5
4 ½ DIGIT , LED DISPLA Y DIGIT A L P ANEL VOL TMETERS
DMS-40PC
Applications
5. Engineering Scaling and Input Attenuation: In many
applications, the input signal must be scaled, or divided down,
before being applied to the DMS-40PC. In some situations, the
input signal exceeds the full-scale range of the meter, and in
other applications, a direct one-to-one relation does not exist
between the input voltage and the desired display reading. For an
example of the second situation, assume an input voltage of
1.0Vdc is required to display a reading of "7500" when applied to
a ±2V input meter. An input divider circuit, constructed with two
series resistors with an output-to-input ratio of 0.75, will scale the
1.0V signal down to 0.75V.
Engineering scaling and input attenuation are discussed in Ap
Note 4 of the DATEL Digital Panel Voltmeter Catalog. Compen-
sating for inputs which have a zero offset is also discussed.
6. BCD Outputs (DMS-40PC-X-RS and -RL-BCD Models Only):
Models with a "-BCD" suffix have 12 extra output pins, labeled
A through L (6 per side), that provide the following information:
multiplexed serial BCD data, digit drive, and polarity indication.
The data present on pins A - L simultaneously drives the meter's
internal LED display. All functions on pins A - L are 5V CMOS
compatible, however, they are only rated to drive one, 74LS
series, TTL load. CMOS logic IC's, for example 74HC or 74HCT
series devices, should be used when more fan-out capability
is required.
As the timing diagram in Figure 5 indicates, 100Hz is the
optimum display scan rate when the BCD outputs are used to
drive external LED displays. Faster scan rates, while permissible,
are not necessary. Slower scanning, however, may result in
noticeable "flickering" of the display. Common anode LED's,
combined with a 74LS247 BCD to seven-segment decoder, is the
simplest way of implementing an external display. A seventy-five
to one-hundred Ohm resistor on each LS247 segment-drive
output provides adequate display brightness.
The functions of pins A - L are listed below:
BCD DATA (pins I - L): Four lines are used for BCD (Binary Coded
Decimal) data outputs, representing the numbers 0 - 9. Positive-logic
convention (a high represents a "1") is used.
DIGIT DRIVE ( pins A - E): These five outputs, when gated with
STROBE, can be used to direct the BCD DATA into external latches.
DIGIT DRIVE outputs may also be connected directly to the bases of
NPN transistors in remote-display configurations. The digits are
scanned right-to-left, i.e., MSD (DIGIT 1 DRIVE) to LSD (DIGIT 5
DRIVE).
BUSY (pin G): This is a status pin that goes high at the start of an
analog-to-digital (A/D) conversion cycle and remains high until the
conversion ends.
STROBE (pin H): A string of five sequential, active-low, STROBE
pulses are output (after BUSY goes low) indicating the end of a
conversion and the availability of new data. The 5 STROBE pulses
occur only once per conversion, or 2.5 times each second. STROBE
can be used to latch the BCD data (on pins I - L) into external
latches. Either edge can be used since STROBE (~ 5µsec wide) is
active only in the center of the corresponding DIGIT DRIVE and BCD
DATA outputs.
POLARITY (pin F): This pin, which is also used inside the DMS-
40PC to drive the negative-sign segment, indicates whether the last
input signal conversion was positive (POLARITY set high) or
negative (POLARITY set low). POLARITY, unlike BCD DATA, is not
multiplexed. For data latching purposes, POLARITY should be
sampled during the STROBE pulse for digit 1 (most significant digit).
Figure 6. BCD Data Timing Diagram
DIGIT DRIVE 1
(MSD)
Data from
Previous Conversion
DIGIT DRIVE 2
DIGIT DRIVE 3
DIGIT DRIVE 4
DIGIT DRIVE 5
(LSD)
STROBE OUT
BCD DATA
(4 LINES)
Approx. 5µsec
5 Strobes Only Per Conversion
Approx. 2msec
Approx. 2msec
Approx. 8msec
BUSY
1
MSD
23
4
5
LSD
12
3
4
5
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