What it is
The Siemens 6ES7720-3AC31-0YC0 is a PG 720 P programming device — a portable industrial computer built for field and shop-floor PLC programming, diagnostics, and commissioning. It carries a Pentium P133 processor, a 1.44 GB hard drive, and a 5-slot expansion chassis for I/O or communication cards.
What the ratings mean for fit
The 1.44 GB hard drive is the factory-fit storage for the PG 720 P — enough for the Windows-based STEP 5 or STEP 7 installation and project files typical of that era. The 5-slot expansion gives room for a CP 5412 (PROFIBUS) card, a memory card, or a serial interface module. The P133 processor runs the HMI and programming software at the speed it was designed for; no faster CPU was offered in this chassis. The PG 720 P is a field-service tool, not a panel-mount PC — it sits on a workbench or straps into a service vehicle.