Data link layer device for a serial communication bus

摘要

According to the IEEE1394 bus protocol, priority is given to isochronous data packets. Data transfer is done in transfer cycles under the control of a cycle master. It depends on the allocated bandwidth for isochronous data how much transport capacity is available in a transfer cycle. To managed the mixed data transfer in one cycle it is specified that the bus nodes not having isochronous data to transfer need to wait with their transmission requests until the end of the isochronous data transfers in the cycle indicated with a sub-action gap. The invention aims to improve the efficiency of data transport for the case that none of the bus nodes need to transfer isochronous data. The data link layer devices according to the invention includes means for checking whether isochronous data is to be transferred and if not they switch over to a no cycle master state, in which the local cycle synchronization events are ignored. The nodes need not wait for a sub-action gap after a local cycle event before drawing asynchronous transmission requests.