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FEATURES
• Compatible with the Bell 202T standard.
• Operate over 2- or 4-wire voice-grade
leased or private lines.
• For point-to-point or point-to-multipoint
polling networks.
• Prevent data and carrier-detect errors
over noisy lines.
• Anti-streaming capability.
• Choose between AC- and DC-powered
Modem 202T models.
An industrial application for the Modem 202.
OVERVIEW
Back in the day, Bell 202 was the top-speed modem
standard, boasting 1200-bps rates that were four times faster
than the poky Bell 108 standard. Still widely used in many
industries, Bell 202 modems like our Modem 202 or Modem
202T keep data moving along. Order one of these modems
to expand an existing industrial, System Control and Data
Acquisition (SCADA), or metering application, or to replace
an old UDS/Motorola
®
202T Modem. In fact, the Modem 202T
is a pin-for-pin, form, fit, and function replacement for that
tried-and-true modem. Just think what it can do for your
legacy system!
The Modem 202 (MD845A-R2) and the Modem 202T
(MD1970A, MD1970A-DC) function the same way. However,
the Modem 202 supports speeds up to 1200 bps over
unconditioned lines; the Modem 202T supports speeds up
to 1800 bps over conditioned lines.
Widely installed and a prerequisite for many leased-line
applications, both the Modem 202 and the Modem 202T
support voice-band frequency shift keyed (FSK) modulation
within asynchronous point-to-point or multipoint systems
that use voice-grade 2-wire, half-duplex or 4-wire, full duplex
dedicated or leased lines. These can be loaded or unloaded
circuits, equal to or better than the ITU (formerly CCITT)
M.1020/1025 or Bell 3002 leased lines.
These modems have many uses.
You can use a Modem 202 or Modem 202T with remote
terminal units (RTUs) and programmable logic controllers
(PLCs) to control and monitor a large number of remote
devices—traffic control lights, for instance. In addition to
SCADA applications, the modems are quite useful for
temperature and pressure controllers in numerous industries,
particularly those in which you expect to add automated
devices.
The water-treatment industry prefers the Modem 202
because it’s compatible with battery systems from 11 to
18 VDC. The Modem 202 is also relied upon heavily by the
U.S. Government for daily commerce.
Noisy lines aren’t a problem either. The Modem 202 and
the Modem 202T have circuitry that prevents data errors
that occur on such lines, particularly those within multidrop
systems where carrier turn-offs and other modems on the
same line can cause noise spikes and other harmful high
frequencies. The modems combine soft-carrier generation
and soft-carrier detection as well as a signal-noise-ratio (SNR)
detector to eliminate such line-noise errors.
The modems also have antistreaming features, which
prevent them from remaining in Transmit Mode when
they’re not actively transmitting data. Great for multipoint
applications, antistreaming prevents a malfunctioning
slave modem from occupying the line indefinitely. When anti-
streaming is active, the Modem 202 or Modem 202T can only
transmit data for up to 56 seconds. Then, they automatically
turn off their transmitter and look for an ON-to-OFF
transition of Request To Send (RTS) before proceeding with
normal operation.
Modem 202T offers diagnostic testing features.
For testing purposes, the Modem 202T has a five-position
rotary switch on its front panel for running analog and digital
loopbacks as well as self-tests. To verify the transmitter and
receiver of the local modem and its connection to a local DTE,
8/22/2007
#16407
724-746-5500 blackbox.com
Programmable logic
controller
Water tank
Water tank
Water tank
Remote
telemetry unit
(RTU)
Remote
telemetry unit
(RTU)
Remote
telemetry unit
(RTU)
Modem 202
(MD845A-R2)
Modem 202
(MD845A-R2)
Modem 202
(MD845A-R2)
RS-232
cable
Dedicated wire
pair(s)
Dedicated wire
pair(s)
RS-232
cable
RS-232
cable
RS-232
cable