E.F. Johnson Company Summit DM 975x, Руководство по обслуживанию

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UNIVERSAL DIGITAL AND DATA INTERFACE
B-7
Revised May 1998
Part No. 001-9750-006
B.7 INTERFACING WITH A DATA MODEM
B.7.1 GENERAL
The following information describes how an
external data modem can be connected to the universal
interface. Described are the lines that are used and
how they should be programmed for proper modem
operation.
B.7.2 MODEM CONTROL LINES
The following control lines should be
programmed as indicated in parentheses for proper
modem operation. For more information on these
lines, refer to Sections B.5 and B.6 and Table 6.
PTT Request N (Data) - This input is used for
transmit/receive arbitration.
Input A (Data ARQ) - This input is also used for
transmit/receive arbitration.
PTT (Tx Sense) - This output indicates to the data
modem the current state of the power amplifier (if it is
keyed or not).
Output A (Clear-to-Send) - This output may be
programmed if desired to indicate to the modem when
the transmit channel is connected and available for
data transmission. Note that this function is restricted
to the Clear Code function when encryption is used
(see description in Table 6).
Output B (Rx Data Group) - This output indicates to
the modem that a call is occurring on a “data” group.
Busy Out (Access Denied or RSSI) - This output can
be programmed for either of the available options. If
“Access Denied” is programmed, it indicates to the
modem that an access failure has occurred because of
a busy or out-of-range condition. If “RSSI” is
programmed, this output can be used to indicate to the
modem a busy condition on a conventional channel.
This can be used to determine channel availability in
the receive mode.
B.7.3 ARQ ARBITRATION
Introduction
ARQ is used on all LTR and Multi-Net data chan-
nels, but not on conventional channels. Its purpose is
to provide an efficient scheme for ACK/NACK
responses to each block of a multi-block message. It
eliminates the normal repeater handshake interval
usually required to change transmitting stations. The
scheme takes advantage of a repeater time-out that
terminates repeater transmission after a 1 second
(minimum) interruption of the repeater received
subaudible data stream.
To use this facility, the transmitting modem, on
reaching end-of-block, briefly pauses the radio RF
transmission (for less than 1 second) to listen for the
ACK/NACK response. The receiving modem, on
detecting end-of-block, inserts its RF transmitted
ACK/NACK response. The transmitting modem
resumes transmission with the next data block. This
sequence is repeated for each data block in the multi-
block message.
To implement this feature it is necessary for the
modem to operate two radio input signals rather than
the single “DATA PTT” signal that might be expected.
This is necessary in order to define the various state
changes that the radio must execute. Therefore, if LTR
or Multi-Net data channels are used, both radio input
signals must be operated even if the ACK/NACK
feature is not used.
The DATA ARQ and DATA PTT lines must be
used to set up a data transmission when operating in
the LTR or Multi-Net modes (this protocol does not
apply to conventional operation). Data transmissions
cannot be set up using any of the other lines.
When a block of data is received, ARQ arbitra-
tion can also be used to provide a quick acknowledg-
ment without the overhead of a repeater handshake.
LTR and Multi-Net repeaters monitor for embedded
data from the transmitting mobile and stop transmit-
ting if this data is not detected for a fixed time (1
second minimum). The ARQ protocol utilizes this
time-out delay to send a short acknowledgment to the