SmarTek Systems SAS-1 Operating Modes and Architecture - Manual (Page 3)

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SAS-1 Operating Modes
The SAS-1 can be set up for a specified fall back or default operating mode. This mode defines
what the SAS does and how it communicates after reset, power up, or a prolonged period of
communication inactivity. The SAS-1 can be set to operate in either a text message mode (Flow
Mode) or a “Loop Equivalent” relay mode (Relay Mode).
In the Flow Mode, the SAS-1 performs vehicle detection and speed estimation processing for all
5 lanes and forms a Traffic Flow Message. The Traffic Flow Message includes vehicle count,
lane occupancy, and average speed for each of the 5 lanes. This message is prepared and made
available every observation period. The observation period is a selectable parameter ranging
from 2 seconds to 220 seconds. If the SAS-1 is set for periodic communication, the Traffic Flow
Message will be output on the SAS-1 data cable every observation period whether anything is
present to receive it or not. If the SAS-1 is set for polled communication, the most recent Traffic
Flow Message will only be output on the SAS-1 data cable if the SAS-1 received a polling
request. This allows multiple SAS-1 units to share a common home run cable or wireless link
without interference or message collision.
In the Relay Mode, the SAS-1 performs vehicle detection and speed estimation processing for
all 5 lanes and forms a bit serial Relay Message. Each bit serial Relay Message includes “loop
equivalent” presence relay state information for 10 time points for each of the 5 lanes. This
message is prepared and made available every 80 milli-seconds. Hence, the bit values (1 or 0) in
this message represent relay state for an upstream and downstream “loop equivalent” for each
lane. If the SAS-1 is set for periodic communication, the bit serial Relay Message will be
output on the SAS-1 data cable every 80 milli-seconds whether anything is present to receive it
or not. If the SAS-1 is set for polled communication, the most recent bit serial Relay Message
will only be output on the SAS-1 data cable if the SAS-1 received a polling request. This allows
multiple SAS-1 units to share a common home run cable or wireless link without interference or
message collision.
SAS-1 Architecture
The SAS-1 architecture is inherently simple and modular since all signal processing, spatial
processing, and detection processing is performed in the SAS-1 unit. This results in a short
simple measurement text message that requires very little communication bandwidth. The SAS-
1 electronics and messaging protocols facilitate a number of connection options for getting the
processed information from the SAS-1 unit (mounted on a highway structure) to a roadside
cabinet or to a central control center. Many different configurations are possible involving
single and multiple SAS-1 units, hard wired and wireless communication, with and without local
controller cabinets. The following sections show four common ways to connect SAS-1 units to
local cabinet controllers and to central control centers.