FC200x - PCI Cards for Lightbus

Lightbus System Description

 
 Icon Note
Note
System description for FC200x / EL6720
The following system description is applicable both for the Beckhoff Lightbus PCI interface card FC200x and for the Lightbus master terminal EL6720.

The Lightbus consists of the Lightbus PCI interface card (in short Lightbus card) and an optical fiber-based fieldbus.

 
 lightbusring200dpi
Lightbus LWL-Ring

The coupling of the Lightbus with the PC is realized on the Lightbus card via a Dual Ported RAM (DPRAM).

Various I/O modules (Bus Coupler, Bus Terminals, Fieldbus Boxes etc.) are available for input and output of the process images, which are connected through the Lightbus in a ring structure. The use of optical fibers results in low susceptibility to interference and a high transfer rate of 2.5 MBaud. Any errors occurring in the optical fiber ring are detected by the Lightbus card and reported to the PC. TwinCAT provides comprehensive diagnostic functions that support fast detection and correction of any errors.

A communication protocol that is optimized for speed and simplicity is specified for the data transfer between the Lightbus card and the I/O modules. The Lightbus card controls the communication on the optical fiber ring. It sends telegrams that pass through the I/O modules in the optical fiber ring and are then received and checked back at the Lightbus card.

A telegram consists of a telegram frame and the telegram content.

 
 lightbus_telegramframe
Lightbus telegram structure

The telegram frame is required for serial data communication and consists of 1 start bit, 6 CRC check bits and 2 stop bits. It is generated and checked independently by the hardware of the Lightbus card. No software support is required.

The telegram content is essentially organized byte-wise. AD0 to AD7 form the address field, through which up to 254 I/O modules can be addressed (the addresses 0x00 and 0x0FF are reserved). The bits CR0 to CR3 specify the telegram type. The following functions can be specified in the telegram:

Functions

 
CR3
CR2
CR1
CR0
Function
Description
0
0
0
0
Read
The addressed module inserts the input information into data fields D0 to D3.
0
0
0
1
Read and write
The addressed module inserts the input information into data fields D0 to D3 and receives the output information.
0
0
1
0
Address initialization
The addressed module receives the content of D0 as the module address and sets D0 to 0.
0
0
1
1
RAM
Special telegram type for the BK2000 Bus Coupler.
0
1
0
0
Address check and count
Each passing module increases the content of D0 by 1. The addressed module moves the content of D0 to D3.
1
0
0
1
Low intensity
The addressed module reduces the transmission intensity by 20%.
1
0
1
1
Broadcast
Special telegram type for the BK2000 Bus Coupler.

The bytes D0 to D3 contain the actual user data. The control field specifies the processing of these user data. The last byte in the telegram contains 6 bits for forming a CRC checksum and 2 reserved bits. A user data length of 50 bits thus achieves a hamming distance of d=3.

The Lightbus consists of a physical ring, which can be split into up to 8 logical rings for processing the process image. A logical ring only operates for selected I/O modules, which are specified via communication description lists (CDLs).

The Lightbus card makes the process image available to the PC via the DPRAM. The DPRAM is divided into three areas:

▪
Data:
Input, output and flags
▪
Communication:
Initialization, testing, analysis and configuration of the Beckhoff Lightbus
▪
Process control:
Updating the Process Image

The DPRAM of the Lightbus cards requires 4 kB per channel in the address space of the PC.