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The unit boots into SmartSpeaker mode by default and can be put into TAP mode via two different

methods:

•

Send out the following 4 bytes using a RS-232 terminal (4800 baud, 8 bits, no parity, 1 stop bit):

0x01,0xFF,0xAA,0x54. This tells the board to switch to ASCII TAP mode. The board will then switch

protocols to 4800 baud, 8 bits, even parity, 1 stop bit. It will reply with a single '>' (0x3E). Since the

response is 0x3E, the parity bit will be set to 1; there is no need to switch protocols until after the

'>' is received since the parity bit will look like a stop bit. Switch the RS-232 terminal to 4800 baud,

8 bits, even parity, 1 stop bit. At this point, normal ASCII TAP commands can be sent to the board.

Note that when TAP mode is activated in this manner, the DSP will not echo characters sent by the

terminal.

•

Set all four DIP switches into the "up" position (away from RCA plugs). Power the PS28 up using

the AC line switch and wait until the amber LED blinks twice and the green LED blinks briefly at

least once (approx. 3 seconds after power up). Then, within twenty seconds, flip ALL four switches

down, then flip switch #4 (closest to the RJ-45 connector) back up. The unit will now communicate

in TAP mode. Note that when TAP mode is activated in this manner, the DSP will echo characters

sent by the terminal.

2.5 Thermistor Temperature Sensor

Sheet 8 of SD254175

RT600 is a negative temperature coefficient device which is used to measure the ambient tem-

perature in the vicinity of the DSP. This measured temperature is used by the DSP in a complex

algorithm which uses the woofer output signal and the PSC_PWM signal (which contains info

about the power supply rail voltage) to compute the power being dissipated in the MOSFETs on

the Power Supply PCB and, hence, their approximate temperature, then adjusts the system gain

over time to keep the unit's operating temperature within safe limits despite the external ambient

temperature or how long/loud the unit has played. This algorithm only "turns the volume down" by

a few dB in cases where the unit has been operated at maximum volume levels with "heavy"

source material.

The resistance of RT600 changes inversely proportional to the temperature of the device, with its

resistance nominally 10k at room temperature (25°C). The circuit formed by two of the comparator

sections of U3 and the associated parts form an oscillator which has an "on" time determined by

C620 charging through R655 and an "off" time determined by C620 discharging through RT600.

This makes the oscillator output a waveform with a duty cycle that is a function of the temperature

of RT600. The DSP samples the output of the oscillator at a constant rate and averages the total

"on" time and total "off" time over 10 cycles of the oscillator to compute the temperature.

2.6 DIP Switches

Sheet 8 of SD254175

The 4-position DIP switch S1 is primarily on the DSP to set the room code of the bass module for

situations where there are multiple speakers on a single zone of a Lifestyle

switches normally ship with all four in the "down" position (towards the PCB). They also serve as a

method of setting the Sharc into TAP mode, but only during the first 20 seconds after power has

been turned on.

Theory of Operation

PS18/28/35 Troubleshooting Guide

media center. These