Electrostatic Transient Protection - Motorola MTM700 Detailed Service Manual

THEORY OF OPERATION (CONTROL HEADS)
Line PTT IRDEC (J0811-6) is used to key up the radio's transmitter. While the PTT button on a con-
nected microphone is released, line PTT IRDEC is pulled to +5 volts level by R0880. Transistor
Q0871 is switched on and causes a low at µP port PA2. When the PTT button is pressed, signal PTT
IRDEC is pulled to ground level. This switches off Q0871 and the resulting high level at µP port PA2
informs the µP about the pressed PTT button. The µP will inform the host radio about any status
change on the PTT IRDEC line via SBEP bus.
When line PTT IRDEC is connected to FLT A+ level, transistor Q0851 is switched on through diode
VR0851 and thereby pulls the level on line ON OFF CONTROL to FLT A+ level. This switches on
the radio and puts the radio's µP in bootstrap mode. Bootstrap mode is used to load the firmware
into the radio's flash memory (See controller sub section for more details).
The HOOK input (J0811-3) is used to inform the µP when the microphone´s hang-up switch is
engaged. Dependent on the CPS programming the µP may take actions like turning the audio PA on
or off. While the hang up switch is open, line HOOK is pulled to +5 volts level by R0883. Transistor
Q0872 is switched on and causes a low at µP port PA1. When the HOOK switch is closed, signal
HOOK is pulled to ground level. This switches off R0883 and the resulting high level at µP port PA1
informs the µP about the closed hang up switch. The µP will inform the host radio about any status
change on the HOOK line via SBEP bus.

Electrostatic Transient Protection

Electrostatic transient protection is provided for the sensitive components in the controlhead by
diodes VR0811 - VR0814. The diodes limit any transient voltages to tolerable levels. The associated
capacitors provide Radio Frequency Interference (RFI) protection.
MTM700 Mobile Radio / Detailed Service Manual 4.3 - 5