Specifications

7-13
INPUT FAIL TIMER (non-programmable)
There is a non-programmable 255 second timer which
also starts timing along with the maximum wait timer
(but is likely much longer than max wait). If the fail timer
times out, the input is considered failed (stuck on) and
will be recorded as such. No further transit priority
service is allowed on that input until the input goes away
and clears the failure (and also clears max wait fail).
The difference between this and the max wait timer is
one of degree and the associated report logged. An
input failure would typically warrant field service, unlike
the more routine max wait cancellations.
MAX EXT = Max Extension Time (0-255 seconds).
This value establishes the time in seconds that the
transit priority phase can extend beyond its normal max
or force off. A value of 0 disallows extension. The
transit priority max extension timer begins timing only
when the transit priority call is still active and the transit
priority phase would otherwise normally terminate either
by max-out or force off (when coordinating). The unit
resumes normal timing if the max extend timer times
out, even if the Transit Priority input is still active. Max
extend typically occurs when the transit priority call
arrives late in the normal TP phase. Max extend timing
during coordination will hold the cycle timer in place*
and will release it when it times out or the input/call
goes away. Offset seeking will result.
*The purpose for holding the cycle timer in place is
so that the cycle timer does not get ahead of the
phasing--which tends to cause subsequent phases to
short time. Holding the cycle timer will indeed cause it to
be out of sync and results in offset seeking, but this is
usually far less detrimental to traffic than short timing
phases.
LIMITS ON MAX EXTEND DUE TO PRE-EXISTING
OFFSET SEEKING CONDITIONS
Shortway offset seeking is a process in which the local
timer adjusts itself to get “in sync” to the master cycle +
programmed offset. It involves timing the local cycle
timer either “fast” or “slow” by 20% as needed. It seeks
fast if the local cycle is behind schedule, and seeks slow
if it is ahead of schedule. This process is routinely used
in the normal operation of coordination to achieve sync.
Transit priority can only stop the cycle timer during TP
max extend timing--which will make it further behind
where it currently is. As a result, the only affect it can
have on offset seeking is to cause it to start seeking
“fast” if not already doing so, or to seek fast for a longer
time if already doing so.
The TP process takes any pre-existing offset
seeking into account by modifying the programmed
max extend timer. The modified value is called the
allowed extend value. This value is determined by
computing how far ahead or behind of sync the unit
currently is:
-If already behind and thus seeking fast, the max
extend period may be reduced. For example, if the
actual local offset is greater* than the programmed
offset by 6 then the local cycle is behind by 6 and is
seeking fast. If the programmed TP max extend is say
10, then only 10-6 or 4 seconds of actual extend time
will be allowed.
*Allowing for cycle roll-overs a local offset of 6 would be
considered “greater” than 89 in a 90 second cycle.
-If already ahead and thus seeking slow, the extend
period may be increased--but not to more than a
built in limit of 1.5 times the programmed extend
value. For example, if the actual local offset is less
than the programmed offset by 6, then the local cycle is
ahead by 6 and is seeking slow. Say the programmed
TP max extend is 10, then 10 + 6 = 16 seconds would
be computed, but this is greater than 1.5 x 10 = 15.
The resulting max extend time would then be 15
seconds.
Note that the TP max extend values are used strictly
as they are programmed during free operation, i.e. a TP
phase in free operation can unconditionally extend up to
its programmed max extend value and no more.
MIN RESV:
Inputs 1-6 = Min Re-service, inputs 1-6 (0-99 min-
utes). This value specifies the minimum time before
any two successive priority transit service calls are
allowed on the same input.
ALL = Min Re-service, all inputs (0-99 minutes). This
value specifies the minimum time before any two
successive priority transit service calls are allowed in
general, regardless of input.
NOTE: The min re-service function has a built in 5
second recognition timer, i.e. the input (call) must be
active for 5 consecutive seconds to enable the min re-
service timer. This prevents a slightly intermittent
detection system from being locked out because of a
brief loss of signal.