Datasheet

BR
VB
BUSL1
GND
BUSL2
TC3
TX
TXI
SC
I
SCcharge
I
SCdischarge
V
T
I = Constant
BUS
V = 20.8 V to 42 V
MAR K
V = V – 10 V
SPACE MARK
V
TXI
V
TX
V
I
BUS
BUS
C
SC
to IC
I
BUS
C = 30 pF typ
BUSL1-BUSL2
V = 25 V, f = 1 MHz
B meas
I =
SCdisharge
I
SCcharge
40 (typ)
TSS721A
www.ti.com ................................................................................................................................................ SLAS222A –APRIL 1999–REVISED SEPTEMBER 2009
Data Transmission, Master to Slave
The mark level on the bus lines V
BUS
= MARK is defined by the difference of BUSL1 and BUSL2 at the slave. It
is dependent on the distance of Master to Slave, which affects the voltage drop on the wire. To make the
receiver independent, a dynamic reference level on the SC pin is used for the voltage comparator TC3 (see
Figure 1).
Figure 1. Data Transmission, Master to Slave
A capacitor C
SC
at pin SC is charged by a current I
SCcharge
and is discharged with a current I
SCdischarge
where:
(1)
This ratio is necessary to run any kind of UART protocol independent of the data contents. (e.g., if an 11-bit
UART protocol is transmitted with all data bits at 0 and only the stop bit at 1). There must be sufficient time to
recharge the capacitor C
SC
. The input level detector TC3 detects voltage modulations from the master,
V
BUS
= SPACE/MARK conditions, and switches the inverted output TXI and the non-inverted output TX.
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