1. Content Overview
This article provides a detailed overview of the communication capabilities of the S7-200 series, focusing on its built-in and expansion communication modules. The CPU200 comes with 1 to 2 integrated RS485 communication ports, which support PPI, MPI, and free port communication. For free port communication, Siemens offers Modbus RTU master and slave libraries, as well as the USS communication library for user convenience.
The first expansion module is the CP243-2, an ASI master communication module that uses two slot resources and supports protocol version 2.1. The second is the EM277, a standard PROFIBUS DP slave module. Behind the EM277 is the EM241, an analog telephone modem module. The last two modules are Ethernet communication expansion modules: CP243-1 and CP243-1IT, which support the S7 protocol. A newer version of the CP243-1 now replaces these, offering IT features like email, HTML, and FTP.
2. PPI Communication
(1) PPI Communication
Initially designed as a point-to-point protocol, the PPI communication protocol operates in a master-slave configuration. The master sends a request, and the slave responds. Over time, it evolved to support multi-master networks. It follows the OSI seven-layer model and uses Token Ring technology for network implementation. PPI uses 1 start bit, 1 stop bit, 8 data bits, and even parity. It utilizes the integrated RS485 port of the CPU200 for programming communication, between PLCs, and with HMI devices.
The CPU200's integrated port has one PG connection resource and three OP connection resources. It supports baud rates of 9.6k, 19.2k, and 187.5k, with up to 32 nodes per segment. The maximum distance is 50 meters, but repeaters can extend it further.
(2) PPI Communication Between CPUs 200
To set up PPI communication between two S7-200 PLCs, one acts as the master and the other as the slave. On the master side, the user configures the Network Read/Write Wizard and uses the generated subroutine. The slave only needs to match the communication rate and set a different address in the system block.
In the S7-200 programming software, navigate to the "Tools" menu, select the command wizard, then choose the Network Read/Write Wizard. Follow the steps to configure how many operations you want to set up—up to 24. Choose the communication port (0 or 1), and define each read/write operation, including the remote and local addresses. Each network transfer can handle 16 bytes of data.
After configuration, the wizard will prompt you to allocate V memory space, typically 12 bytes per operation. Ensure no overlap with existing program memory. The generated subroutine, named "NET_EXE," must be continuously called using SM0.0. The timeout parameter allows setting a delay, and if exceeded, an error is reported.
The wizard supports up to 8 active network commands at once, ensuring smooth operation without conflicts.
3. MPI Communication
(1) MPI Communication
The S7-200's integrated RS485 interface can act as an MPI slave. It enables communication between S7-200 PLCs, with S7-300/400, and HMI devices. The supported baud rates are 19.2kps and 187.5kps. However, unlike PPI, MPI cannot function as a master station.
When communicating between S7-200 and S7-300, the S7-200 doesn't require any specific code. Data should be organized into a continuous V memory area. In the S7-300, the system functions X_GET (SFC67) and X_PUT (SFC68) are used. These are triggered by a signal from the "REQ" input pin. The "DEST_ID" refers to the MPI address of the S7-200, usually set to 2 in examples. The S7-200's V memory is treated as DB1 by the S7-300, so the remote data address should start from VB0.
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