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Tham khảo tài liệu 'advances in robot manipulators part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 432 Advances in Robot Manipulators the long ones from 256 to 507 bytes. It is not possible to send packets of 253 254 or 255 bytes in those cases null data are added and they are sent either as two short packets or one long. As each node can only send data when it has the token no collisions occur when using the ARCNET protocol. Moreover thanks to the token-passing scheme the time a node lasts in sending a message can be computed this is an important feature in industrial networks where it is required that the control events occur in a precise moment. 5.1.1 Specifications for the PA10 system Communication between the motion control board and the PA10 s motor drives is carried out via an optical fiber using the ARCNET protocol. In its original configuration a transmission rate of 10 Mbps is employed the node number of the ARCNET controller IC in the driver cabinet is 254 hexadecimal FE while the one in the motion control board has number 255 hexadecimal FF . As explained in Mitsubishi 2002b the data packets are short and they follow the format which is shown in Figure 4. 0 1 2 3 X 255 Data type Sequence No. Transmission data even number ----Sequence No. address X ----Receiver node number ----Transmitter node number ARCNET Short-packet format Fig. 4. ARCNET data transmission format The first two bytes in each packet indicate the node numbers of the transmitter and receiver respectively next byte specifies the address X in which the data block start in a way that the last byte always be the number 256. The first byte of data identified as Data type in Figure 4 can be either S E or C . Commands S and E indicate respectively the start and end of a control sequence when using these commands no more data is expected. Command C is used to transmit control data which must be done periodically. When executing the S command a timer starts its operation which checks that the time between each C command does not exceeds a preestablished time limit in case of occurring this an .
