relevant current or voltage controlled level to the
patient output sockets.
The delivered stimulus is monitored by the PIC,
and if the pulse is incorrect, the PIC turns off the
oscillator circuit providing the 350V supply and
also discharges the supply. The PIC also sends an
error message to the mainframe indicating an error
has occurred. The stimulus will not start up again
until the PIC is enabled by the mainframe.
The hand held stimulator probe has a stimulus
intensity control built into it, and this control is
connected directly to the PIC. The intensity level is
returned to the mainframe, which sends the
relevant signals back to the stimulator to set the
required level. A pair of LED’s on the probe
indicate the polarity of the stimulus pulse by
illuminating the LED adjacent to the positive
electrode.
4.4 The Evoked Potential Stimulators
A schematic diagram of the Enhanced EP
Stimulator board is shown in .
This board plugs into one of the ISA slots and a
“Plug-n-Play” device provides the necessary
interfacing to the board. A decode PAL converts
the signals from the “Plug-n-Play” device to make
them compatible with the “H16 Interface” LCA.
A second PAL acts as the DSP Interface. This
device processes the signals from the DSP
Daughter board (that are connected to the EP
Stimulator via a ribbon cable) and generates audio
and visual stimulus timing and control signals. It
also decodes signals from the EP Stimulator board
to generate status signals that are returned to the
DSP Daughter board.
Auditory Stimulator
The eleven half sinewaves corresponding to the
eleven auditory stimulus frequencies are stored in
the Waveform Generator LCA. The auditory
stimuli are controlled by this LCA and the Z80
processor. The “white noise” signal is digitally
generated within the LCA and is then either
switched directly to the auditory amplifier or
filtered before being connected to the amplifier.
The digital sinewave data is clocked into a data
latch and then converted into an analogue signal by
a dual DAC.
The sinewave and/or noise signals are selected by
analogue switches controlled by a shift register that
is loaded from a Z80 controlled latch. A
microphone input or external input are also
buffered and connected to these switches.
The selected sinewave and/or noise signals are
then fed to dual digital attenuators (LOGDACs).
The noise signal has a switched resistive -34dB
attenuator in the line before the LOGDACs.
The output from the LOGDACs are summed
together before being connected to the power audio
amplifiers – a switched resistive -30dB attenuator
is inserted in the sinewave signal path prior to the
summing junction.
Switched feedback around the audio amplifiers
provides an amplification factor of 0dB or +24dB.
The left and right audio signals are then switched
through various resistive networks to provide 0dB,
-50dB or -100dB of attenuation before being
connected to the audio transducer.
The audio output is also monitored to compare the
output energy delivered into the transducer with a
level set by a serial DAC controlled by the Z80. If
the auditory output delivered to the patient exceeds
the set limit, the stimulus is immediately shut off,
the audio amplifiers are muted, and the -100dB
output attenuators are switched in. The board then
has to be reset, and the code downloaded again in
order to continue with any tests.
Visual Stimulator
Blocks of information for the various complex
visual stimulus patterns are programmed into the
“H16 Interface” LCA. The pattern data is then
written into a RAMDAC via the Z80 data bus at
addresses set by the Z80 address bus and the
palette address bus.
The video pattern is then output from the
RAMDAC as red, green, and blue (RGB) signals
to the monitor along with the vertical and
horizontal sync. signals from the LCA. Various
leads are available to connect the RGB output to
the different options of video monitor e.g. VGA
monitor, monochrome monitor etc.
4.4.1
PC I/O panel
This is mounted in the lowest 5
3
" disk drive bay of
the tower and has three cables which exit the front
of the PC tower from behind a clipped on metal
cover. This board connects and switches the
isolated DC power from the PSU to the EMG,
DSP and EP stimulator cards in the PC tower. The
+5V generated by the internal supply in the PC
tower is monitored by a TL7702BCD chip. Only
when this +5V has settled will the four Mosfets be
turned on. These are two type MTB30N06VL (or
IRF5305) devices and two MTB30P06VL (or
IRF9Z34) devices which switch the +24V,-24V,
+12V and -12V to the DSP and EP stimulator
boards.
Issue A May 1998
TECHNICAL OVERVIEW
Chapter 4 Page 6
Synergy Mobile Workshop Manual