SIMATIC COMMUNICATION PROCESSOR 6ES7341-1CH02-0AE0 6ES7 341-1CH02-0AE0 6ES7341-1CHO2-OAEO

Siemens 6ES7341-1CH02-0AE0 | SIMATIC S7-300 CP 341 — RS422/485 Serial Communications Processor, 0.3–115.2 kbit/s, ASCII / 3964(R) / RK512 / Modbus RTU, 15-Pin Sub-D, Includes Config CD

Overview

The Siemens 6ES7341-1CH02-0AE0 is the CP 341 with RS422/485 interface — the serial communications module that adds a dedicated industrial serial port to the SIMATIC S7-300 PLC.

The CP 341 was the standard solution for S7-300 serial communication requirements that the CPU's own MPI/PROFIBUS interface could not address: connecting to a weighing terminal that speaks Modbus RTU, exchanging data with a barcode scanner over RS485, controlling a serial printer for production logging, or communicating with a legacy drive amplifier that predates fieldbus protocols.

The practical necessity the CP 341 addresses is that most industrial environments contain a mix of communication technologies. Modern PROFIBUS DP and PROFINET devices connect cleanly to the S7-300's native interfaces.

But not everything in a factory is modern — and even in new installations, many field devices (barcode readers, scales, label printers, weighing systems, temperature controllers, energy meters) communicate via RS485 Modbus RTU or simple ASCII serial, not PROFIBUS or Ethernet.

For every one of these devices that needs to exchange data with the S7-300, the CP 341 provides the electrical interface and protocol handling — offloading serial communication processing from the CPU so that serial data exchange does not occupy CPU scan time.

The RS422/RS485 (X.27) physical interface supports both full-duplex (RS422, four-wire: separate transmit and receive pairs) and half-duplex (RS485, two-wire: shared transmit/receive pair) operation.

RS422 full-duplex is used for point-to-point connections where the maximum cable length and noise immunity benefits of differential four-wire transmission are needed, and where the 3964(R) and RK512 protocols are applicable.

RS485 half-duplex supports multi-drop bus configurations — connecting the S7-300 CP 341 as one node among multiple devices sharing the same two-wire bus — which is the topology required for Modbus RTU networks and multi-device ASCII configurations.

Key Specifications

Protocol Suite — Four Communication Modes

ASCII is the simplest protocol — the CP 341 transmits and receives ASCII character strings with configurable start/stop characters, data length, and parity.

ASCII mode is appropriate for devices that communicate with human-readable text: barcode readers that output scanned code strings, weight terminals that transmit ASCII-formatted readings, serial printers, and any device that uses a simple text-based protocol. 

ASCII works with both RS422 and RS485 connections and across the full 0.3–115.2 kbit/s speed range.

3964(R) (only on RS422) is a point-to-point protocol designed for PLC-to-PLC and PLC-to-peripheral communication.

It adds block transfer with header, end-of-text acknowledgement, and error detection to a serial link, providing reliable, structured data exchange between two endpoints. 3964(R) is the protocol used by the historical SIMATIC S5 point-to-point connection and remains relevant for communication with legacy Siemens equipment.

RK 512 (only on RS422) is a request-response protocol that supports reading and writing data areas in a remote SIMATIC S5 or compatible device.

It operates as a layer above 3964(R) and provides memory-mapped access — the S7-300 CPU program reads or writes to specific data addresses in the remote device using RK 512 service requests.

This protocol bridges the CP 341's serial link into the older S5 data exchange model.

Modbus RTU is not a built-in driver in the CP 341's firmware — it requires loading a separate Modbus master (6ES7870-1AA01-0YA0) or Modbus slave (6ES7870-1AB01-0YA0) driver onto the CP 341.

Once loaded, the Modbus driver handles the Modbus RTU framing, timing, and function code implementation, while the S7-300 CPU program exchanges data through the CP 341's shared memory interface using STEP 7 function blocks.

Modbus RTU on RS485 is the dominant protocol for energy meters, VFDs from many manufacturers, temperature controllers, and industrial instrumentation — the Modbus driver transforms the CP 341 into a capable Modbus master or slave without requiring any additional hardware.

