AN2677/D
6 Digital Satellite Equipment Control (DiSEqC ) using MC68HC908QT/QY
The DSU in VFC mode is sensitive to tone burst signaling. This control signal
enables switching between LNB-A and LNB-B, respectively. This is described
in the DiSEqC bus specification from Eutelsat (see References).
DiSEqC Control If the DSU receives a DiSEqC message from the IDU, it decodes it and
switches between satellite A or satellite B inputs according to the received
control command.
Table 1 summarizes all states that can be used within this small DSU. It’s
possible to receive all programs from two different satellites in the horizontal
and vertical polarization, both in the low and high bands. The detailed
description of the message format is in the DiSEqC bus specification from
Eutelsat (see References).
DiSEqC Slave Unit
Description
The schematic is shown in Figure 3.
The MC68HC908QT4 MCU is powered by the supply net, which comprises L3,
C6, R6, D4, C4, and C5. The supply voltage ranges from 4.5 Vdc to 5.0 Vdc,
depending on the kind of zener-diode.
The MCU receives control messages through pin 5 (IRQ). The 22-kHz net
comprises L3, C6, R6, C7, R8, and the Q3 circuit.
The outputs from the MCU are pins 2 and 3. These outputs control the two dc
switches according to the control messages received. The first dc switch
comprises Q1, Q2, D1, D2, and associated components. The second dc switch
comprises Q4, Q5, D3, D6, and associated components. The desired LNB is
powered from the IDU through the Q1 (Q5 opposite channel) and low-pass filter
L1, C1, and L2 (L5, C8, and L4 opposite channel). The IDU (satellite receiver)
produces the desired power supply, +13 V or +18 V respectively, and
superimposes the 22-kHz signal if needed.
Table 1. States of DiSEqC Slave Unit
State
LNB
A/B
Polar
H/V
Band
L/H
Msg 1
1 A V L
$E0 $10 $22 ($23, $03, $00)
or $E0 $10 $38 $F8
2 A V H
3 A H L
4 A H H
5 B V L
$E0 $10 $26 or $E0 $10 $27
or $E0 $10 $38 $F4
6 B V H
7 B H L
8 B H H
Freescale Semiconductor, I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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