EVAL-AD421EB

REV. A

–2–

Link Options

There is one link option on the evaluation board which should

be set for the required operating setup before using the board.

The function of this link option is described below.

LK1

Function

This option is used to control the selectable

regulator on the AD421 that is used to power

the AD421 itself and the digital buffers on the

board.

Position

Function

A

With this link in position A, the LV pin on the

AD421 is connected to COM, in this position

the regulated voltage for the AD421 and the

digital buffer is set to 5V.

B

With this link in position B, the LV pin on the

AD421 is connected to V

regulated voltage for the AD421 and the digital

buffer/level shifter is set to 3V.

C

With this link in position C, the LV pin on the

AD421 is connected through a 0.01uF capacitor

to V

the AD421 and the digital buffer/level shifter is

set to 3.3V.

EVALUATION BOARD INTERFACING

Interfacing to the evaluation board is via a 36-pin Centronics

connector, SKT1 using a standard parallel printer port cable.

The pinout for the SKT1 connector is given in Figure 1 and its

corresponding pin designations are given in Table I. The

evaluation board should be powered up before a cable is

connected to the connector.

The digital interface on the AD421 consists of just three wires:

DATA, CLOCK and LATCH. The interface connects directly

to the serial ports of commonly-used microcontrollers without

the need for any external glue logic. On this evaluation board

the PC via the printer port is used to emulate a microcontroller.

Data is loaded MSB first into the input shift register of the

AD421 on the rising edge of the CLOCK signal and is trans-

ferred to the DAC latch on the rising edge of the LATCH

signal. This data can take two forms; normal 4-20mA data and

alarm current data. The first form is where the AD421 operates

over its normal 4mA to 20mA output range with 16-bits of

resolution between these end-points. The second form allows

the user to program a current value outside this range as an

indication from the transmitter than there is a problem with the

transducer i.e. tranducer burnout. The AD421 counts the

number of clock pulses which it receives between LATCH

signals as a means of determining whether the data clocked in is

4-20mA data or alarm current data.

If there are 16 rising clock edges between successive LATCH

pulses then the data to be loaded to the input shift register is

assumed to be normal 4-20mA data.

If there are more than 16 rising clock pulses between successive

LATCH pulses then the data to be loaded to the input shift

register is assumed to be alarm current data.

Table I. SKT1 Pin Designations

1

NC

No Connect. This pin is not connected

on the evaluation board.

2

NC

No Connect. This pin is not connected

on the evaluation board.

3

LATCH

Latch Input. The signal on this pin is

buffered in 5V operating mode and

level translated in both 3.3V and 3V

modes using a 74HC4050 before being

applied to the LATCH pin of the

AD421. On the rising edge of the latch

signal

data is loaded from the input

register to the DAC register and the

DAC output is updated.

4

NC

No Connect. This pin is not connected

on the evaluation board.

5

CLOCK

Serial Clock. The signal on this pin is

buffered when using a V

level translated when V

before being applied to the CLOCK

pin of the AD421. Data on the DATA

pin is clocked into the AD421 shift reg-

ister on the rising edge of this clock in-

put.

6-8

NC

No Connect. These pins are not con-

nected on the evaluation board.

9

DATA

Serial Data Input. Data applied to this

pin is buffered when using a V

and level translated when V

or 3V before being applied to the

AD421's DATA pin. The serial data

applied to the DATA pin is written to

the input shift register on the part. Data

from this input shift register is trans-

ferred to the data register on the rising

edge of the LATCH signal.

10-18

NC

No Connect. These pins are not con-

nected on the evaluation board.

19-25

DGND

Ground reference point for digital input

signals. Connects to the COM plane on

the evaluation board.

26-36

NC

No Connect. These pins are not con-

nected on the evaluation board.

Figure 1. SKT1 Pin Configuration, Pin View.

1

18

19

36