m908 User Manual, Rev. G
4.2.4 Clock and Control Connections
REMOTE CONNECTOR:
DB15 The m908 RCU handles all system control. Connection to
the m908 RCU is via this DB15 connector, which carries RS485 serial data, DC power and
headphone signals. The m908 ships with a high quality 25’ cable. Custom cables can be
assembled by the user according to the diagrams in the
section of this manual.
While the serial data can travel over 1000 feet we do not recommend cables longer than 50’ for
headphone use. If you need a longer, cable contact your Grace Design dealer or us directly. Do
NOT use an off the shelf DB15 cable as the pinout will be incompatible and may cause damage
to your system.
WORD CLOCK IN:
BNC 75Ω or 47kΩ (setup menu selected). The m908 can accept a
standard 5V/75Ω word clock signal from an external clock generating unit. This might be a
stand-alone clock source or via, for example, the word clock output from your digital audio
workstation. The m908 locks to the incoming word clock with an ultra low jitter PLL. The PLL
has a fast lock mode which rapidly acquires lock and then switches to a high jitter rejection mode
with a loop bandwidth of 0.5Hz. This provides exceptional jitter rejection for the DAC and ADC
sample clocks. In the event of a dropout or loss of incoming word clock signal, the intelligent
PLL will remain at the last known valid frequency. When the signal is restored the PLL will
smoothly re-acquire lock.
WORD CLOCK OUT:
BNC 75Ω. The word clock output allows the user to synchronize
other digital audio equipment to the m908. When the system menu setting "clock output" is set to
external, this connector output is a buffered copy of the signal on the word clock IN jack. This
mode is useful for "daisy chaining" to other units in a system. When "clock output" is set to
system clock this connector outputs a buffered copy of the internal m908 word clock. The output
is buffered and is designed to drive a 75Ω line.
ETHERNET:
RJ45. This is a standard 1000 Base-T Ethernet interface. Currently it is
reserved for factory diagnostics.
USB HOST:
USB A connector. This host connector is intended for
.
EXT. TALKBACK:
1/4" TRS Jack. External talkback control. This input allows the
connection of an external switch, such as a footswitch, for remotely activating the ACU talkback
mic input, or the built in talkback mic on the RCU. The input is a TRS jack and is used with a
“normally open” switching device. Note that when using this jack the associated talkback
function can be assigned in talkback setup. See the remote talkback
at the end of
this manual for connection details.
4.2.5 Thermal Management
The m908 ACU requires adequate ventilation to maintain proper operating temperature.
Under normal operating conditions the m908 will dissipate 30-35W of power depending on
user settings and hardware options. To protect the m908 from excessively high internal
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