Integration with the S7-300 Program — P_SEND and P_RCV

From the S7-300 CPU program's perspective, the CP 341 is accessed through two standard function blocks from the SIMATIC point-to-point communication library: P_SEND (send a data buffer over the serial port) and P_RCV (receive data from the serial port into a data buffer).

These blocks are called from the CPU's ladder or structured text program, specifying the data source/destination, data length, and communication parameters.

The CPU program writes data into a send buffer in the S7-300's data blocks, calls P_SEND, and the CP 341 picks up the data and transmits it over the serial interface — the CPU does not manage the serial timing, handshaking, or protocol framing.

Similarly, when data arrives on the serial interface, the CP 341 buffers it and signals the CPU via the P_RCV block's status outputs; the CPU reads the received data from its data block buffer. This architecture offloads all serial processing from the CPU scan cycle, ensuring that slow serial communication (at 9600 baud, 100 characters takes approximately 100ms) does not add a visible delay to the CPU's program execution.

FAQ

Q1: Can one S7-300 CPU use multiple CP 341 modules simultaneously?

Yes. Multiple CP 341 modules can be installed in the same S7-300 rack (subject to the rack's slot count and power supply capacity), each providing an independent serial port. Each CP 341 occupies one rack slot and draws power from the module's 24V supply and the backplane.

The CPU accesses each CP 341 independently using separate P_SEND / P_RCV function block instances, with the addressing distinguishing which CP the data exchange targets.

This allows an S7-300 to simultaneously maintain multiple independent serial links — one for a barcode scanner, one for a Modbus RTU energy meter, one for a serial printer — without additional hardware.

Q2: What is the difference between the RS422 and RS485 modes, and how is the mode selected?

RS422 is a four-wire full-duplex differential interface — separate transmit and receive wire pairs, allowing simultaneous bidirectional communication. It supports point-to-point connections at up to 1200m and enables the 3964(R) and RK512 protocols.

RS485 is a two-wire half-duplex differential interface — a shared bus pair that alternates between transmission and reception.

It supports multi-drop configurations (multiple devices on one bus segment) and Modbus RTU, but cannot use 3964(R) or RK512. 

The mode is selected in the CP 341's configuration in STEP 7 — the physical 15-pin Sub-D connector wiring must match the selected mode (four wires for RS422, two wires for RS485).

Q3: The datasheet mentions transmission rates up to 115.2 kbit/s. Does the Modbus RTU driver support this speed?

Modbus RTU can operate at any standard baud rate supported by the CP 341, including 115.2 kbit/s. However, Modbus RTU's practical bus speed is typically limited by the capabilities of the devices on the bus — many Modbus field devices (energy meters, older VFDs, legacy instruments) support only up to 19.2 or 38.4 kbit/s.

The CP 341 configuration must be set to a baud rate supported by all devices on the RS485 bus. 

For RS485 multi-drop networks, the lowest common baud rate supported by all connected devices should be selected.

Q4: Does the CP 341 include Modbus drivers in the standard package, or must they be purchased separately?

The standard delivery of 6ES7341-1CH02-0AE0 includes the configuration package CD and the built-in ASCII, 3964(R), and RK512 drivers.

Modbus RTU is not included — it requires separately purchased loadable driver software: catalog number 6ES7870-1AA01-0YA0 for Modbus Master (RTU) and 6ES7870-1AB01-0YA0 for Modbus Slave (RTU). 

These Modbus drivers are loaded onto the CP 341 from STEP 7 as part of the hardware configuration process.

Both Modbus and the standard drivers can coexist on the same CP 341 — but only one protocol is active at a time for any given communication session.

Q5: The CP 341 is discontinued. What is the Siemens current alternative for RS485 serial communication on an S7-300?

Siemens has discontinued the CP 341 series. For the current SIMATIC S7-300 platform, serial communication is now typically handled by the CM PtP RS422/485 (SIMATIC ET 200M communication module) when operating in distributed configuration.

For new machine designs, Siemens recommends migrating to the S7-1500 family with the CM PtP communication modules (e.g., 6ES7541-1AB00-0AB0) which provide RS422/RS485 serial communication with CM PtP function blocks in TIA Portal.

For maintenance of existing S7-300 CP 341 installations, the module remains available through the industrial surplus market, and Siemens service continues to support the CP 341 within the S7-300 product lifecycle.