There are several instances where it may be necessary to operate multiple 1401s in the same experimental setup. For example, more waveform input channels may be needed when running the Spike2 software, than are available with a single, fully expanded 1401. The synchronising option lets the user link several 1401 systems to overcome this problem.
Linking separate systems raises the issue of timing. When used independently, each 1401 can be started with the same trigger pulse, but then runs at a clock rate derived from its own internal crystal clock.
The accuracy of the internal clock is 50 parts per million or better over time and operating temperature. This means that two 1401s started with the same start pulse could be up to 1 second different after 6 hours. For many applications, this may not be significant, but for experimental procedures where multiple 1401s are in use, it may make it difficult to line up events that had been recorded by different 1401s.
To address this problem, the Power1401 and the Micro1401-4, 3 and 2 have factory-fitted options to synchronise multiple 1401s so that their crystal clocks keep exactly in step.
When fitted, the synchronising option adds a rear-panel connector into which a daisy-chain cable plugs. The wiring in the cable determines which 1401 has the "master" clock and which are "slave" units. A slave 1401 detects an incoming clock signal and measures its frequency. If it is satisfied that the external signal is an acceptable clock, it will switch its reference clock to use the external frequency instead of an internal crystal. This process is automatic and requires no intervention from the software. The clock signal is regenerated in each 1401 to pass on to the next unit. Cable lengths can be up to 2 metres between 1401s.
The number of 1401s that may be synchronised in this way is 3 or 4, determined by the cabling. To cater for synchronising more 1401s, CED plans to introduce an external synchronising box that will act as a "master" and produce clocks for up to 8 "slaves". This unit will also have facilities for locking to user-supplied external frequency sources such as DAT machines and video genlocks.
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