Jands HP6 User Manual

Equipment Description 3-11
Revision 2 - 26 April, 2000 HP SERIES DIMMER TECHNICAL MANUAL
3.7 Front panel card
(
Figure 3.4
The front panel PCB holds all front panel switches and LEDs, including the LOAD and
DRIVE LEDs which are fed by the Output PCB. Additionally, the control electronics for
the switches and LEDs and the encoder circuitry are also on the front panel PCB. To
minimise flat cable size, the CPU data bus and required CPU select lines are connected to
the front panel card to control the latches for the LEDs and buffers for the
switches/encoder circuitry. The majority of components on this card are Surface Mount
devices.
3.7.1 Switches
The switches are read individually via tri-state 74HC245 buffers (IC4, IC5, IC6). Each
switch when pressed (closed) pulls the buffer input high, otherwise a 10K resistor (RN1,
RN2, RN3) pulls it to ground.
Note that switch faults will most likely be caused through wear in the switches
themselves.
3.7.2 LEDs
Due to the large number of front panel LEDs (including the four 15 segment displays),
all except the Phase LEDs and Drive/Load LEDs are driven in a matrix, the seven rows
of which are driven by a 74HC574 latch and BCX17 transistors (IC1/Q1-7/R1-R7). The
16 columns are driven by two 74HC574 latches (IC2-3/R8-23). The matrix is driven with
only one row on at any time, each row being on for approximately 2ms out of 14ms. A
failure in the matrix could result in a row being dead (eg. Digit 1 or Green Select LEDs
off) or a column being dead (eg. segment 4 of all digits failed), else the fault will be a
single LED dead. It is important when tracing matrix faults to refer directly to the circuit
diagram.
Note that the bicolour “Select” LEDs are sensitive to excess soldering temperature and
should be replaced with care.
An RC delay network C5/R24 holds the display off for a short period during startup to
ensure the CPU has time to initialise the matrix before the display becomes active. This
prevents damage to the display LEDs through over current and ensures that the large
current required to feed all segments does not occur.
An RC decoupling network C11/R25 minimises the amount of noise generated by the
LED multiplex circuit that is fed into the 5 volt rail.
3.7.3 Encoder
The encoder output consists of two signals 90 degrees out of phase. These signals have
already been filtered on the CPU card, and so are then squared by a 40106 Schmitt
(IC8:A/:B/:C). The signals are used to control the 4516 up/down counter (IC9) such that
when the encoder is rotated clockwise the counter counts up,and when the encoder is
rotated counterclockwise the counter counts down. Note that any time the encoder is
rotated, pulses should appear on pins 10 and 15 of the counter IC9.

Jands HP6 Specifications

General

Brand	Jands
Model	HP6
Category	Dimmer
Language	English