SSI encoder interface for ET 200S
The Siemens 6ES7138-4DB02-0AB0 is a single-channel SSI absolute encoder module for the ET 200S distributed I/O system. It reads 25-bit synchronous serial interface data from one encoder and also provides one digital input with comparison value capability. The module fits a 15 mm wide slot in the ET 200S terminal block assembly and draws 0.8 W typical from the backplane bus.
Cable length limits by clock rate
Maximum shielded cable length depends on the SSI clock frequency: 320 m at 125 kHz, 160 m at 250 kHz, 60 m at 500 kHz, 20 m at 1 MHz, and 8 m at 2 MHz, with the cable twisted in pairs and shielded. For a typical 1 MHz clock the 20 m limit is generous for most machine-mounted encoder runs, but at 2 MHz the 8 m ceiling means the encoder must be close to the cabinet.
Digital input and sensor supply
The module accepts 2-wire sensors (Type 2) with a signal '1' threshold of +11 to +30 V and a signal '0' range of -30 to +5 V. The permissible quiescent current for signal '0' is up to 2 mA. Maximum load from L+ without load is 34 mA.