What this module does in an S7-300 rack
The Siemens 6ES7338-4BC01-0AB0 is a signal module for the SIMATIC S7-300 family that reads position values from up to three SSI (synchronous-serial interface) absolute encoders. It includes two FREEZE inputs that latch the encoder values simultaneously on a trigger, which is useful for synchronized position capture on a moving axis. The module supports isochronous mode, meaning it can exchange process data with the CPU at a fixed bus cycle, keeping the position read deterministic for motion-control applications.
Supply and signal-level details
Rated supply is 24 V DC (L+ minus 0.8 V), with a permissible range of 20.4 V to 28.8 V DC. The module draws up to 160 mA from the 5 V backplane bus and up to 100 mA from the load voltage L+ (unloaded). Power dissipation is typically 3 W. Signal thresholds are defined: a logic "0" spans -3 to +5 V, and a logic "1" spans 11 to 30.2 V. The input current for a "1" signal is typically 9 mA. The minimum transition time from "0" to "1" is 300 µs.
Cable length limits by baud 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, and 20 m at 1 MHz. For standard encoder inputs (non-SSI), the maximum shielded cable length is 600 m. The module also supports 2-wire sensors (BERO) and provides a diagnostic alarm. The 20-pin front connector is required and sold separately.