akator wrote:Mike, just verifying — you're saying it's a bad idea to keep multiple carts in the HEScard because of possible damage? Is this theoretical or verified?
You first need to check whether the HEScard (de-)activates the cartridges by switching the +5V power rail.
That given, there's not just (as you say) a theoretical but verified possibility of damage to the cartridges, mainly because of
latch-up: when voltages are applied to (input) pins of logic chips that either exceed Vss+0.7 V or are below Vcc-0.7 V, that can trigger a parasitic thyristor present in CMOS circuits. Once triggered, that thyristor draws excess current and acts as a short as long as even a small difference voltage remains present. Normally only the complete removal of power on all pins stops this. Needless to say, a short like this - internal within a chip! - can easily destroy that chip within a few seconds.
With the supply removed, Vss of the chips effectively is 0 V, so the inputs only allow for applied voltages in the range -0.7 V .. +0.7 V. Now, the address lines, the data lines and other control signals (select, output enable, read/write) are still connected. The VIC-20 puts signal voltages outside that range on the inputs during operation (an output on high-level is anything above +2.4 V up to +5 V for TTL or NMOS). The only thing that limits the current is the TTL and NMOS circuitry normally only used inside the VIC-20. High-level outputs with TTL or NMOS only source currents in the 2-digit µA range. The latch-up nonetheless happens and pulls down all signals where it happens. If one of the outputs that 'overdrives' an unsuspecting input is itself a CMOS output, that can source a lot more current and then all bets are off.
The rule that
first the supply voltages and
then the signals are applied is one of the first rules learned in practical courses in digital electronics, and is motivated here.
In short: the continued use of a multiport expander that merely switches the supply voltages is no good idea.
Use at your own risk. The correct method is keeping the supply connected but disconnecting all relevant control signals (BLKx, RAMx, I/Ox) and pulling them high (i.e.
inactive) on the cartridge side where not connected.