SMSC COM20020 manual

buffer address 2 contains a zero or non-zero value. The format of the buffer is shown in Figure 8. Address 0 contains the Source Identifier (SID); Address 1 contains the Destination Identifier (DID); Address 2 (COUNT) contains, for short packets, the value 256-N, where N represents the number of information bytes in the message, or for long packets, the value 0, indicating that it is indeed a long packet. In the latter case, Address 3 (COUNT) would contain the value 512-N, where N represents the number of information bytes in the message. The SID in Address 0 is used by the receiving node to reply to the transmitting node. The COM20020 puts the local ID in this location, therefore it is not necessary to write into this location.

Please note that a short packet may contain between 1 and 253 data bytes, while a long packet may contain between 257 and 508 data bytes. A minimum value of 257 exists on a long packet so that the COUNT is expressable in eight bits. This leaves three exception packet lengths which do not fit into either a short or long packet; packet lengths of 254, 255, or 256 bytes. If packets of these lengths must be sent, the user must add dummy bytes to the packet in order to make the packet fit into a long packet.

Once the packet is written into the buffer, the microcontroller awaits a logic "1" on the TA bit, indicating that a previous transmit command has concluded and another may be issued.

Each time the message is loaded and a transmit command issued, it will take a variable amount of time before the message is transmitted, depending on the traffic on the network and the location of the token at the time the transmit command was issued. The conclusion of the Transmit Command will generate an interrupt if the Interrupt Mask allows it. If the device is configured for the Command Chaining operation, please see the Command Chaining section for further detail on the transmit sequence. Once the TA bit becomes a logic "1",

the microcontroller may issue the "Enable Transmit from Page fnn" command, which resets the TA and TMA bits to logic "0". If the message is not a BROADCAST, the COM20020 automatically sends a FREE BUFFER ENQUIRY to the destination node in order to send the message. At this point, one of four possibilities may occur.

The first possibility is if a free buffer is available at the destination node, in which case it responds with an ACKnowledgement. At this point, the COM20020 fetches the data from the Transmit Buffer and performs the transmit sequence. If a successful transmit sequence is completed, the TMA bit and the TA bit are set to logic "1". If the packet was not transmitted successfully, TMA will not be set. A successful transmission occurs when the receiving node responds to the packet with an ACK. An unsuccessful transmission occurs when the receiving node does not respond to the packet.

The second possibility is if the destination node responds to the Free Buffer Enquiry with a Negative AcKnowledgement. A NAK occurs when the RI bit of the destination node is a logic "1". In this case, the token is passed on from the transmitting node to the next node. The next time the transmitter receives the token, it will again transmit a FREE BUFFER ENQUIRY. If a NAK is again received, the token is again passed onto the next node. The Excessive NAK bit of the Diagnostic Status Register is used to prevent an endless sending of FBE's andNAK's. If no limit of FBE-NAK sequences existed, the transmitting node would continue issuing a Free Buffer Enquiry, even though it would continuously receive a NAK as a response. The EXCNAK bit generates an interrupt (if enabled) in order to tell the microcontroller to disable the transmitter via the "Disable Transmitter" command. This causes the transmission to be abandoned and the TA bit to be set to a logic "1" when the node next receives the token, while the TMA bit remains at a logic "0". Please refer to the Improved Diagnostics section for further detail on the EXCNAK bit.

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