6

LT1785/LT1785A

LT1791/LT1791A

TEST CIRCUITS

Figure 1. Driver DC Test Load

Figure 2. Receiver Timing Test Load

A

B

R

R

1785/91 F01

RECEIVER

OUTPUT

1k

S1

S2

TEST POINT

1k

1785/91 F02

state. If receiver enable RE and driver enable DE are both

in the disable state, the circuit goes to a low power

shutdown state. Placing either RE or DE into its active state

brings the circuit out of shutdown. Shutdown state is not

entered until a 3

µs delay after both RE and DE are disabled,

allowing for logic skews in toggling between transmit and

receive modes of operation. For CAN bus operation the DE

pin is used for signal input to place the data bus in

dominant or recessive states.

DI: Driver Input. TTL level logic input. A logic high at DI

causes driver output A or Y to a high state, and output B

or Z to a low state. Complementary output states occur for

DI low. For CAN bus applications DI should be tied low.

GND: Ground.

Y: Driver Output. The Y driver output is in phase with the

driver input DI. In the LT1785 driver output Y is internally

connected to receiver input A. The driver output assumes

a high impedance state when DE is low, power is off or

thermal shutdown is activated. The driver output is pro-

tected from shorts between

±60V in both active and high

impedance modes. For CAN applications, output Y is the

CANL output node.

Z: Driver Output. The Z driver output is opposite in phase

to the driver input DI. In the LT1785 driver output Z is

internally connected to receiver input B. The driver output

assumes a high impedance state when DE is low, power is

off or thermal shutdown is activated. The driver output is

protected from shorts between

±60V in both active and

high impedance modes. For CAN applications, output Z is

the CANH output node.

A: Receiver Input. The A receiver input forces a high

receiver output when V(A)

≥ [V(B) + 200mV]. V(A)≤ [V(B)

– 200mV] forces a receiver output low. Receiver inputs A

and B are protected against voltage faults between

±60V.

The high input impedance allows up to 128 LT1785 or

LT1791 transceivers on one RS485 data bus.

The LT1785A/LT1791A have guaranteed receiver input

guaranteed to be in a high state for 0V inputs.

B: Receiver Input. The B receiver input forces a high

receiver output when V(A)

≥ [V(B) + 200mV]. When V(A)

≤ [V(B) – 200mV], the B receiver forces a receiver output

low. Receiver inputs A and B are protected against voltage

faults between

±60V. The high input impedance allows up

to 128 LT1785 or LT1791 transceivers on one RS485 data

bus.

The LT1785A/LT1791A have guaranteed receiver input

guaranteed to be in a high state for 0V inputs.

4.75V