44210/49210-0798 <90-00025>
P
AGE
5
RS-422 and RS-485 Receivers/Drivers
The RS-422 and RS-485 interfaces are differential receivers and
drivers. They are used when the modem is located greater
distances from the data terminal equipment. The RS-422 and
RS-485 interfaces do not use the RTS handshaking lines. When
using the RS-485 interface, only a TXD receiver and RXD driver
is available. When the TXD receiver recognizes a driver from the
terminal equipment, DTR is activated on the modem. When a
carrier is detected on the facility side, the CD signal on the
modem enables the RXD driver. Using this interface (RS-485)
requires only two pairs of wires for TXD and RXD, and no wires
are required for DTR and CD. The circuit board has two strap-
ping points to force RTS and DTR to the active (ON) state if
these handshaking lines are not used by the data terminal
equipment connected to the modem. If handshaking is used, the
straps should be in the OFF position. Strapping points for
termination of the differential receivers are located on the
circuit board and must be placed in the TERM position for the
interface being used. For interfaces not being used, the termina-
tion straps must be in the OFF position. Front panel LEDs are
provided on the module to indicate RXD, TXD, CD, and DTR
activity.
MPI Inputs
Six MPI inputs directly interface the microprocessor on the
44210 using edge connector pins. When an MPI input becomes
active, the microprocessor's input goes to a low state, causing
the microprocessor to perform a dial-out. One of the MPI inputs
(pin 22) is TTL/CMOS-compatible and is activated by grounding
(ground to 1.5 VDC) and inactive when left open (3.0 to 5.0
VDC). The other five MPI inputs (pins 46, 48, 52, 54, and 56) are
ground activated (ground to -2.0 VDC) and inactive when left
open or at minus battery voltage (-21 to -56 VDC).
Modem and Controller
This circuit converts data to tones and tones to data between the
line-side facility (tones) and the data terminal equipment (DTE).
The controller contains a full Hayes
tm
-compatible command set
in internal ROM and controls all of the modem's functions. The
modem has PSK and FSK modulators and demodulators for both
300 and 1200 baud operation. It also contains a DTMF generator
and call-progress-mode filters. The audio output of the circuit
may be monitored by edge connector pins or front panel jacks.
Ring Detect/Line-Seize Circuit
This circuit detects ringing tones and seizes the circuit and
holds the line active for the modem and controller.
CIRCUIT DESCRIPTION
