4

®

ISO508

OPERATION

ISO508 has two modes of operation: synchronous and asyn-

chronous. The mode is selected by the CONT pin. CONT

held low selects synchronous operation while CONT held

high will select asynchronous operation. In synchronous

mode the user has control of the time at which the data is

transmitted across the barrier, while in asynchronous mode

the data is continually transmitted across the barrier under

the internal logic control. The input latches and output

latches are level sensitive.

SYNCHRONOUS MODE

With CONT held low, the input data is transmitted across the

barrier under the control of LEI. When LEI is held low, no

data is passed to the input buffer and no barrier transmission

takes place. When LEI goes high, data is transferred to the

input buffer. On the falling edge of LEI the data is then

transmitted across the barrier.

FIGURE 1. Data Transfer—Synchronous Mode.

If LEI is taken high when transmission is in progress, the

input data is recaptured in the input buffer without interfer-

ing with the transmitting data. If LEI then goes low while

transmission is still in progress the current transmission will

be terminated and restarted with the new data. The last full

data transmission will not be affected.

The output data changes under the control of LEO. If LEO

is high at the end of a transmission, the output data will

change immediately the transmission is complete. If LEO is

low at the end of transmission, the output data will change

when LEO goes high. In both cases all data bits will change

together guaranteeing the specified skew performance. Trans-

mitted data can be ignored selectively, if desired, by main-

taining LEO low until the desired data is available.

ASYNCHRONOUS MODE

When CONT is held high, the internal transmission circuit is

in control and will initiate data transmission asynchronously

at 1MWords/s i.e., 1Mbps on each of the 8 channels.

The asynchronous mode runs continuously under the inter-

nal controls. LEI can serve as an input data latch, but will not

control the data transmission across the barrier. As in the

synchronous mode, LEI and LEO can be used to select of

ignore input or output data respectively.

FIGURE 2. Data Transfer—Synchronous Mode Restart.

FIGURE 3. Data Transfer—Asynchronous Mode.

n–1

INPUT DATA

LEI

BARRIER

LEO

DATA n

DATA n+1

DATA n–1

OUTPUT DATA

DATA n

ACTIVE

ACTIVE

n–1

INPUT DATA

LEI

BARRIER

LEO

DATA n+1

DATA n+2

DATA n

DATA n–1

OUTPUT DATA

DATA n+1

ACTIVE

ACTIVE

ACTIVE

n–1

INPUT DATA

LEI

BARRIER

LEO

DATA n

DATA n+1

DATA n+2

OUTPUT DATA

ACTIVE n–1

ACTIVE n

ACTIVE n

ACTIVE n+2

DATA n–1

DATA n

n+2