Synchronous-Serial Interface for absolute Encoders TWK-ELEKTRONIK GmbH · D-40041 Düsseldorf · PB 105063 · Heinrichstr. 85 · Tel +49/211/632067 · Fax +49/211/637705 · e-mail: info@tw

Advantage of the SSI over parallel interfaces Significantly less expenditure for cabling: In the case of 24 bit encoders, only 4 lines are needed for the transmission of data instead of 24. Expenditure for cabling and interface components does not depend on the length of the data word. Screening out of noise is achieved through the clock and data signals being transmitted synchronously and symmetrically via twisted pair lines. Multiple transmission of each data word provides an automatic plausibility check. Absolute Encoder and reception electronics are separated with opto-couplers rendering...

Example: Absolute Encoder with 4096 positions / 360°, 4096 revolutions and Gray tree as output code Idle state

Single transmission In the case of a single transmission, i.e. when the current position data is read out once, the clock sequence can be terminated after the transmission of the LSB since only zeroes will follow this. Wait time Clock sequence Idle state Idle state (Parallel / serial converter) P/S Data + Data word Least significant bit (S1) Bit (S 1) Multiple transmission In the case of multiple transmission, i.e. when the current position data is read out a number of times, the clock sequence is designed in accordance with the schematic below. Clock sequence 1st transmission Intermediate clock...

TTL / HCMOS Logik / Logic 91R 100R Takt IN-/ Clock IN6N137 (Optokoppler/Optocoupler ) LED z.B. LR3360 (rot) Siemens oder ähnliche LED e.g. LR3360 (red) Siemens or similar Output circuit 47nF 100pF Data OUT+/ Data OUT+ Data OUT-/ Data OUT- TTL / HCMOS Logik / Logic The output circuit is in the form of a differential line driver which fulfils complaying with RS 422 / RS 485. The differential, symmetrical design ensures a high degree of reliability in respect of noise. The use of opto-coupler inputs means that earth loops are not required and this in turn reduces the sensitivity to noise still further....

Maximum data transmission rate [MHz] maximum Baud rate in acc. with RS 422 The maximum achievable data transmission rate (clock frequency) is set for the drivers and reception electronics used as well as for the transmission protocol in accordance with the RS 422 standard. It must be one half of the values stated in the standard for the Baud rate. Both limit curves are shown in the adjoining graph. It is shown below how the maximum data transmission rate can be achieved by means of measures on the reception electronics side by taking into account the different transit times. maximum clock frequency...

Accordingly the above values can be inserted into the formula tTD (ns) = 300 ns + 2 x 6.5 ns/m x lCA (m) Example: For a cable of length 200 m the total delay is tTD = 300 ns + 2 x 6.5 ns/m x 200 m = 2900 ns = 2.9 µs In the schematic below the significance of the total delay time t for transmission is represented for the above example. The clock TD frequency selected is 300 kHz, namely the maximum permitted for a cable length of 200 m. This clock frequency is equivalent to a clock cycle time (tT) of 3.3 µs. 11.4 Significance of the time by which the evaluation (reading-in) of the data as transmitted...