44214-0898 <90-00005>
P
AGE
3
CIRCUIT DESCRIPTION
F
ig. 1 shows the 44214 9600 bps Modem functional schematic.
The circuit contains:
♦
87C51 Microcontroller
♦
Loopback capability
♦
Modem Controller
♦
Reset circuitry
♦
Asynchronous Serial Interface
♦
Analog Interface
87C51 Microcontroller
The microcontroller is used to configure and operate the modem
controller and operates at 12 Megahertz. This processor contains
a one-time programmable ROM and 128 bytes of data RAM. A
two-position switch (S4) configures the microprocessor to oper-
ate in either originate or answer mode. When configured in
originate mode, the modem constantly transmits an origination
tone on the analog line. When in answer mode, it continually
monitors the line for an origination tone and responds if one is
detected. For two modems to communicate over the orderwire,
one device must be in originate and the other in answer mode.
Loopback
Both the analog and digital sides of the 44214 can be set to
loopback. A two-position switch (S3) is used to enable or disable
the analog side. A second two-position switch (S5) is used to
enable or disable the digital side.
Modem Controller
The modem controller provides all the modem functions. It
includes the analog modulation/demodulation, analog to digital
conversion, asynchronous serial data signal control, and configu-
ration. Receiving configuration information from the micro-
controller, the modem controller drives the asynch digital
interface as well as the analog line interface. The transformers
are 600-to-442 ohm and provide facility-side to circuit isolation.
Reset Circuit
The modem can be reset by a push button (S1), which can be
accessed through the front panel. There is also a watchdog
circuit that will reset the modem if there is no activity from the
microprocessor for 1.2 seconds.
N
OTE
:
Upon power-up, the modem con-
figured for answer-mode will
transmit a continuous answer
tone and will respond if an origi-
nate tone is detected. If an origi-
nate tone is not detected within
10 seconds of power-up, it will
stop transmitting the answer
tone and will wait. Upon receiv-
ing an originate tone, it wil start
transmitting an answer tone
again.
