What this CP 340 does on the S7-300 backplane
The Siemens 6ES7340-1CH00-0AE0 is a CP 340 communication processor for the S7-300 rack, providing a single isolated RS422/485 interface for point-to-point serial links to drives, printers, barcode readers, or other controllers running ASCII or 3964(R) protocols. It draws 165 mA from the 5 V DC backplane bus and dissipates 0.85 W — within the typical S7-300 slot budget, provided the rack power supply is sized for the full module complement.
Serial link constraints — cable length, baud rate, and protocol
The RS422/485 interface runs at 19.2 kbit/s maximum with 3964(R) protocol, dropping to 9.6 kbit/s with ASCII or the printer driver — the limiting factor is the UART and firmware overhead, not the transceiver. Maximum cable length is 1 200 m at the lower baud rates, which covers most plant-floor runs between control cabinets and remote I/O or field devices, provided the cable is terminated per RS485 practice. The ASCII buffer holds 1 024 byte, and the 3964(R) buffer is also 1 024 byte — adequate for short telegrams and parameter blocks, but a multi-drop poll cycle with long data frames may require the CP 341 if the buffer ceiling is hit.
Printer support — a niche but documented capability
The CP 340 can drive HP-DeskJet, HP-LaserJet, and IBM-Proprinter printers via the serial port at 9.6 kbit/s, and also accepts user-defined printer drivers — useful for legacy line-reporting stations where a PC-based print server is not available.