LT1785 Linear Technology, LT1785 Datasheet - Page 6

• Manufacturer Part Number: LT1785
• Description: 60V Fault Protected RS485/RS422 Transceivers
• Manufacturer: Linear Technology

TEST CIRCUITS
LT1785/LT1785A
LT1791/LT1791A
PIN
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 s 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
6
U
FUNCTIONS
U
Figure 1. Driver DC Test Load
A
B
U
V
OD
R
R
1785/91 F01
V
OC
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
thresholds –200mV < V
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)
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
thresholds –200mV < V
guaranteed to be in a high state for 0V inputs.
V
4.75V V
output drive swing. V
low ESR capacitor directly at Pin 8 (V
CC
[V(B) – 200mV], the B receiver forces a receiver output
: Positive Supply Input. For RS422 or RS485 operation,
RECEIVER
OUTPUT
CC
Figure 2. Receiver Timing Test Load
5.25V. Higher V
TEST POINT
C
RL
CC
should be decoupled with a 0.1 F
TH
TH
1k
< 0. Receiver outputs are
< 0. Receiver outputs are
CC
S1
S2
input voltages increase
CC
1k
).
1785/91 F02
V
CC