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Interface (Data Transmission)
Texas Instruments Incorporated
Cascading of input serializers boosts
channel density for digital inputs
By Frank Dehmelt,
Systems Engineer Analog Products
Introduction
Programmable logic controllers (PLCs) play an integral
role in industrial automation. They allow inputs from
digital as well as analog sensors and provide outputs to
drive actuators. The digital inputs represent a significant
share of those I/Os, accepting inputs from end switches,
proximity switches, fuel sensors, light barriers, over-
temperature sensors and many others.
The traditional approach
There are several types of digital inputs; the
IEC-61131-2-standard defines those most commonly used.
Traditionally, digital inputs used discrete components
and required a parallel processor interface. Current
limitation was achieved by a series of high-power resistors.
Resistor-capacitor (RC) filters reduced bouncing of
mechanical switches, while a per-channel optocoupler
connected to the parallel processor interface. This design,
however, requires bulky components, many isolation
channels, and a large footprint host controller to allow
for the parallel inputs. It also creates significant power
dissipation.
With a typical resistor chain providing about 2.2 k
A new approach
TI’s SN65HVS88x product family addresses these limita-
tions and more. The digital input serializer (as the name
implies) serializes the inputs into a single SPI data stream
and reduces the number of isolators by 50%.
The resistors and LEDs shown in Figure 1 are required
by the IEC-61131-2-standard; they can be omitted for
inputs that do not require conformance with this standard.
Regardless, the integrated current limit allows use of a
lower power resistor.
The input current is fed to an output pin, which allows
to drive an external LED to indicate the current state
of the input. Without the LED, this pin simply connects
to ground.
The HVS88x family allows for the cascading of several
devices, all sharing the same SPI interface. For a 32-chan-
nel input, it still provides a four-channel isolation, saving
87% of ISO channels.
And what about power dissipation with a 32-channel
interface? We previously calculated a worst-case dissipa-
tion of 400 mW/channel totaling almost 13 W: this is too
much for a PLC slice which is about the size of a deck of
cards. The HVS88x family allows the designer to set cur-
rent limitation anywhere between 200 µA and 5.2 mA.
For a type-1 or type-3 switch, choose a limit in the 3-mA
range, limiting the per-channel dissipation to 90 mW at
30 V. This reduces power dissipation by more than 75%
vs. a discrete approach.
,
the current at the nominal 24 V rises to 11 mA and results
in power consumptions of 260 mW or 400 mW at 30 V.
Considering that this dissipation may occur simultaneously
for all input channels — along with the bulky components
and the processor interface — it severely limits channel
density.
Ω
Figure 1. 8-channel digital input using HVS882 and ISO7241
SN65HVS882
0 V
24 V
V
CC
5VOP
3 V - ISO
VREG
Host
Controller
V
CC
ISO7241
SIP
I
IN
0:7
8
Optional
R
IN
0:7
V
CC1
V
CC2
RE 0:7
8
LD
Load
SCLK
STE
SOMI
8
CLK
2
DB 0:1
CE
RLIM
SOP
TOK
GND
GND1
GND2
GND
TEMP
16
Analog Applications Journal
High-Performance Analog Products
www.ti.com/aaj
3Q 2008
Texas Instruments Incorporated
Interface (Data Transmission)
The designer can further reduce the number of external
components by using the integrated debounce filter, set to
filter pulses of less than 3 ms or 1 ms in duration. For the
fastest acquisition of glitch-free switches, bypass the filter
as well.
The parts operate from the 24-V nominal field supply
and generate the internally used 5 V themselves. This
supply is also available to drive external circuitry such as
the field side of the isolation barrier on the SPI interface.
The HVS88x family allows high-density digital inputs by
serialization, cascading, a significant reduction of power
dissipation, and elimination of external components.
Production material, samples, and evaluation boards
are available.
Table 1. The SN65HVS88x digital input serializer family
digital isolator
PARAMETER
SN65HVS880
SN65HVS882
Serialization
Yes
Yes
Cascading
Yes
Yes
Current Limitation
Yes
(0.2 to 5.2 mA)
Yes
(0.2 to 5.2 mA)
Debounce Filter
Yes
(0 ms, 1 ms, 3 ms)
Yes
(0 ms, 1 ms, 3 ms)
V
CC
18 V to 30 V
10 V to 34 V
Undervoltage Indicator
Yes (~15 V)
No
5-V Output
Yes
Yes
Related device
ISO721 – High-speed digital isolator
www.ti.com/iso721
Input Voltage Range
0 V to 30 V
0 V to 34 V
Temperature Range
–40°C to 85°C
–40°C to 125°C
Over-Temperature Protection
Yes
Yes
Related Web sites
interface.ti.com
www.ti.com/sn65hvs882
Figure 2. Simplified 32-channel digital input with cascading,
using four HVS88x serializers and one ISO7241 digital isolator
Host
Controller
ISO7241
4 x SN65HVS880
INA
Load
OUTB
INB
STE
OUTC
INC
SCLK
IND
OUTD
SOMI
Serializer
Serializer
Serializer
Serializer
17
Analog Applications Journal
3Q 2008
www.ti.com/aaj
High-Performance Analog Products
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