TTI QPX1200 Service Manual page 17

Interface Board
The interface board allows the unit to communicate externally via USB or RS232. IC5 is an ARM7
processor which is programmed in-circuit using the standard JTAG port accessed at PJ5.
IC3 and IC6 generate the required supply rails. PJ3 provides an RS232 connection; IC1 and IC2
provide the necessary level shifting and inversion to convert the UART's 5V 'mark' and 0V 'space' to
RS232's –5V mark and +5V space. They also allow for data to pass through from one unit to
another for TTi's Addressable RS232 Chain (ARC). USB is controlled directly by the processor; Q1
switches R22 to provide a soft connect/disconnect feature.
PJ3 provides a shared connection between the boards in the system; Q2 and IC4 allow the
processor to pull down the data line (pin) allowing it to go high only if none of the processors in the
system want to pull low. The firmware operates a master-slave system to ensure there are no data
clashes.
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