The ISA 220, 428, 430 and 430 MkII can be fitted with optional digital converter cards. If your unit has one of these cards fitted and you are connecting digitally to other equipment (via AES, SPDIF or ADAT) then the ISA unit must be in sync with all other digital audio devices in the signal chain. Wordclock is the name of the signal which keep devices in sync - this can be embedded within the digital audio stream (e.g. within a SPDIF signal) or can be carried between devices via a 75 Ohm BNC connection. The digital cards for all of the units listed above feature two BNC connectors - one is a wordclock input and the other a wordclock output.
There are two ways to configure the ISA in a digital audio system:
1. Wordclock master - in this configuration the ISA generates the clock signal so you do not need to connect a wordclock cable to it. If the receiving device can sync to a wordclock signal embedded in a digital audio stream (virtually all audio interfaces do) then you do not need to connect the wordclock out of the ISA to anywhere. Instead connect the digital audio output of the ISA to the digital input on the receiving device and set it to sync to its digital input. If the digital audio signal chain in your set up is simple e.g. ISA -> sound card -> computer, then it is best to use the ISA as wordclock master.
2. Slave to wordclock - in this configuration you need to connect a BNC wordclock cable to the wordclock input of the ISA so that it can sync to an external master clock. You will need to select 'Ext' as the sync setting on the front of the ISA, or 'Ext S/C' if the clock signal is 256x 'Superclock' (NOTE: the ISAs will not generate Superclock, only receive it). The signal seen at the ISA's 'wordclock in' connector will be
regenerated at its 'wordclock out' connector.
NOTE: it is important to set the sample frequency and bit depth settings on the front of the ISA to the same settings for the rest of your system, even if the ISA is slaving to a wordclock signal.