Synchronous Serial Interface

The Synchronous Serial Interface, Eng. Synchronous Serial Interface (SSI ) is an interface for absolute absolute encoder ( measuring systems ). It allows to obtain, by a serial data transmission is an absolute information concerning the position. The Synchronous Serial Interface was originally developed by Max Stegmann GmbH (since 2002 part of the Sick group), but is used by a number of other manufacturers.

It is particularly suitable for applications where reliability and robustness in industrial environments are required. The Synchronous Serial Interface is very simple, there are only two pairs of wires ( for the clock and data ) needed and used more as a shift register and a one-shot in the sensor is little of the same for the control needed. This enables a cost-effective structure. SSI also allows you to connect up to three sensors to a common clock. This allows the reading of multiple sensors at a defined time. Compared to other interfaces for position measurement systems offers the Synchronous Serial interface, the following additional advantages:

• Reduced cabling
• The cost of cabling and interface technology is independent of the length of the data word
• By synchronous and symmetrical clock and data signals over twisted pair cable, the shielding is achieved against interference.
• To avoid ground loops a complete galvanic isolation is used by optocouplers.

Thus, the transmission line lengths can be customized, the switching frequency is adjustable by software. In addition, the number of data bits is programmable via software, so that a degree of flexibility in terms of resolution is possible. The data value can be done either in Gray code or binary code. To back up data, a parity bit can also be transferred. Some sensors use a data bit as alarm bit, for example, in optical Wegmessern indicate the failure of the light emitting diode or an interruption of the supply voltage.

Transmission

In the sensor, a shift register is permanently loaded with the current measured value. If a data value is to be read, the controller outputs a clock bundle on the clock line. The first falling edge triggers a one-shot in the sensor, which switches the shift register from the parallel loading into serial output. With each subsequent rising clock edge now, a data bit is output. If the least significant bit is received, the clock is stopped. The monostable multivibrator, which was always retriggered by the clock pulses falls below the switching time back to the ground state and again allowed the acquisition of measured values ​​in the shift register. The data line is held up to that point to a low level.