ISL3150E-59E INTERSIL [Intersil Corporation], ISL3150E-59E Datasheet - Page 2

• Manufacturer Part Number: ISL3150E-59E
• Description: Add a Loss of Signal (LOS) Indicator to Your RS-485 or RS-422 Transceiver
• Manufacturer: INTERSIL [Intersil Corporation]

Using Full-Failsafe Receivers for
LOS Detection
This full-failsafe feature can be exploited to detect the LOS bus
condition. The advantage of using full failsafe receivers for LOS
detection is that the V
condition. If the network is set up so that a LOS causes the
V
detect the LOS.
Adding an Additional Full-Failsafe Rx Allows
LOS Detection
Figure 2 illustrates circuitry that can implement an LOS detection
function. The transceiver block can be any RS-422 or RS-485 IC
with a full-failsafe Rx (see Table 1 for a list of Intersil full-failsafe
Rx devices), and at least one termination resistor (R
to collapse the bus voltage when the bus is not actively driven.
Adding a second full-failsafe Rx - with its inputs connected
opposite to those of the primary Rx - enables the LOS circuit to
distinguish between the V
1 bus state. When the bus is actively driven, RXD and RXD will be
in opposite states (see Figure 3). The only time that RXD and RXD
are in the same state is when the bus voltage is 0V, because both
Rx interpret this as a logic high input. ANDing the two Rx outputs
yields an active high LOS signal (see Figure 4). Propagation delay
differences (skews) between the transceiver Rx and the
secondary Rx may cause glitches on the LOS signal. Adding a low
pass filter (R
FIGURE 3. Rx OUTPUT VOLTAGES RELATIVE TO THE BUS VOLTAGE
ID
LOS
=0V condition, then the full-failsafe function can be utilized to
-2
-4
5
4
3
2
1
0
4
2
0
C
F
F
R
and C
F
FIGURE 2. LOS DETECTION CIRCUIT
RXD
F
in Figure 2) filters out these glitches.
ID
=0V condition is now a valid input
ID
RXD
=0V bus state and the normal Data =
TIME (1µs/DIV)
RO
RO
D I
2
ISL3280E
R
R
D
V
B
A
BUS
A/Y
B/Z
RXD
RXD
Application Note 1593
R
T
T
) is required
+
V
-
BUS
A good choice for the secondary Rx is the ISL3280E. It is a single
Rx in a micro package (5 Ld SOT-23), has the required
full-failsafe function, operates up to 20Mbps, and features
IEC61000-4-2 ESD levels. Additionally, the 3V-5V supply range,
and the -40°C to 125°C operating temperature range allow for
the ISL3280Es use in most RS-485 and RS-422 applications.
Los Detect May Not Work For All Rx
Even if present at each full-failsafe Rx node, the LOS detect
circuit may not properly indicate all the possible LOS conditions.
As previously mentioned, a termination resistor is required to
force the V
the RS-422 Standard allows only one termination resistor on the
bus, and the RS-485 Standard allows only two. Thus, only one or
two of the LOS detectors can have the termination resistors. This
is fine for the case where a multi-drop (only one driver on network
with multiple receivers) network loses power to the driver, as the
one termination resistor forces the V
thus on each Rx input. But if an Rx without the termination
resistor becomes disconnected from the bus, its V
0V, so the LOS circuit may not indicate the LOS condition. A
possible solution is to include a larger value (e.g., 1kΩ) resistor
across the inputs of the all Rx that don’t have termination
resistors. Whether or not this is acceptable depends on the
transceiver family used, the criticality of the termination
resistance value (slow, slew rate limited transceivers aren’t as
termination sensitive), and the number of nodes on the network.
Multi-point systems offer another challenge. As mentioned
previously, there are multiple drivers on the bus, so there are
periods of inactivity when one driver disables before another
enables. If the idle period is long enough, the bus voltage might
collapse to 0V, causing the LOS to momentarily activate. If this is
a problem, then the user might have to implement a timing
circuit to ensure that the LOS signal has been active long enough
to indicate a real LOS condition.
FIGURE 4. AND GATE OUTPUT AND FILTERED LOS INDICATOR
5
4
3
2
1
0
5
4
3
2
1
0
ID
=0V condition that indicates a LOS. Unfortunately,
SIGNAL
TIME (1µs/DIV)
ID
AND
=0V on the whole bus, and
RXD
RXD
ID
February 1, 2011
may not be
LOS
AN1593.0