LTC1485IS8#TR Linear Technology, LTC1485IS8#TR Datasheet - Page 9

• Manufacturer Part Number: LTC1485IS8#TR
• Description: IC TXRX BUS/LINE RS485 LP 8SOIC
• Manufacturer: Linear Technology
• Type: Transceiverr

Specifications of LTC1485IS8#TR

• Number Of Drivers/receivers: 1/1
• Protocol: RS422, RS485
• Voltage - Supply: 4.75 V ~ 5.25 V
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

A
AC Cable Termination
Cable termination resistors are necessary to prevent un-
wanted reflections, but they consume power. The typical
differential output voltage of the driver is 2V when the
cable is terminated with two 120
33mA of DC current to flow in the cable when no data is
being sent. This DC current is about 10 times greater than
the supply current of the LTC1485. One way to eliminate
the unwanted current is by AC-coupling the termination
resistors as shown in Figure 13.
The coupling capacitor must allow high frequency energy
to flow to the termination, but block DC and low frequen-
cies. The dividing line between high and low frequency
depends on the length of the cable. The coupling capacitor
must pass frequencies above the point where the line
represents an electrical one-tenth wavelength. The value
PPLICATI
Rt = 120
Rt = 470
Rt = 47
DX
Figure 13. AC-Coupled Termination
C = LINE LENGTH (FT) • 16.3pF
Figure 12. Termination Effects
O
DRIVER
U
PROBE HERE
S
120
I FOR ATIO
C
U
R
t
W
RECEIVER
RECEIVER
resistors, causing
1485 F12
1485 F13
U
RX
RX
of the coupling capacitor should therefore be set at 16.3pF
per foot of cable length for 120 cables. With the coupling
capacitors in place, power is consumed only on the signal
edges and not when the driver output is idling at a 1 or 0
state. A 100nF capacitor is adequate for lines up to 400 feet
in length. Be aware that the power savings start to de-
crease once the data rate surpasses 1/(120 • C).
Receiver Open-Circuit Fail-Safe
Some data encoding schemes require that the output of
the receiver maintains a known state (usually a logic 1)
when the data is finished transmitting and all drivers on the
line are forced into three-state. The receiver of the LTC1485
has a fail-safe feature which guarantees the output to be in
a logic 1 state when the receiver inputs are left floating
(open-circuit).
If the receiver output must be forced to a known state, the
circuits of Figure 14 can be used.
Figure 14. Forcing “0” When All Drivers Are Off
5V
130
110
5V
5V
110
120
130
1.5k
1.5k
120
100k
C
RECEIVER
RECEIVER
RECEIVER
LTC1485
1485 F14
RX
RX
RX
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