Service manual

2-1
March 2000
Part No. 001-3139-502
8000 SERIES RECEIVER DECODER MODULE (RDM)
SECTION 2 8000 SERIES RECEIVER DECODER MODULE (RDM)
2.1 GENERAL
The Receiver Decoder Module (RDM) is
designed for either operation with the 800 MHz or 900
MHz receivers and is located in the receiver drawer.
Refer to the Multi-Net 8000 Repeater Service Manual,
Part No. 001-8000-377 or Multi-Net 8900 Repeater
Service Manual, Part No. 001-8900-379 for detailed
information on the circuit description and alignment.
The RDM is powered from 13.8V DC. The RDM
operates in the temperature range of -30°C to +60°C
(-22°F to +140°F) with 10%-90% humidity.
The RDM decodes the subaudible data and four
levels of Received Signal Strength Indication (RSSI)
plus the condition of the squelch line, open or closed,
from the receiver. In the Remote Site, information is
then passed to the RDM, to the RMM, to the Local
Site CDM and then to the RVM. In the Local Site, the
RDMs are hard wired to the RVM.
The Multi-Net System is capable of handling 30
channels and the Voter System is capable of handling
32 voted receiver sites, the maximum number of
RDMs is 960.
2.2 INITIAL RECEPTION
The RDM RSSI is squelch plus four additional
levels to the squelch settings. The opening squelch
setting is 6 dB SINAD plus four squelch settings at
10 dB, 15 dB, 20 dB and 25 dB, ± 2 dB.
The initial RSSI level is determined by averaging
the RSSI during the initial decode time. The averag-
ing is done by sampling during the subaudible
"WORD" transmitted continuously from the mobile.
The "WORD" is 225 milliseconds in length, the RSSI
is sampled every 10 milliseconds during this
"WORD" period. The initial reception will be sent
directly to the Receiver Voter Module (RVM) via a
1200 baud FSK blank and burst signaling on the main
audio line. This also indicates what Home, Group ID,
Unique ID, priority and status has been received.
2.3 RSSI UPDATES
The RDM continues to sample the RSSI every
10 milliseconds and averages the RSSI for each
“Word”. When this average has been determined, the
RDM sends the updated RSSI information to the
RMM. The upper 4-bits of the byte are unused or 0.
The lower 4-bits of the byte are the RSSI information,
see Table 2-1.
The updated RSSI information is sent via a sec-
ondary RS-232 line (refer to Figure 2-1). The path is
the RSSI update path where the RDM connects to a
Receiver Multiplexer Module (RMM). This path is a
1200 baud, RS-232 line.
The audio line is a direct connect path between
the RDM and RVM. This line is also an audio 1200
baud line with FSK blank and burst data on initial
decode.
Table 2-1 RDM RSSI INFORMATION
Hex Value DESCRIPTION
7
6
5
4
Strongest RSSI signal level
Weakest RSSI signal level a call will be
started at
3
2*
1
0*
Weakest RSSI level before a call will be
dropped.
Time-Out no update received
No Data received from RDM (generated
by RMM only)
Turn-off Code received from mobile
* Idle state.