4.2 - 6 THEORY OF OPERATION (ENHANCED CONTROL HEADS)
USB
There are 2 USB connections on the Remote Mount Back Housing Board. The first USB is shared
with the front GCAI USB and the D- and D+ lines are switched ON the main board. The second USB
is provided by the ISP1105 USB transceiver U302. It is interfaced to the OMAPs USB1 port. USB
power supply is separately switched for both USBs via Q306 and Q307. Detection and switching is
controlled by the I2C port expanders U501 and U502.
Audio
Audio is on 2 wires for both directions and balanced and is routed from and to the Milan transceiver
via the Gabel Hybrid U201. The Gabel Hybrid serves to transform the audio from unbalanced to bal-
anced in order to reduce noise susceptibility on the remote cable. Receive or Handset Audio is
routed to the main board and via 10uF to the 25 pin connector J105. Transmit or MIC audio is multi-
plexed by U203 and U204. Depending on PTT, these switches determine where the transmit audio
is coming from. There are 3 sources: Front GCAI, Rear GCAI ON 25 pin connector and Rear MIC,
also from the 25 pin connector J105.
1-Wire
The 1-Wire bus is routed from the 25 pin connector to the main board. It is shared between front and
back GCAI and switched ON the main board.
Bus+/SCI_TX
Both BUS+ and SCI-TX serial communication lines have Pull-Ups to 5V. These 5 Volt needs to be
turned ON/OFF by the OMAP in order to detect the ON/OFF status of the radio transceiver. This is
done via the I2C port expander and Q503.
Analogue 4-level Input:
An analogue 4-level input pin is available on the 25 pin connector (U503). The 4 levels are preset by
resistor dividers R540 - R551 and detected by comparators U503. The 4 states can be read by I2C
expander 501.
I2C Port Expander settings and In/Output Matrix:
The heart of the Remote Mount Back Housing Board is the two PCA9535BS port expanders U501
and U502. The GPIOs and most detection and switching circuitry on the Back Housing Board are
provided by these Port Expanders. The PCA9535BS provides 16 general purpose In/Output pins. It
is interfaced to the OMAP’s I2C bus via SDA (data) and SCL (clock) line. Both expanders are shar-
ing the same interrupt, provided by an extra line GPIO_REQ.
Under normal operating condition, the second port expander U502 has only outputs and static
inputs (HW ID) connected to it, so no interrupt is generated by U502. All Inputs that generate an
interrupt are connected to U501.
Data Expansion Head Enhanced
Overview
The Data Expansion Head Enhanced (PMLN4908_) is a hardware interface to the MTM800 TETRA
mobile radio. It gives access to the radio via three front side connectors. The following interfaces
and protocols are provided:
- 8-wire RS232 Interface (9-pin D-type connector)
6866539D31.book Page 6 Tuesday, March 20, 2007 6:32 PM
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