The transmission card Xilinx chip

The transmission card xilinx chip performs three main tasks. It controls the transfer of data between the TAXIchips on the transmission line and the FIFOs to the port controller. Also it can insert the CCITT header error check CRC to cells being transmitted and check and remove the CRC from received cells. The final task is to control hardware handshaking over the transmission line using a simple Xon/Xoff protocol.

 
Table 1:  Pins on trans chip

Table 1 gives details of the pins on the xilinx chip. Most are used to control the fifos or TAXIchips. The other pins are discussed below.

To ensure correct behaviour in the presence of disconnected or noisy transmission lines the xilinx chip will detect the ``violation'' signal from the receiving TAXIchip and stop accepting data. The line has to be clear from violations for 64 character times before data will again be accepted. While seeing violations, or in the recovery period an indication is put out on the ``xilinxM2'' line.

Special pins

This section discusses the special pins on the trans chip. Note that commands and cell formats are as in the proposed Fairisle/Yes/Maybe protocol. Both the Soc and Sno cell formats are supported, and the transmission format may be changed from cell to cell.

• txNoCrc. This pin is used to enable or disable the insertion of the CRC into the transmitted data stream. If this pin is low then a cell will be started with a Soc command and the CCITT CRC of the first four (ie header) bytes will be inserted as the fifth in the cell. If the pin is high then the Sno command is used to start the cell and no CRC is inserted. The state of this pin should only be changed between cells, if it is altered during transmission of a cell then the cell being transmitted may be corrupted.

• txNoHand. This pin is used to enable or disable the Xon/Xoff handshaking. If this pin is set then the chip will never generate or respond to Xoff. It will always send Xon as a response to a Held command from the other end.

• rxDataNext. This pin is used to indicate valid data on the rxfD[0-7] lines. It is intended for use when the trans chip is not being used to drive a fifo directly. The output changes on the falling edge on rxClk to indicate the validity of data after the next rising edge. If rxDataNext is high after falling rxClk then there will be valid data placed on the lines after the next rising edge, otherwise the data is undefined.

• rxCrcFailbar. This pin will go low if the received cell contains a CRC and the CRC does not match that in the cell. The first few bytes of the cell will have been presented on the output lines, but no further bytes will be output until the start of cell for the next cell. It is expected that the receiving system will be able to deal with such short cells.

• rxNoCrcbar. This pin will go low if a cell is received containing no CRC (ie one which starts with an Sno command) and go high if a cell containing a CRC (ie starting with Soc) is received.

• xilinxM2. This pin has two uses. Until the xilinx chip has been configured (or during reconfiguration) it is the M2 input line, which indicates the programming method. For serial master mode programming the line should be pulled low through a resistor. Once configured the line becomes an output and indicates a violation on the reception line, or the recovery period following a violation. The output is driven low to indicate the violation state. On the Fairisle card this signal is required for the driver of the ``violation'' LED.

Handshake

The hardware handshake is done on a per cell basis. If the receive fifo is over half full when a start of cell command is received, and handshaking is enabled, then an Xoff command will be sent. An Xon command will be sent when the fifo falls below half full following transmission of an Xoff, or if a Held command is received and the fifo is less than half full.

If enabled in response to an Xoff the transmission system will stop just before sending a start of cell command. It will then send the Held command every 8th character time, with Sync commands in between, until Xon is received.

The handshaking is of no use over long links (where many cells are stored in the transmission medium) and is of doubtful value between switches. However it may be of use on direct links between hosts, and on host-switch links.

Notes

The transmission TAXIchip command line c[3] is not driven by the xilinx, and should be tied low. This limits the xilinx to generating commands 0-7, but most of the others should be avoided anyway.