Hi, Craig
It is indeed a serial-in parallel-out buffer (there are a few more details in Pulse engine docs available under Download section at PoLabs homepage).
Pulses are generated differently when used with 'Internal pulse generator' vs. 'External pulse generator'. Anyone that follow the development of features of PoKeys devices can see that we try to implement as many features in the devices themselves and it was the same case here. Unfortunately, due to pin compatiblity requirement between USB and Ethernet versions, more axes with the internal pulse generator were simply not possible and external board was offered for our general purpose I/O devices (PoKeys57U and PoKeys57E), while the PoKeys57CNC has it already built-in.
ps: there are also not enough pins on the PoKeys57E to connect all 8 axes (with step, dir, enable signals and inputs for limit and home switches...) and have anything else connected to the device - PoKeys57CNC has all those, signals for LCD, pendant, encoders, analog signals, relay and OC outputs... and both USB and Ethernet
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