Key Digital ES381 - Manual (Page 9)

Repeat this for all except the last counter to be stacked.
For the last counter in the stack, do the same as above but use the
short length connector. This time, the pins are shorter so they stop
well inside the counter’s case. This counter MUST be the bottom
counter in the stack. The reason for this is that if you use the long
connector, the pins could be shorted out accidentally, or you may
touch the pins when the counter is connected to a high voltage and
you could get an electric shock! Similarly, if you plan to use only
one counter, make sure there is no stack connector fitted or the
shorter connector type is fitted.
This is a safety critical point so please note it carefully.
Now you can assemble the stack.
Elan Digital Systems Ltd. 9 USBcount50 USER’S GUIDE Iss1