The following TAXIchip command characters are used:
With the exception of Soc/Sno, commands may be sent at any time. Receiving hardware should be able to continue assembling a cell even if the data component is interrupted by a command. This is useful for several reasons. Firstly it enables data to be injected onto the link at a slower speed than the TAXIchip is being clocked - the chip will automatically insert Sync commands between data items in this case. Secondly it enables the handshake commands (which apply to the other side of the link) to be sent in a timely manner even if the sender is also transmitting data.
The handshake commands Xoff and Xon are generated by a receiving node to request the transmitter to stop or continue sending. Since the link distance is short this simple protocol can be used, for links with several cells delay in the transmission media other techniques would be required. The Xoff command requests that the transmitter stops sending at the end of the current cell or before. Thus a transmitter may choose to respond to an Xoff by stopping immediately, but the cell seems the natural unit to use for this activity. A transmitter which has been stopped may continue to send all commands except Soc/Sno. When a transmitter is stopped it should periodically send Held commands to prevent loss of an Xon from stalling the system. A receiver should send the Xon command when it has sent an Xoff and is now prepared to receive another cell, or if it sees a Held command and is prepared to receive a cell. The Xon/Xoff handshaking is only advisory - thus Xoff is a suggestion that the transmitter stop to avoid data loss, but it may be ignored by simple implementations.
The Taxi use of 4B/5B and NRZI encoding ensures that the transmitted data will always have the same average amount of high and low times, so the DC balance of the line is ensured. The Taxi command set has this property when averaged over the transmission of a single character. However some of the command characters have few transitions, so there use is not recommended - these characters have been avoided when selecting the codes used above.