TM 11-7025-234-23
b. Control Board (A4). Data from the receiver is logically controlled for
loopback and other test purposes. This board also controls the configuration and
training mode of the receiver and generates control signals to the terminal.
Received data and commands are sent from control board to the interface.
c. Interface Board (A3).
Data commands and clock information are sent through
the interface board.
The remote loopback option allows a central site to initiate audio and dc busback
tests of the local and remote modems and telephone lines in the absence of any
operator at the remote site. This RLB consists of two tone filters and detector
circuits located on the remote loopback board (A8). The switches and indicators
for this function are mounted on the front edge of the board. Control of audio and
dc busback is full-duplex. Either end of a point-to-point system can control the
other modem.
a. Remote Loopback Board Description. Various loopback diagnostics are
performed by the RLB. They consist of remote loopback of modem in the dc loopback
and remote loopback in audio loopback mode. After test use of the loopback
function is complete, simultaneous detection of both tones at the remote modem
releases both functions. This also restores the modems to normal data modes.
b. Remote Loopback Board Operation. The remote loopback board has two switches
and two status indicators. One switch is a DC/AUDIO control. In AUDIO position
the SEND switch commands the AUDIO loopback tone to be sent, and similarly, when in
the DC position, the SEND switch commands the DC loopback tone to be sent. The
DC/AUDIO also determines which of the two loopbacks the status indicator will
monitor. The bottom switch is normally in the LPBK (middle) position. Pressing
the switch to the SEND position momentarily causes the applicable loopback tone to
be sent, dependent on the top switch. Pressing the bottom switch down to CLEAR
position momentarily causes both tones to be sent. This clears the loopback.
A multiport modem contains a built-in multiplexer which enables two or more
separate data streams to be combined for transmission over a single circuit. A
multipoint modem is basically a modem designed to achieve fast polling acquisition
times on multipoint lines.
Multipoint, or multidrop lines, usually are installed for applications that require
interactive terminal access from a number of remote locations into a central
computer facility. This type of line may link a number of keyboard-display
terminals, or they may be installed to provide remote terminal access to a
centralized data base for one particular application. An example would be an
inventory control system where terminals are located at many warehouses and are
used to report shippings and arrivals so that organization inventories are
continuously updated. For this application, each terminal uses a small fraction of
the total time available to all terminals connected to the line to complete a
transaction, and the terminal is addressable and can recognize messages for which
it is a recipient.
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