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COMMUNICATION TECHNOLOGIES
MODERN ENERGY-METER
NETWORKS CORRECT CROSS-WIRE FAULTS AUTOMATICALLY
By Thomas Kugelstadt, Texas Instruments
Worldwide energy metering networks use
differential data transmission based on
RS-485 technology to span long distance
data links. To overcome large ground
potential differences between remote bus
nodes, transceivers are galvanically isolated
from their node circuitry.
An e-meter network is a master-slave system
with a host processor (master) that is located
in a control center. The master addresses
multiple slave nodes which are located in the
end user premises along the bus.
A single network typically comprises up
to 60 nodes, making the potential for
cross-wire faults of the twisted-pair bus
cable during installation rather high. To
assure reliable data transmission, modern
transceivers apply automatic polarity
correction (POLCOR) of the bus signal
polarity. Figure 2: Polarity correction timing after power-up
Figure 1 shows a typical e-meter network
with POLCOR transceivers.
Each slave transceiver contains an internal
timer reversing the polarity of transmit and
receive data, if the receiver output state
is logic low for a specified time span. Due
to the wide operating temperature range
of POLCOR transceivers, this time varies
between a minimum of 44 ms (t FS-min ) and a
maximum of 78 ms (t FS-MAX ).
The master node determines the signal
polarity on the bus via a failsafe-biasing
resistor network (R FS and R T ). The transceiver
in the master node requires no polarity
correction. Polarity correction might, therefore, be
initiated by a constant bus voltage present for
as short as 44 ms duration. Hence, data strings
of consecutive 0-bits must be shorter than
44 ms to prevent false polarity correction.
The slave transceivers, however, do require
integrated polarity correction to sense and,
if necessary, correct the bus signal polarity
during bus idling. That is when no node is
actively driving the bus.
On the other hand, polarity correction
automatically initiated after power-up
requires the bus-idle voltage to be present
for at least 78 ms to ensure the polarity
correction process is complete.
After the initial correction the bus polarity
status is latched within the transceiver
and maintained for all following data
transmissions. This means that switching
between drive and receive modes will not
lose the bus polarity status.
Figure 2 shows polarity correction after a
power-up sequence. During power-up the
receiver output, R, is undetermined. Once
the slave node supply, VSS, is stable, the
bus must idle for at least t FS-MAX to ensure
the completion of a polarity correction.
Because of the cross-wire fault, the
positive bus voltage at the master’s failsafe
network, V AB-M , appears negative at the
transceiver input. Thus, after completion
of t FS-MAX the transceiver’s internal polarity
is switched to invert receive and transmit
data. Hence, the negative input voltage,
VAB-S, is converted into a positive output
voltage. Long data strings of ones and zeros
exceeding the minimum polarity
correction time t FS-MIN also can trigger
a polarity correction. To prevent such
incidents, e-metering standards such as
DL/T645 add a fixed-bit pattern to the
transmit data within the driving node and
subtract it from the output data in the
receiving node.
Figure 1: Typical e-meter bus with polarity-correcting transceivers
22 Bus loading
Because an e-meter bus uses galvanically
isolated transceivers, ground potential
differences between bus nodes do not
METERING INTERNATIONAL ISSUE - 5 | 2014