Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 134 of 265
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15.1 UTC Introduction
In UTC mode, operations are controlled and monitored by the central computer of
an Urban Traffic Control system.
Stage changes are effected by the application of forces and demands. The demands
may either be local or simulated by the computer. During UTC mode maximum
green timers normally have no effect and are held in a RESET state.
Instation equipment at the central computer office communicates via telephone lines
to an Outstation Transmission Unit (OTU) housed within the controller cabinet.
Control signals are normally transmitted as two 8-bit control words and monitoring
signals are returned as two 8-bit reply words.
15.2 UTC Interface
Interfacing of the control and reply signals between the OTU and the controller may
be via physical parallel inputs and outputs (see section 29).
Communication between a Siemens Integral TC12-OTU and the controller is via the
controller’s extended system bus, see section 37. Communication between a semi-
integral Siemens Gemini
2
unit (for MOVA or UTMC for example) is via the enhanced
serial link, see section 36.3. Neither of these interfaces use controller digital I/O for
the UTC interface, but the control/reply bit principle is still used.
The logic states of the control and reply bits at the controller are as follows:
CONTROL REPLY
ACTIVE:
CLOSED (1) OPEN (0)
INACTIVE:
OPEN (0) CLOSED (1)
(Any unused reply bits are usually set to the inactive state)
Note that since the controller’s outputs are ‘normally open-circuit’, the reply bit
outputs are energised by the controller when they are inactive in order to close the
output contacts. They are then released when, for example, the associated stage is
active in the case of ‘G’ bits, see section 15.3.2, or when the controller is switched
off.
However, the reply bits on a stand-alone pedestrian stream (section 15.3.6) must
use ‘normally closed-circuit outputs’ so that they are inactive when the controller is
switched off. The controller has a number of ‘change-over’ type outputs that include
both a ‘normally open contact’ and a ‘normally closed contact’, such that when the
output is energised, the ‘normally open contact’ is closed and the ‘normally closed
contact’ is opened. Therefore, if the controller’s normal stage confirm outputs are
used to generate the GX and PC confirms, the output sense needs to be inverted so