MA28139

In RT mode, power up resets the OBT and causes MA28139 to deselect both busses. Two watchdog counters monitor the Nominal l-bus and the Redundant l-bus. If either bus becomes active, that bus will be selected. If the selected bus stops, the OBT watchdog times out and resets both the OBT and the user. If both busses become active, the Nominal bus will be selected in preference to the Redundant one. A change in bus selection will always result in the OBT and the user being reset. Responses from the user are always returned on the selected bus. Setting 'SIMUL' high causes both BATs to drive both the Nominal and the Redundant busses irrespective of the current bus selection. The time-out period may be set to any desired number of bits by varying the 'LOSC' frequency. The OBT derives all timing from, and is synchronous with, the selected l-bus. The OBT demodulates the l-bus to the DBI and decodes commands to the IUB.

The CTpRTn mode pin causes the modem circuits and the RTU Kernel to be either cascade or isolated. If CTpRTn is low (RT mode), the RIRSYNC, CLK, DATA and VAL signals are routed to the RTU Kernel and the associated pins act as outputs; responses from the RTU Kernel are ORed with those from the external RRTDATA and RRTEN inputs and can be independently monitored on the DATARRT and ENRRT pins. In this mode any reset caused by the Clock Detector watchdogs is also combined with the power up reset input.

If CTpRTn is high (CT mode), the modem and RTU Kernel functions are isolated to permit the MA28139 to be used as either a modem within the CTU or an RTU Kernel interfacing to an external modem where the RIRSYNC, CLK, DATA and VAL pins act as inputs. The right-hand multiplexer bank is switched to the upper position so that the CT drives the OBDH via the CIT and CBT (if used) pins and receives responses/telemetry via the CRR and CBR (if used) pins. Note: in CT mode, BAT1 must be connected to the l-busses.

In RT mode, the CITSEL, MOD, CLK, SYNC and INV pins of MA28139 are disabled and the clocks are supplied by the l-bus BAR in response to the selected bus. In CT mode, the Clock Detector is functional and drives the TlMEOUTn pin but is unable to cause internal reset on time-out; in this mode the CT mustsupply all clocks and select the operational bus.