GE Industrial Solutions POWER LEADER PMCS Network Architecture Guide - Manual (Page 25)

Power Management Control System
Chapter 2 – Network Design
15
2–5 Modbus Wiring Rules – Diagrams
The Modbus network protocol has wiring rules and limits
on the number of IEDs that may be attached.
This section describes in greater detail the rules you must
follow when designing a Modbus network.
WARNING: Network wiring and grounding
rules described herein apply primarily to
commercial/industrial installations. Substation
installations will exist in the presence of
dangerously elevated ground potential relative to
points outside of the station grid as well as large
electromagnetic induction fields. Additionally,
large ground faults can elevate substation
ground potentials. Follow local utility best-
practices/safety procedures to prevent
risk of
shock/electrocution to personnel and damage to
equipment that could result in a loss of
protection and communications.
NOTE: It is important to take future
expandability into consideration when designing
a network configuration. This is particularly so
when the network is near its maximum number
of IEDs or maximum cable length. Adding IEDs
to a network after it has been installed may
require rewiring the network.
CAUTION: Wire-run distances mentioned in the
configuration rules assume application above
grade or in conduit. For below-grade
applications, refer to Section 3–1, Wiring
Requirements.
Regardless of which platform is supporting the RS-485
networks (Ethernet Gateway, RS-485 card, or RS-232/RS-
485 converter), the following rules apply to each
individual RS-485 network.
1. Each RS-485 network may support up to 31 Modbus
IEDs. Figure 7 illustrates this rule. (See the
exception below Figure 6.)
31 RS-485 IEDs maximum;
PMCS Host PC, Ethernet Gateway or MSP
always located at one end of Modbus network.
Host
Figure 7. Network illustrating Modbus Rules 1 and 2.
Exception to Rule 1: Some types of IEDs must be
wired on dedicated private serial network segments,
one IED per serial Modbus line.
2. The host (or Ethernet Gateway) must always be
located at one end of any Modbus segment. It may
not be located in the center of a Modbus network.
Figure 7 shows the correct placement of the host
(PC or Ethernet Gateway).
3. All Modbus IEDs on a single RS-485 network must
communicate at the same baud rate. If IEDs with
different communication speeds are connected to
the same RS-485 network, the whole segment will
communicate at the speed of the slowest IED. Figure
8 illustrates this rule. (Communication speeds for
supported IEDs are listed in Table 1.)
Figure 8. Network illustrating Modbus Rule 3.
4. Each RS-485 network must be properly terminated
at both ends of the cable run after the final IED.
(See Section 3–1 for details on termination.) Figure
9 illustrates this rule.