1-331

Figure 14. Typical Logic High Enable Propagation Delay vs.

Temperature.

Figure 15. Typical Logic Low Enable Propagation Delay vs.

Temperature.

Figure 16. Test Circuit for Common Mode Transient

Immunity.

Applications

Data transfer between equipment

which employs current loop

circuits can be accomplished via

one of three configurations:

simplex, half duplex or full

duplex communication. With

these configurations, point-to-

point and multidrop arrangements

are possible. The appropriate

configuration to use depends

upon data rate, number of

stations, number and length of

lines, direction of data flow,

protocol, current source location

and voltage compliance value,

etc.

Simplex

The simplex configuration,

whether point to point or multi-

drop, gives unidirectional data

flow from transmitter to

receiver(s). This is the simplest

configuration for use in long line

length (two wire), for high data

rate, and low current source

compliance level applications.

Block diagrams of simplex point-

to-point and multidrop

arrangements are given in

Figures 17a and 17b respectively

for the HCPL-4200 receiver

optocoupler.

For the highest data rate per-

formance in a current loop, the

configuration of a non-isolated

active transmitter (containing

current source) transmitting data

to a remote isolated receiver(s)

should be used. When the current

source is located at the trans-

mitter end, the loop is charged

Alternatively, when the current

source is located at the receiver

end, the loop is charged to the

full compliance voltage level. The

lower the charged voltage level

the faster the data rate will be. In

the configurations of Figures 17a

and 17b, data rate is independent

of the current source voltage

compliance level. An adequate

compliance level of current

source must be available for

voltage drops across station(s)

during the MARK state in multi-

drop applications or for long line

length. The maximum compliance

level is determined by the trans-

mitter breakdown characteristic.

A recommended non-isolated

active transmitter circuit which

can be used with the HCPL-4200

in point-to-point or in multidrop

20 mA current loop applications

is given in Figure 18. The current

source is controlled via a

standard TTL 7407 buffer to

provide high output impedance of

current source in both the ON