Echelon LONWORKS PLT-22, Руководство пользователя

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amplifier is driving a low impedance line and its loaded output voltage is
somewhat less than 7Vp-p. For proper node operation this condition must be met
over the full range of worst-case component tolerances (including I
DD5
drain), AC
line voltage, and temperature.
Having chosen a storage capacitor to ensure adequate supply voltage after one packet,
the power management circuitry must then be enabled to prevent the erratic operation
of a node that transmits so frequently that its energy-storage capacitor cannot fully
recharge between transmissions. If the V
A
supply were to droop too far due to closely
spaced packet transmissions, a linear regulator used to generate V
DD5
would
eventually drop out of regulation. This would cause the node to experience a power-on
reset. Since it is difficult, if not impossible, for a developer to guarantee that a node
will not transmit too frequently, the PLT-22 transceiver’s power management circuitry
specifically covers this case.
The power management circuitry continuously monitors the V
A
supply voltage.
If the V
A
supply drops below the lower power management threshold (nominally
7.9V), the PLT-22 transceiver will prevent the Neuron Chip from transmitting
until the energy storage capacitor has sufficiently recharged to allow
transmission of a complete packet. The Neuron Chip and the PLT-22 transceiver
automatically work in concert to ensure the transmission of any waiting packets
once the capacitor has fully recharged. If a high packet transmission duty cycle
causes V
A
to drop too low, in turn causing the packet transmission to be aborted
prior to completion, the transceiver automatically signals the Neuron Chip to re-
transmit the packet, independent of the LonTalk protocol service in use. Even
when unacknowledged service is employed, a packet that is interrupted by V
A
droop will be re-transmitted once the power management system determines
that the supply has fully recharged.
The power management circuitry of the PLT-22 transceiver automatically
adjusts the amount of time that it inhibits transmission based on the node’s
actual recharge characteristics. This adaptive feature allows energy storage
nodes to typically transmit without constraint or intervention from the power
management circuitry. When a node powered by an energy storage supply has
worst case component tolerances and is exposed to low AC line voltage, the
PLT-22 transceiver’s power management circuitry actually measures the supply
recharge rate and calculates a suitable transmit hold-off time. The formula used
to make this calculation is three times the time required for the supply to charge
from its lower power management threshold (nominally 7.9V) to its upper power
management threshold (nominally 12.0V). Figure 5.1 illustrates examples of an
energy storage node operating under both typical and worst case conditions.
(1) The maximum packet length adequate for most applications is 34 bytes (this is
the maximum length for nodes which use default buffer sizes, input and output
network variables of 15 bytes or less, and non-explicit messaging). If the
application does not require both group addressing and 6 byte domains, then 32
bytes becomes the maximum. For the primary carrier frequency, a 32 byte
packet corresponds to a maximum transmission duration of 74.6ms.
Calculating the maximum transmission duration for a packet at the secondary
carrier frequency is somewhat more complicated due to the combination of error
correction and data compression used with that carrier frequency. If we
consider a case where message traffic satisfies three further common
5-4 Power Supplies for the PLT-22 Transceiver