Datasheet

BR
VB
BUSL1
GND
BUSL2
I
CS3
R
RIS
RIS
CS3
TC4
RX
RXI
I
MS
V = Constant
BUS
I
MC
I
SPACE
I
MARK
V
I
BUS
BUS
V
RXI
V
RX
I
BUS
to IC
I = I + I
MC MS CS3
20
I
(mA)
MC
0.1
15
5
10
R (k )
RIS
W
0
Typical
V = Voltage on pin RIS
R = Programming resistor
I = Programmable current
I = Modulation current
I = Modulation supply current (220 µA typ)
RIS
RIS
CS3
MC
MS
R =
RIS
=
V
RIS
I
CS3
V
RIS
I – I
MC MS
0.2 0.5
1
TSS721A
SLAS222A –APRIL 1999–REVISED SEPTEMBER 2009................................................................................................................................................ www.ti.com
Data Transmission, Slave to Master
The device uses current modulation to transmit information from Slave to the Master while the bus voltage
remains constant. The current source CS3 modulates the bus current and the master detects the modulation.
The constant current source CS3 is controlled by the inverted input RXI or the non-inverted input RX. The current
source CS3 can be programmed by an external resistor R
RIS
. The modulation supply current I
MS
flows in addition
to the current source CS3 during the modulation time.
Figure 2. Data Transmission, Slave to Master
Because the TSS721A is configured for half-duplex only, the current modulation from RX or RXI is repeated
concurrently as ECHO on the outputs TX and TXI. If the Slave, as well as the Master, is trying to send
information via the lines, the added signals appear on the outputs TX and TXI which indicate the data collision to
the slave (see figure 6).
The bus topology requires a constant current consumption by each connected slave.
To calculate the value of the programming resistor R
RIS
, use the formula shown in Figure 3.
Figure 3. Calculate Programming Resistor R
RIS
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