And I must regrettably disagree with my esteemed colleague.:-)
1) From the TI datasheet, +/-50mA may be listed but it is the absolute maximum Icc current. Absolute maximum sink or source current per output pin is listed at +/-25mA.
More important is the sink/source current while maintaining TTL voltage spec. This is a far more modest +/-6mA at 0.33V and 3.84V at a 4.5V Vdd. Using Ohm's Law, this gives a 55 Ohm sink resistance and a 110 Ohm source resistance. Scaling these impedances to 16mA required by the drives gives 0.88V for a logic '0' and 2.74V for a logic '1'. This is a less than sterling performance.
Now let's look at a TI 74ACT541 datasheet and compare. A 24mA sink current gives 0.44V on the output pin while a 24mA source current gives 3.8V with a 4.5V Vdd. Scaling gives 0.3V at 16mA sinking and 4V when sourcing 16mA. This is acceptable performance.
The 74ACT part will reliably drive a G201 or G320, the 74HCT part will not. FYI: I never use 'typical' 25C specs when I design; I always use the guaranteed minimum specs at temperature (-40C to +85C in this case). 'Typical' means "most parts will be this this good'" so I worry about the remainder that won't be.:-)
2) Symmetrical sink/source current drive capability is optimum for driving cables. Cables are primarily capacitive loads at step pulse frequencies and it's current that charges and discharges this capacitance. It does no good to have a 74LS part that can sink 23mA when it can't source more than 400uA. The 'weak sister' is the source current which will set a pathetically low maximum frequency because of its miserable ability to 'charge' the cable capacitance.
Use 74ACT parts. They are as cheap as 74HCT or 74LS parts so why not?
For your edification and amusement, please see the attached .gif picture. It shows an HEDS-9100 encoder with 74ACT14 driving a 100' shielded cable containing four 22-gage wires. The signal shown is at the G320 end which is running at 100K step pulses per second. The signal is clean as a whistle when it comes out of the cable.
Mariss |