3-37

(UR pin 21) outputs are latched on the falling edge of

LATCH/HOLD and remain in that state until the end of the

next conversion cycle. In addition, digits 1 through 4 are

blanked during overrange. All three of these pins are “weak

outputs” and can be overridden with external drivers or pull-

up resistors to enable their input functions as described in

the Pin Description table.

Instant Continuity

A comparator with a built-in 200mV offset is connected

directly between INPUT HI and INPUT LO of the ICL7129

(Figure 7). The CONTINUITY output (pin 27) will be pulled

high whenever the voltage between the analog inputs is less

than 200mV. This will also turn on the “CONTINUITY”

annunciator on the display. The CONTINUITY output may be

used to enable an external alarm or buzzer, thereby giving

the ICL7129 an audible continuity checking capability.

Since the CONTINUITY output is one of the four “weak out-

puts” of the ICL7129, the “continuity” annunciator on the dis-

play can be driven by an external source if desired. The

continuity function can be overridden with a pull-down resistor

connected between CONTINUITY pin and DGND (pin 36).

Display Configuration

The ICL7129 is designed to drive a triplexed liquid crystal

display. This type of display has three backplanes and is driven

in a multiplexed format similar to the ICM7231 display driver

family. The specific display format is shown in Figure 8. Notice

that the polarity sign, decimal points, “LOW BATTERY”, and

“CONTINUITY” annunciators are directly driven by the

ICL7129. The individual segments and annunciators are

addressed in a manner similar to row-column addressing. Each

backplane (row) is connected to one-third of the total number of

segments. BP1 has all F, A, and B segments of the four least

significant digits. BP2 has all of the C, E, and G segments. BP3

has all D segments, decimal points, and annunciators. The seg-

ment lines (columns) are connected in groups of three bringing

all segments of the display out on just 12 lines.

Annunciator Drive

A special display driver output is provided on the ICL7129

which is intended to drive various kinds of annunciators on

custom multiplexed liquid crystal displays. The ANNUNClATOR

DRIVE output (pin 3) is a squarewave signal running at the

backplane frequency, approximately 100Hz. This signal

plane outputs BP1, BP2, and BP3. Figure 9 shows these

four outputs on the same time and voltage scales.

Any annunciator associated with any of the three backplanes

can be turned on simply by connecting it to the ANNUNClA-

TOR DRIVE pin. To turn an annunciator off connect if to its

backplane. An example of a display and annunciator drive

scheme is shown in Figure 10.

+

-

BUFFER

500k

Ω

IN HI

COMMON

IN LO

V

200mV

+

-

+

-

CONTINUITY

TO DISPLAY

DRIVER

(NOT LATCHED)

FIGURE 7. “INSTANT CONTINUITY” COMPARATOR AND

OUTPUT STRUCTURE

a

b

c

d

f

g

e

a

b

c

d

f

g

e

B1, C1, CONTINUITY

BACKPLANE

CONNECTIONS

BP1

BP2

BP3

A1, G1, D1

F1, E1, DP1

B2, C2, LOW BATTERY

A2, G2, D2

F2, E2, DP2

F4, E4, DP4

A4, G4, D4

B4, C4, BC5

F3, E3, DP3

A3, G3, D3

B3, C3, MINUS

f

e

a

b

c

d

f

g

e

a

b

c

d

f

g

e

LOW BATTERY CONTINUITY

c

a

b

c

d

f

g

e

a

b

c

d

f

g

e

f

e

a

b

c

d

f

g

e

a

b

c

d

f

g

e

LOW BATTERY CONTINUITY

c

FIGURE 8. TRIPLEXED LIQUID CRYSTAL DISPLAY LAYOUT FOR ICL7129

ICL7129