I

=

SCdisharge

I

SCcharge

40 (typ)

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

MARK

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

TSS521

www.ti.com

SLAS980 – SEPTEMBER 2013

Functional Description

Data Transmission, Master to Slave

The mark level on the bus lines VBUS = MARK is defined by the difference of BUSL1 and BUSL2 at the slave. It

depends on the distance from 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 (CSC) at the SC pin is charged by a current (ISCcharge) and is discharged with a current (ISCdischarge)

where:

(1)

This ratio is necessary to run any kind of UART protocol independent of the data contents (for example, 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 CSC. The input level detector TC3 detects voltage modulations from the master

(VBUS = SPACE or MARK conditions) and switches the inverted output TXI and the noninverted output TX.

Copyright © 2013, Texas Instruments Incorporated

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