Basically I need some demultiplexers to switch between 24 outputs. The data inputs would be connected to counters for the addresses. I'm currently using 74ls138 3-8 line demultiplexers. Is this possible?
You can use 3 74HC138 8 channel decoders or 2 74HC154 16 channel decoders. Both IC's will drive 15mA loads at speeds of 20 MHZ. Both IC's are available in many package types. If your already working with counters then you know how to wire up the address selects for these IC's.
If the active low outputs need to be off while changing addresses, use the clock for the counters and connect it to the /EN pin of the decoders. If the counters count on the rising edge of the clock, the decoders will not have the addressed pin go low until the clock goes low. If this is not an issue you can wire the /EN pin of the decoders to ground.
NOTE: The 74LSxx series is from 30 years ago and has limited drive current (1mA). The output voltage cannot be higher than about 3.6 volts. They need 5.0 volts for power while the 74HCxx series will work down to 3.0 volts.