Closure Limits

The high switch density attained in certain System 40/45/50 cards, particularly high-density matrix types, necessitates close packing of relays and airflow is quite restricted. If excessive numbers of relays were energised for a prolonged period overheating could occur. For example, in model 40-531 simultaneous energisation of all 256 relays would yield a power dissipation of around 17W. In BRIC units the situation is even more extreme - see BRIC Operation. To guard against this danger the software driver places a limit on the number of crosspoints that can be energised simultaneously. The limits imposed by the driver are set with regard to operating temperature levels and will not cause any difficulty for typical matrix usage, where only a small proportion of crosspoints are simultaneously ON. A sub-unit's closure limit can be discovered using the pipx40_getClosureLimit function.

The software driver provides a method of disabling this protection. Calling the function pipx40_setDriverMode with the bit pipx40_MODE_UNLIMITED set allows an unlimited number of crosspoints to be energised simultaneously. This feature should be used with EXTREME CAUTION. Although it may be safe to energise larger numbers of crosspoints where ON times are short and duty cycle is low, it must be borne in mind that if the user's program were to halt in the ON state (for example at a breakpoint when debugging) the danger of overheating is present.

Some models incorporate fuses to protect against simultaneous activation of a hugely excessive number of channels. These are self-resetting in moderate overload, and operation will be restored when the fault condition clears.