What the EM253 does – and what trips people up
The 6ES7253-1AA22-0XA0 is the SIMATIC S7-200 EM253 positioning module – a 200 kHz pulse train generator for controlling stepper or servo drives in open-loop positioning tasks. It accepts five P-reading digital inputs (STP, RPS, LMT+, LMT-, ZP) and provides four drive interfaces configurable as RS 422/485 or 5 V DC differential outputs. Cable lengths matter per input type – shielded runs up to 100 m on STP, RPS, LMT+, LMT-; ZP is limited to 10 m shielded. Unshielded, the four limit/ref inputs reach 30 m, but ZP is not recommended unshielded. This is a common source of intermittent faults during commissioning. Power draw: 190 mA from the 5 V backplane bus, plus up to 300 mA from the 24 V L+ (130 mA if using 12 V DC). Plan the S7-200 power budget accordingly – the CPU supplies the backplane bus, and the external 24 V feeds the inputs and encoder supply.
CPU compatibility – which S7-200 it fits
The EM253 module count per CPU is limited by the backplane bus power and addressing. Maximum modules: 5 with a CPU 226 or 226XM, 3 with a CPU 224, and only 1 with a CPU 222. These are absolute limits – exceeding them may cause erratic bus behavior or power failure. This is the only dedicated stepper/servo pulse output module for the S7-200 family. It is specific to S7-200 rack builds.