GAI-Tronics IPE2500A, IPE2500A-MLS Paging Encoder / Desktop Controller User and Installatoin Manual - Manual (Page 64)

Theory of Operation IPE2500A and IPE2500A-MLS Paging Encoder/Desktop Controller
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Multi-Line Select Module
The reference designators listed in this section refer to the multi-line select schematics unless otherwise
stated.
The transmit and receive audio paths to and from the desk set are routed to the NSL PCBA through P802.
Control lines for four individual radio stations are brought into the desk set onto terminal strip J801.
Each line is selected individually through K800–K803. The relays are driven from the microcontroller
on the desk set via the SPI bus register/latch V801. The SPI bus interfaces on P800-8, 10, 11, 12, and 13.
Each channel has an adjustable line activity detector that is also interfaced to the desk set microcontroller
via the SPI bus at V809.
Receive audio balance for all four lines is adjusted with pot 1 to pot 4 and the resultant audio is switched
to either selected (select) or background (unselect) status through audio gates V804 and V805. The
unselect and select audio is mixed and sent to the desk set PCBA through driver V808 and T806.
Reset Circuit
Both the DSP and the microcontroller use the EPROM, U2. When coming out of reset, the DSP must
boot its software from the EPROM. In doing so, it asserts U1-21, BMS, low, causing the microcontroller
to remain reset.
After the DSP has completed loading its software from the EPROM, it no longer requires access to the
EPROM. The DSP then disasserts BMS, allowing the microcontroller to come out of reset. The
microcontroller then executes its software from the EPROM.