The SeaDaq is a network synchronised instrument. All of the analog and digital inputs are acquired at exactly the same instance in time subject to slight delays due to the propogation delay caused by the network cable and processing hardware. The ATM/SONET network transmits two sources of clock and it is these clocks that are used to synchronise the network. The two clocks are called Byte Frame Clock & Sonet Frame Clock. The Byte Frame clock runs at 19.44MHz and the SONET Frame clock at 8KHz. The SeaDaq's synchronisation system utilises both of these signals to synchronise the ADC acquisition rate. Because of the high precision in the timing clocks the analoge and digital inputs between different nodes are synchronised.

Figure 1 shows a typical network layout. Note. No external timming signals are reqiured by the SeaDAQ instruments other than those generated by the ATM interfaces. The timing information from the two clocks is interleaved into the data stream from where it is recovered by the Sonet node interfaces built into the instruments. The internal Sonet Node software can also correct the data stream for the propogation delay caused by the network cables.

The above diagram shows the timing acquisition for 3 nodes on a network. The inter-node delay is represented by 'I'. This is comprised of I1, the delay due to processing inside the node, and I2 the signal delay in propagation between nodes. The delay between channels is represented by N. Typically the inter-channel spread inside a node is around 10ns, as is the processing delay I1. The delay in the cable is dependant on the length of the cable, but typically the propagation is 0.15m per ns as the signal travels at approximately half the speed of light. When calculating the timing delays, one should also consider the delay between the sensor and the node, since this signal also travels at around half the speed of light.

Figure 3 shows the timing delays for a 3 instrument network. The distance between the first two nodes is 5m and the distance between nodes two and three set to the maximum distance of 100m. The 650 ns delay between nodes 2 and 3 and the 40 ns delay betweens nodes 1 and 2 is calibrated out in the driver software.