700271-0001 Rev. G 05/13 5
2 Definitions
100 mA Loop:
Also known as the municipal loop or city loop. This is the wire that runs in and out of the
telegraph boxes and carries the telegraph code. Voltages on these loops can range from
just a few volts to as much as 140 volts dc. Power supplies at the head end are
programmed to maintain 100 mA on the loop, and do so by varying the voltage on the
lines.
Box Round
Is one complete transmission of the box number plus any additional codes.
Bulldog or Bulldogging the Line
A box is said to be “bulldogging” when it transmits on the loop that already has another
box transmitting on it. When a zone on a panel is tripped the panel looks to the loop to
see if any other boxes are currently transmitting. If another box is currently transmitting,
it will wait for a preprogrammed time period after which it will bulldog the loop and
transmit over the other box.
It was a feature of the older windup boxes that needed to transmit within a certain period
after the hook was pulled or risk winding down without sending the alarm.
Form Four
Power supply cards designed to power telegraph loops.
Ground Mode
This is a backup method for transmitting an Alarm when an open loop condition exists. If
a telegraph transmitter needs to make a transmission and detects an “Open Loop”
condition, it will then switch to Ground Mode in an attempt to get the transmission
through. At the same time, when an Open Loop condition occurs, the Form Four cards at
the head end detect the open and apply a positive voltage to both sides of the loop and
references earth ground as the negative side of the loop. The telegraph box sends the
transmission coding the loop to earth ground.
Open Loop
Current must flow through the loop at all times in order to transmit an alarm. Any break
in the loop stops the flow of current, creating an “Open Loop”. When an Open Loop
occurs, the Form Four card will detect the absence of current flow and enter “Ground
Mode”.
PNIS (Positive Non-Interfering Successive)
Each box on a telegraph loop monitors the telegraph for coding activity and waits for a
“quiet” before proceeding to send its telegraph transmission.
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