SMSC COM20020 manual Receive Sequence

The third possibility which may occur after a FREE BUFFER ENQUIRY is issued is if the destination node does not respond at all. In this case, the TA bit is set to a logic "1", while the TMA bit remains at a logic "0". The user should determine whether the node should try to reissue the transmit command.

The fourth possibility is if a non-traditional response is received (some pattern other than ACK or NAK, such as noise). In this case, the token is not passed onto the next node, which causes the Lost Token Timer of the next node to time out, thus generating a network reconfiguration.

The "Disable Transmitter" command may be used to cancel any pending transmit command when the COM20020 next receives the token. Normally, in an active network, this command will set the TA status bit to a logic "1" when the token is received. If the "Disable Transmitter" command does not cause the TA bit to be set in the time it takes the token to make a round trip through the network, one of three situations exists. Either the node is disconnected from the network, or there are no other nodes on the network, or the external receive circuitry has failed. These situations can be determined by either using the improved diagnostic features of the COM20020 or using another software timeout which is greater than the worst case time for a round trip token pass, which occurs when all nodes transmit a maximum length message.

Receive Sequence

A receive sequence begins with the RI status bit becoming a logic "1", which indicates that a

previous reception has concluded. The

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microcontroller will be interrupted if the corresponding bit in the Interrupt Mask Register

is set to logic "1". Otherwise, the microcontroller must periodically check the Status Register. Once the microcontroller is alerted to the fact that the previous reception has concluded, it may issue the "Enable Receive to Page fnn" command, which resets the RI bit to logic "0" and selects a new page in the RAM buffer. Again, the appropriate buffer size is specified in the "Define Configuration" command. Typically, the page which just received the data packet will be read by the microcontroller at this point. Once the "Enable Receive to Page fnn" command is issued, the microcontroller attends to other duties. There is no way of knowing how long the new reception will take, since another node may transmit a packet at any time. When another node does transmit a packet to this node, and if the "Define Configuration" command has enabled the reception of long packets, the COM20020 interprets the packet as either a long or short packet, depending on whether the content of the buffer location 2 is zero or non-zero. The format of the buffer is shown in Figure 8. Address 0 contains the Source Identifier (SID), Address 1 contains the Destination Identifier (DID), and Address 2 contains, for short packets, the value 256-N, where N represents the message length, or for long packets, the value 0, indicating that it is indeed a long packet. In the latter case, Address 3 contains the value 512-N, where N represents the message length. Note that on reception, the COM20020 deposits packets into the RAM buffer in the same format that the transmitting node arranges them, which allows for a message to be received and then retransmitted without rearranging any bytes in the RAM buffer other than the SID and DID.

Once the packet is received and stored correctly in the selected buffer, the COM20020 sets the RI bit to logic "1" to signal the microcontroller that the reception is complete.