Comtech EF Data SMS-758 - Manual (Page 105)

SMS-758 Modem Protection Switch Theory of Operation
MN/SMS758 Rev. 3 4–45
4.7 Address Decoder/Driver
The address decoder/driver is a 10.5” x 3.5” module (Figure 4-18) that fits in slot 4 of
the front upper section of the switch chassis. Its functions include address decoding of
the external address bus, external data bus buffering, serial communications interface,
modem fault interface, and fault outputs.
4.7.1 Specifications
Address Decoding 2K bytes of address bus decoded to 24 discrete channels.
Data Bus Buffering 8 bits to all peripheral devices.
Communication Interface Remote interface: RS-485 or RS-232C.
255 addresses, 110 to 9600
baud.
Relay-remote interface: 4 lines (parallel).
Modem Control Interface: RS-485.
255 addresses, 9600 baud.
(Dual ACIAs on the AS/1048
module enable simultaneous
communication on both
Remote and Modem control
interfaces.)
Modem Fault Interface 10 lines for muxed modulator and demodulator faults.
Fault Outputs Controller Fault: Form C.
System Fault: Form C.
Demod Signal Fault: Form C.
4.7.2 Theory of Operation
The address decoder/driver interfaces the M&C data bus, address bus, control lines, and
serial interface. Addresses of the peripheral devices, both on and off of this board, are
latched and decoded to select each device. The data bus is bidirectionally buffered to and
from the M&C board. The data bus and decoded address lines are buffered off this board
as output to the other modules.
The address decoder/driver formats the serial data communication to and from the M&C.
All interface drivers and receivers for external communication are on this module.
The fault lines from the interface switch modules are routed to this board. The M&C
reads the faults at regular intervals to update its own registers.
The M:N summary, demodulator signal, and controller fault relays are on this board. The
M:N summary and demodulator signal fault relays are controlled by the M&C directly.
The controller fault is controlled by a “watchdog” timer. The “watchdog” timer must be
reset at regular intervals by the M&C, or the controller fault will set.