September 26, 2003
6881096C77-O
3-6
Theory of Operation: W4, W5, and W7 Control Heads
3.2.5
Reset
On initial power-up, the RESET line to U1-1 is held low by U4 for approximately 120 milliseconds
before going high. This resets U1 and starts it operating properly. Then, a pulse train generated at
U1-38 sends tickle pulses to the watchdog timer circuitry in U4 and sends a STROBE signal to the
VF display driver, U101. Capacitor C6 charges to about 3.3 volts and triggers a reset as described
above. Any time a reset occurs, U4-9 goes high, causing radio circuitry in the command board to
also reset. Should radio circuitry initiate the reset, pin 23 of the control unit PC board connector is
forced high, triggering the same events described above. The control head can reset the radio
command board, and vice-versa, via the reset line. See
Table 3-3
.
3.2.6
Bus Busy
The control head prepares to send data on the serial bus when U-15 (BUSY OUT) goes low, causing
U4-10 to go high. This signals the radio command board that the control head has data to send.
U4-11 (BUSY IN) goes low whenever U4-10 goes high. This informs the control-head microcomputer
that the serial bus is in use. See
Table 3-3
.
3.2.7
Serial Bus Data
See
Table 3-3
for Serial Data Bus Logic. Two lines, BUS+ and BUS-, send data between the control
head and the radio command board. Under the normal condition of inactivity, BUS+ is high and BUS-
is low. This is the “inactive state” on the serial bus. When the two lines invert (BUS+ goes low, BUS-
goes high), the serial bus is in the “active state.”
The control head uses the TX DATA line (U1-33) to send data over the serial bus. When TX DATA
goes low, the bus is placed in the active state by U4; when TX DATA goes high, the bus returns to
the inactive state.
Data is sent to the microcomputer on the RX DATA line (U1-32). Whenever the serial bus is in the
active state, RX DATA is low. An inactive state on the bus causes RX DATA to go high.
Table 3-3. Serial Data Bus Logic
Data Line
No Bus Activity
(Normal
Receiving Data)
Bus Activity
(Sending or
Receiving Data)
In Reset*
Busy
Low
High
Low
BUSY IN
High
Low
High
BUSY OUT
High
Low*
High
BUS+
High
Low and High
High
BUS-
Low
High and Low
Low
TX DATA
High
Low and High**
High
RX DATA
High
Low and High
High
RESET
Low
Low
High
RESET
High
High
Low
* Bus activity will occur immediately after a reset.
** Sending only.
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