Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 138 of 265
priority than UTC and would cause the computer to release control unnecessarily
and then require computer operator intervention to restore UTC control.
Ideally the other conditions should be used with an ‘MC’ or ‘RR’ reply bit (see
section 15.4.4) although the availability of bits in the reply words will be the deciding
factor.
15.3.3 Demands - Common (DX), Stage (D1, D2 etc.), Pedestrian (PX)
The demand control bits simulate local demands, i.e. detectors or pedestrian push
buttons.
The ‘DX’ control bit normally demands all phases and extends all extendible phases.
The ‘D1’, ‘D2’, etc., control bits normally demand and extend (where required) the
phases within the specified demand dependent stage.
On a stand-alone pedestrian stream, the ‘D’ bit that demands the pedestrian stage is
usually named ‘PX’.
The demands may be latched or unlatched as required, although normally they are
latched.
Demands for pedestrian phases normally illuminate the WAIT indicators, although
they may be programmed not to if required.
Timing delays associated with call/cancel facilities are not applicable, although
demands may still be unlatched.
Any demands dependent on other demands may still be effective.
The demands may be programmed so that they are only effective if the appropriate
‘F’ bit is present with the ‘D’ or ‘DX’ bit.
If required, it is possible to program the controller so that demands for phases may
be ignored for certain stages.
‘D’ bits may be used during CLF, VA, Bus Priority or Emergency Vehicle modes as
well as UTC mode.
15.3.4 Stage Demand Confirm (SD1, SD2 etc.)
An ‘SD’ reply bit may be returned to indicate the presence of a demand, either
locally or from a ‘D’ bit, for a phase within the specified demand dependent stage,
ignoring demands for phases which are inhibited by red lamp monitoring.
Any phase that appears in more than one demand dependent stage will cause the
‘SD’ bit for each of the stages to be returned whenever a demand is present, unless
the demand has been programmed to be ignored.