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P300H P300 Series Modem Installation and Operating Handbook Page 203
8 Disable the Upper Temperature Limit
The specified upper internal temperature limit of the modem is 60C. Within three degrees of this
the unit will raise a Unit Warning (& deferred alarm relay) “Warning: Int' temp 57C, Tx CARRIER
WILL MUTE outside operating range 0-60C”. Above an internal temperature of 60C the carrier
mutes and a Unit Fault is raised (& Prompt Unit alarm relay), “Unit Fault: Tx carrier muted, unit
temp 61C outside operating range 0-60C”. Setting Fault Mode to 8 disables both the carrier muting
and the Unit Fault / Prompt alarm above 60C, however the warning/deferred alarm above 57C is
still activated. Modem operation outside the specified internal temperature range is not
guaranteed, and so this option should not be used on satellite.
9 Internal Test Mode (Disable Differential Encoder and Decoder)
10 Extended 1 for 1 for Antenna Diversity, Initially On Ships (Software >=V2.05)
The normal 1 for 1 redundancy system will cope well with each modem in the 1 for 1 pair being
connected to a different antenna (eg at opposite ends of a ship on stabilised platforms). The
modems will automatically switch traffic to whichever one remains operational. Should one of the
antennas loose its line of sight to the satellite it will cause the on-line modem receive to fail and the
modems 1 for 1 partner to come on line, conveniently also switching the Tx to the other antenna.
However one specific customer wished to integrate a `block` signal from the Antenna Control Unit
(ACU) into the 1 for 1 system, to guarantee the Tx on either antenna would be muted when the
corresponding ACU indicated `block` (ie when the antenna movement limits set on the ACU
indicated the antenna was starting to point at the ship itself, or more importantly the crew !).
The solution is to link the `block` signal from the antenna directly to the modems external Tx Inhibit
signal on pin 7 of the alarms connector (grounded when blocked, also connect a ground on pin
15) AND to set Fault Mode to 10. With the connection made but without the Fault Mode set to
10 the Tx to either antenna will be muted when the corresponding ACU `block` signal is active. It
will not however immediately switch the traffic over to the other modem in the 1 for 1 pair (ie not
switch the traffic to the other antenna, as the external Tx inhibit is not considered a fault by the 1
for 1 logic). Without Fault Mode set to 10 then it would be possible for the ACU to indicate `block`,
muting the Tx carrier (because the antenna has just passed the ACU movement limit where the
beam is approaching the ship) and for the Rx carrier to remain OK until the receive signal is fully
obstructed by the ship. If this happens there will be a break in transmission from the ship until the
receive totally fails, finally causing a Rx fault which will make the modems 1 for 1 partner take over,
switching both Tx and Rx to the other antenna.
When Fault Mode is set to 10, the External Tx Inhibit is additionally considered as a fault by the
1 for 1 logic (although it has no effect on the normal LCD display of modem faults). Now, apart
from the ACU `block` signal immediately muting the Tx carrier, the 1 for 1 modem pair will also
change over to the standby modem causing the traffic to be immediately routed to the other
antenna (provided the standby modem has no faults and the ACU of the other antenna is not itself
indicating `block` causing the standby modem External Tx Inhibit to be active). Only when the ACU
indicates `not blocked` can the traffic switch back to the first antenna. Should both ACU's indicate
`block` then Tx from both antenna will be muted, with the last one to indicate `not blocked`
remaining on-line (as far as the 1 for 1 is concerned) providing receive data. Full traffic will switch
back to the first modem which has no faults, ie where the modem has a good receive signal and
the ACU indicates `not blocked`. When both ACU's indicate `not blocked` (ie most of the time) the
normal modem 1 for 1 redundancy operates.
This might take a few reads to fully understand, but it is an elegant solution to the problem requiring
nothing other than a data `Y` cable, a standard 1 for 1 cable and a two wire link (ground &
`blocked`) from each ACU to the corresponding modem !
A word of advice on installation. DO NOT put the modems at opposite end of the ship and run the
1 for 1 and data `Y` cables along the entire ships length. Instead co-locate the modems, keep the
1 for 1 cable and data `Y` cable short, and run the modem IF along the length of the ship. If the