TR-4800E
Tally Router
Page - 14 Revision 1.1
3.7 GENERAL PURPOSE I/O MODES
The TR-4800E tally router is designed to have flexible General Purpose I/O to allow them to be used
for a wide range of purposes from driving all types of equipment to illuminating lamps, and reading the
state of switches. The GPIs and GPOs can be wired in Wet or Dry mode to interface with both
powered and unpowered devices. The GPIs have a wide voltage range and high sensitivity (see Table
2-4). The GPO relay contacts can support a relatively high amount of current (see Table 2-7).
3.7.1 Tally Wet and Dry Terminology
The Tally Router is named after the most common use which is driving tally lights on cameras. In a live
recording studio with multiple cameras, each camera has a tally light which indicates it is the one that is
currently being broadcast. Typically the different cameras are switched between from the control
center. The main video router switches the video feed between the cameras, and the tally router
follows by switching the corresponding tally light GPOs.
Throughout this manual the “wet” and “dry” terminology is used to describe whether a GPO and GPI
provides/requires an external voltage source to operate. This terminology is arcane and confusing
outside its narrow scope of usage. The term “Wet” means that voltage is present and “Dry” means no
voltage, always from the perspective of the output port (General Purpose Output).
When connecting a GPO to a GPI, remember “Wet” to “Wet” and “Dry” to “Dry”.
Never connect “Wet” to “Dry” or vice-versa.
A General Purpose Output has two contacts marked “A” and “B”. A “Dry” GPO opens these two
contacts when OFF, and closes them when turned ON. No voltage is present. A “Wet” GPO always
has +WET voltage on the “A” contact. The “B” contact is open when the GPO is OFF, and closes to –
WET when the GPO turns ON. See sections 2.8.1 and 2.9.1 for more information.
A “Wet” GPO provides voltage to the downstream device.
A “Dry” GPO does not provide voltage to the downstream device.
The General Purpose Inputs are used to read the state of another device. This may be a passive
device such as a pushbutton or switch, but is often connected to a GPO. A “Wet” GPI is named such
that it is wired to connect to a “Wet” GPO. This means a “Wet” GPI does not provide its own voltage
source. A “Dry” GPI is meant to interface to a “Dry” GPO. This means that it provides a +WET voltage
which the external device must return in order to activate the GPI. See sections 2.8.2 and 2.9.2 for
more information.
A “Wet” GPI requires an external voltage to be applied in order to activate it.
A “Dry” GPI requires only an external contact closure to return its own voltage to activate it
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