A5191HRT_11 ONSEMI [ON Semiconductor], A5191HRT_11 Datasheet - Page 8

• Manufacturer Part Number: A5191HRT_11
• Description: HART Modem
• Manufacturer: ONSEMI [ON Semiconductor]

Functional Description
Highway Addressable Remote Transducer (HART) field
instruments and masters. The modem IC contains a transmit
data modulator with signal shaper, carrier detect circuitry, an
analog receiver, demodulator circuitry and a crystal
oscillator, as shown in the block diagram in Figure 1.
and generates a sine shaped FSK modulated signal at the
analog output TxA. A digital “1” or mark is represented with
a frequency of 1200 Hz. A digital “0” or space is represented
with a frequency of 2200 Hz. The used bit rate is 1200 baud.
input, filters it with a band-pass filter and generates 2 digital
signals: RxD: Received Data and CD: Carrier Detect. At the
digital output RxD the original modulated signal is received.
CD outputs the Carrier Detect signal. It goes logic high if the
received signal is above 100 mVpp during 4 consecutive
carrier periods.
using either a simple external resonator or an external clock
source.
Detailed Description
Modulator
TxD input and generates the FSK modulated signal at the
TxA output.
1200 Hz. A logic “0”or space is represented by a frequency
fs = 2200 Hz.
RTS
TxD
The A5191HRT is a single-chip modem for use in
The modulator accepts digital data at its digital input TxD
The demodulator receives the FSK signal at its analog
The oscillator provides the modem with a stable time base
The modulator accepts digital data in NRZ form at the
A logic “1” or mark is represented by a frequency fm =
Figure 5. Modulator Block Diagram
t
“1” = Mark
BIT
Controlled
Oscillator
1.2 kHz
Numeric
= 833 ms
Figure 6. Modulation Timing
Shaper
Sine
“0” = Space
t
BIT
MODULATOR
2.2 kHz
= 454 m s
TxA
FSK_OUT
http://onsemi.com
t
8
continuous mode preventing abrupt phase shifts when
switching between mark and space frequency. The control
signal Request To Send RTSB enables the NCO. When
RTSB is logic low the modulator is active and A5191HRT
is in transmit mode. When RTSB is logic high the modulator
is disabled and A5191HRT is in receive mode.
Shaper block to a trapezoidal signal. This circuit controls the
rising and falling edge to be inside the standard HART
waveshape limits. Figure 7 shows the transmit-signal forms
captured at TxA for mark and space frequency. The slew
rates are SR
3300 V/s at the space frequency. For AREF = 1.235 V, TxA
will have a voltage swing from approximately 0.25 to 0.75
V
0.5 V
0.5 V
Demodulator
and reconstructs the original modulated signal at the RxD
output. Figure 8 illustrates the demodulation process.
frame with Start, Stop, 8 Data – and 1 Parity bit. The
communication speed is 1200 baud.
FSK_IN
RxD
V
V
DC
The Numeric Controlled Oscillator NCO works in a phase
The digital outputs of the NCO are shaped in the Wave
The demodulator accepts a FSK signal at the RxA input
This HART bit stream follows a standard 11-bit UART
Figure 7. Modulator shaped output signal for Mark
TxA
TxA
IDLE (mark)
.
0
0
Start
t
BIT
“0”
and Space frequency at TxA pin.
m
= 1860 V/s at the mark frequency and SR
Figure 8. Modulation Timing
LSB
D0
“1”
SR
“1” = Mark; f
“0” = Space; f
D1
“0”
s
= 3300 V/s
D2
“1”
1
8 data bits
1
SR
D3
“0”
m
m
s
=1.2 kHz
= 1860 V/s
=2.2 kHz
D4
“0”
D5
“1”
D6
“0”
MSB
D7
“1”
2
2
Par
“0”
0.5 V
Stop
t
BIT
0.5 V
IDLE (mark)
t (ms)
t (ms)
s
=