For my OnStep telescope mount controller (visit my group for more information.) This design is built around the Teensy3.5/3.6 There are four stepper drivers (one for RA/Azm, one for Dec/Alt, one for a focuser, and one for a rotator/second focuser.) There's a power jack, fuse, and power switch connection. There are several connectivity options: USB, two I2C ports (w/common footprints for the DS3231 RTC and BME280.) There are two serial ports. Serial4 is brought out on a Molex connector (w/power.) Nearby there is a limit sense input (to stop the mount.) Serial1 is on a WeMos D1 Mini (ESP8266) foot-print for Wifi support. There is a D-Sub connector DB9 that has home SW inputs, PEC input, and two channels of A/B encoder inputs (which run to the ESP8266.) There's also a modular connector for an ST4 guider interface (with optional 5V power.) And finally, there's a mini speaker output for an alert buzzer.