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MCP2510-IST Datasheet(PDF) 8 Page - Microchip Technology |
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MCP2510-IST Datasheet(HTML) 8 Page - Microchip Technology |
8 / 76 page ![]() MCP2510 DS21291C-page 8 Preliminary 2000 Microchip Technology Inc. 2.4 Error Frame An Error Frame is generated by any node that detects a bus error. An error frame, shown in Figure 2-4, con- sists of two fields, an error flag field followed by an error delimiter field. There are two types of error flag fields. Which type of error flag field is sent depends upon the error status of the node that detects and generates the error flag field. If an error-active node detects a bus error then the node interrupts transmission of the current message by generating an active error flag. The active error flag is composed of six consecutive dominant bits. This bit sequence actively violates the bit stuffing rule. All other stations recognize the resulting bit stuffing error and in turn generate error frames themselves, called error echo flags. The error flag field, therefore, consists of between six and twelve consecutive dominant bits (generated by one or more nodes). The error delimiter field completes the error frame. After completion of the error frame, bus activity returns to normal and the inter- rupted node attempts to resend the aborted message. If an error-passive node detects a bus error then the node transmits an error-passive flag followed by the error delimiter field. The error-passive flag consists of six consecutive recessive bits, and the error frame for an error-passive node consists of 14 recessive bits. From this, it follows that unless the bus error is detected by the node that is actually transmitting, the transmission of an error frame by an error-passive node will not affect any other node on the network. If the transmitting node generates an error-passive flag then this will cause other nodes to generate error frames due to the resulting bit stuffing violation. After transmission of an error frame, an error-passive node must wait for six consecutive recessive bits on the bus before attempting to rejoin bus communications. The error delimiter consists of eight recessive bits and allows the bus nodes to restart bus communications cleanly after an error has occurred. 2.5 Overload Frame An Overload Frame, shown in Figure 2-5, has the same format as an active error frame. An overload frame, however can only be generated during an lnter- frame space. In this way an overload frame can be dif- ferentiated from an error frame (an error frame is sent during the transmission of a message). The overload frame consists of two fields, an overload flag followed by an overload delimiter. The overload flag consists of six dominant bits followed by overload flags generated by other nodes (and, as for an active error flag, giving a maximum of twelve dominant bits). The overload delimiter consists of eight recessive bits. An overload frame can be generated by a node as a result of two conditions. First, the node detects a dominant bit during the interframe space which is an illegal condition. Sec- ond, due to internal conditions the node is not yet able to start reception of the next message. A node may generate a maximum of two sequential overload frames to delay the start of the next message. 2.6 Interframe Space The lnterframe Space separates a preceeding frame (of any type) from a subsequent data or remote frame. The interframe space is composed of at least three recessive bits called the Intermission. This is provided to allow nodes time for internal processing before the start of the next message frame. After the intermission, the bus line remains in the recessive state (bus idle) until the next transmission starts. |
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