LAUREL LWIFI, User Manual, Page: 11 / 34

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7. DATA CACHING, DATA RATES & READ RATES
Cached operation is a key feature of LWIFI and LWIFIX boards. Using the Custom
ASCII protocol at 19200 baud and address 1, these boards poll the host meter every
16.666 msec , except when the host meter is an analog input meter or a scale/weight
meter programmed for 50 Hz noise rejection, in which case data is polled every
20.000 msec . The latest values are written into cache. Using the Modbus protocol, an
external Modbus Master can then read the cached data as fast as every 10 msec .
These time intervals apply regardless of the meter
’s display update rate, and
whether the meter is an analog input DPM or weight/scale meter, or a pulse input
counter/timer. If the
“Cache Setting” is set to “Displayed Measurement Only,” the
meter
’s single displayed value is written into cache every
16.666 msec or 20.000
msec . If the
“Cache Setting” is set to “All Measurements,” a set of 6 values is written
into cache every 100 msec or 120 msec .
Data can be read from cache by an external Modbus Master as fast as every 10
msec whether the
“Cache Setting” is set to “Displayed Measurement Only” for a
single value or to
“All Measurements” for a set of 6 values. WiFi data rates can be up
to 72.2 Mbits/sec. USB data rates are fixed at 38400 baud for Modbus operation.
The fastest rate at which updated values can be read via Modbus is paced by the
measurement intervals of the meter. Unchanged values are read by the external
Modbus Master when data is read from cache at a rate faster than that at which
readings are updated into cache.
Measurement intervals for an analog input meter (model numbers starting with L1-
L4 or LW) are every 16.666 msec or 20.000 msec, depending on the meter
’s 60 Hz or
50 Hz noise rejection setting. Each 17
th
interval is used to zero the meter, so that the
measurement is then not updated. For counters (model numbers starting with L5-L8)
used in totalizing mode, the measurement interval (or display update interval) is a
gate time which can be user programmed from 10 msec to 199.99 sec. For counters
used in frequency/rate mode, the measurement interval (or display update interval) is
gate time + 30 msec + 1-2 signal periods.
Internal communications between the LWIFI and LWFIX boards and the host meter
always use the Custom ASCII protocol, 19200 baud and address 1. These para-
meters need to be entered into the meter from its front panel and can be read by
Instrument Setup (IS) software.
USB communications between an LWIFI or LWFIX board and an external Modbus
Master use the Modbus RTU protocol, 38400 baud and address 1. These parameters
cannot be changed and cannot be read by Instrument Setup (IS) software.
WiFi communications between an LWIFI or LWFIX board and an external Modbus
Master use the Modbus TCP protocol at a baud rate allowed by IEEE 802.11 b/g/n.
Addressing uses an IP address assigned by the WiFi router and discovered by our
Network Setup utility.