above, to avoid ground loops.
Correct polarity is also essential. The MM200 IEDs must be wired with all the positive (+)
terminals connected together and all the negative (–) terminals connected together. Each
relay must be daisy-chained to the next one. Avoid star or stub connected configurations.
The last device at each end of the daisy-chain should be terminated with a 120 ohm
¼ watt resistor in series with a 1 nF capacitor across the positive and negative terminals.
Observing these guidelines will ensure a reliable communication system immune to
system transients.
The uncovered communications cable shield connected to the common terminal should
not exceed 1" (2.5 cm) for proper EMC shielding of the communications cable.
TWISTED PAIR
+
SHIELD
-
C
UP TO 32 MM200
OR OTHER IEDs,
MAXIMUM CABLE
LENGTH OF
Z T (*)
1200 m (4000 ft.)
MM200 MOTOR MANAGEMENT SYSTEM – INSTRUCTION MANUAL
CHAPTER 2: INSTALLATION
MM200 IED
RS485 +
OPTOCOUPLER
RS485 -
DATA
COMMON
IED
RS485 +
RS485 -
COMMON
IED
RS485 +
RS485 -
LAST
COMMON
DEVICE
888745A1.CDR