Printed in the Federal Republic of Germany
TR-Electronic GmbH 2018, All Rights Reserved
03/19/2021
TR-ECE-BA-DGB-0141 v03
Page 43 of 67
3 SSI information
The SSI procedure is a synchronous serial transmission procedure for the measuring
system position. By using the RS422 interface for transmission, sufficiently high
transmission rates can be achieved.
The measuring system receives a clock sequence from the control and answers with
the current position value, which is transmitted serially and is synchronous to sent
clock.
Since the data transfer is synchronized by the start of the sequence, it is not
necessary to use single-step codes such as Gray code.
The clock signals clock+ and clock- can be received via opto coupler (see Figure 1) or
RS422 (see Figure 2) depending on the device. As standard, the clock signals are
received via an input circuit with opto coupler to protect against damage due to
interference, potential differences or polarity reversal. The data signals Data+ and
Data- are generally transmitted via RS422 (see Figure 3).
Parity bits or checksums can be added to detect faulty transmissions. The simplest
measure is to read in the values twice with the data bits being repeated after 26 clock
pulses of a sequence. However, this has the disadvantage of considerably increasing
transmission times.
Figure 1: Principle SSI input circuit with opto coupler
Figure 2: Principle SSI input circuit RS422
Figure 3: Principle SSI output circuit RS422
