Cables carry these supplies to the stimulator
boards, the riser card is not used to carry these
voltages.
Connection are also made for the other signals that
are required by the Connection Unit and Control
Panel.
4.5 The Two Channel Headbox
Figure 4.5 shows a schematic diagram of the
headbox assembly with both the Pre-amplifier and
Control boards.
4.5.1
The Control Board
Communication with the mainframe is made via an
RS-232 link, which is used to set up the various
headbox parameters e.g. sensitivity.
A switching regulator that is synchronised with the
amplifier sampling system, is used to drive the
primary of an isolating transformer. The
transformer output is rectified and regulated to
provide the required isolated d.c. supplies.
Isolation of the various control and data signals is
via several opto-isolators.
The headbox is controlled by a PIC
micro-controller that interfaces with the RS-232
port. The PIC also sends data in the form of a
serial stream to shift registers on the pre-amplifier
board to set up the amplifier, and controls the
switching of the calibration signal.
The analogue patient signals from the
Pre-amplifier are buffered on this board to provide
differential signal inputs to the ADC, which
digitises and serialises the signals under control of
the PAL. The serialised data stream is returned to
the mainframe along with a clock and strobe
(frame) signal from the PAL, to be processed by
the DSP.
4.5.2
The Pre-amplifier Board
The serial data stream from the PIC on the Control
Board is converted into parallel data by shift
registers, and these signals control the switching of
the gain and filters.
The patient signals are routed through input
switches to the fixed-gain input amplifiers and then
buffered to provide a screen driver. The amplified
signals are then fed through high pass filters and
switched-gain amplifiers before being sent to the
Control Board.
When the amplifier is switched off by operation of
a switch on the headbox, the mainframe is allowed
to switch on the Impedance Check/Amp Cal
oscillator circuit.
In Amp Cal mode (selected from the mainframe),
the oscillator signal is connected to the
Pre-amplifier inputs through switched attenuators
controlled by signals from the mainframe via the
PIC.
In Impedance Check mode, the oscillator signal is
connected directly to the Pre-amplifier inputs (and
hence any impedance across the input) via user
operated push-button switches. These switches also
indicate to the mainframe (via the PIC) timpedance
check is being made. The oscillator signal is
attenuated according to the level of impedance
across the input (the patient) and is then processed
as a normal patient signal by the DSP to determine
the level of signal attenuation. The DSP then
returns a signal to the headbox and the PIC outputs
a signal to turn on the relevant impedance level
indicator LED.
Three rotary switches on the board are used for
setting the Sweep Speed, Gain, and Trigger Level.
They operate by sending pulses to the PIC as they
are rotated, and this information is returned to the
mainframe. The Trigger Level switch also has a
momentary push-button action that sends a signal
to the PIC and mainframe to turn the trigger on
and off.
The temperature probe amplifier monitors the
changes in the resistance of the probe, and outputs
an analogue signal that varies with changes in
temperature. This signal is input directly to the PIC
ADC which processes the signal into digital data
that is fed back to the DSP.
4.6 The 5 Channel Headbox
This headbox comprises a Control Board,
Electrode Board, and Amplifier Modules that plug
in to the Electrode Board. Block diagrams for
these boards are shown separately.
4.6.1
The Control Board
Communication with the mainframe is via an
RS-232 link, which is used to set up the various
headbox parameters e.g. sensitivity.
A switching regulator that is synchronised with the
amplifier system, is used to drive the primary of an
isolating transformer. The transformer output is
rectified and regulated to provide the required
isolated d.c. supplies. An extra output from the
transformer is rectified and regulated to provide
the +5V supply for the non-isolated circuitry.
Isolation of the various control and data signals is
made via several opto-isolators.
The headbox is controlled by a PIC
micro-controller that interfaces with the RS-232
port. The PIC also sends data in the form of a
serial stream to shift registers on the amplifier
Issue A May 1998
TECHNICAL OVERVIEW
Chapter 4 Page 8
Synergy Mobile Workshop Manual