USB ⇆ Nexstar AUX Relay for Celestron Mounts

Control Celestron Mounts via AUX Protocol (without Handcontrol)

Based on the excellent NexStar AUX Command Set documentation and previous projects

1. NexStar Direct Telescope Mount Control
2. An Arduino-based controller for Celestron CG5-series equatorial mounts
3. Homebrew Mount-USB Connection for AUX-Port

an USB ⇆ AUX relay is developed. The circuit (50k resistor and 1N4148 diode) is explained in 1) and also depicted in 2) and emulates a Tri-State logic.

The used microcontroller in this project is a Seeed Studio XIAO SAMD21 ARM Cortex-M0+ 32bit 48MHz microcontroller(SAMD21G18) with 256KB Flash, 32KB SRAM and Arduino library compatible.

The final USB ⇆ Nexstar AUX relay dongle is depicted below:

Compiling the Firmware

Install PlatformIO and run

$ pio run -t upload
...
...
Auto-detected: /dev/ttyACM0
Forcing reset using 1200bps open/close on port /dev/ttyACM0
Waiting for the new upload port...
Uploading .pio/build/seeed_xiao/firmware.bin
Atmel SMART device 0x10010005 found
Erase flash
done in 0.922 seconds

Write 27816 bytes to flash (435 pages)

[====                          ] 14% (64/435 pages)
[========                      ] 29% (128/435 pages)
[=============                 ] 44% (192/435 pages)
[=================             ] 58% (256/435 pages)
[======================        ] 73% (320/435 pages)
[==========================    ] 88% (384/435 pages)
[==============================] 100% (435/435 pages)
done in 0.173 seconds

Verify 27816 bytes of flash with checksum.
Verify successful
done in 0.036 seconds
CPU reset.

Tracing AUX Packets

For tracing transmitted and received AUX packets attach the TX/RX pins of USB-TTL PL2303TA 3.3V cable to pins 0 and 1 of the MCU. The MCU uses soft serial communication for tracing as the USART communication is already occupied for the AUX communication. In addition make sure the PlatformIO config file platformio.ini contains build_flags = '-DAUX_DEBUG', that is,

[platformio]
default_envs = seeed_xiao

[env:seeed_xiao]
platform = atmelsam
board = seeed_xiao
framework = arduino
monitor_speed = 19200
monitor_port = /dev/ttyUSB0
build_flags = '-DAUX_DEBUG'

Celestron uses the AUX protocol for communicating with all devices connected to the mount. That is, one can see e.g. the AUX packets transmitted and received to/from the handcontrol:

$ pio device monitor
[00544457ms,aux] Nexstar+ HC -> ALT MC, cmd MTR_GET_MODEL: AUX packet <0x3b, 0x03, 0x0d, 0x11, 0x05, 0xda> (checksum 0xda)
[00544603ms,aux] ALT MC -> Nexstar+ HC, cmd MTR_GET_MODEL: AUX packet <0x3b, 0x05, 0x11, 0x0d, 0x05, (0x14 0x85), 0x3f> (checksum 0x3f)
[00544736ms,aux] Nexstar+ HC -> AZM MC, cmd MTR_GET_MODEL: AUX packet <0x3b, 0x03, 0x0d, 0x10, 0x05, 0xdb> (checksum 0xdb)
[00544882ms,aux] AZM MC -> Nexstar+ HC, cmd MTR_GET_MODEL: AUX packet <0x3b, 0x05, 0x10, 0x0d, 0x05, (0x14 0x85), 0x40> (checksum 0x40)
[00545015ms,aux] Nexstar+ HC -> AZM MC, cmd MTR_GET_VERSION: AUX packet <0x3b, 0x03, 0x0d, 0x10, 0xfe, 0xe2> (checksum 0xe2)
[00545182ms,aux] AZM MC -> Nexstar+ HC, cmd NXS_GET_FW_VER: AUX packet <0x3b, 0x07, 0x10, 0x0d, 0xfe, (0x07 0x0f 0x20 0x4e), 0x5a> (checksum 0x5a)
[00545321ms,aux] Nexstar+ HC -> AZM MC, cmd MTR_IS_APPROACH_DIR_NEG: AUX packet <0x3b, 0x03, 0x0d, 0x10, 0xfc, 0xe4> (checksum 0xe4)
[00545463ms,aux] AZM MC -> Nexstar+ HC, cmd MTR_IS_APPROACH_DIR_NEG: AUX packet <0x3b, 0x04, 0x10, 0x0d, 0xfc, (0x00), 0xe3> (checksum 0xe3)
[00545598ms,aux] Nexstar+ HC -> ALT MC, cmd MTR_IS_APPROACH_DIR_NEG: AUX packet <0x3b, 0x03, 0x0d, 0x11, 0xfc, 0xe3> (checksum 0xe3)
[00545740ms,aux] ALT MC -> Nexstar+ HC, cmd MTR_IS_APPROACH_DIR_NEG: AUX packet <0x3b, 0x04, 0x11, 0x0d, 0xfc, (0x00), 0xe2> (checksum 0xe2)
[00545875ms,aux] Nexstar+ HC -> Focuser, cmd FUCUS_GET_FW_VER: AUX packet <0x3b, 0x03, 0x0d, 0x12, 0xfe, 0xe0> (checksum 0xe0)
[00546003ms,aux] Nexstar+ HC -> Focuser, cmd FUCUS_GET_FW_VER: AUX packet <0x3b, 0x03, 0x0d, 0x12, 0xfe, 0xe0> (checksum 0xe0)
[00546132ms,aux] Nexstar+ HC -> Focuser, cmd FUCUS_GET_FW_VER: AUX packet <0x3b, 0x03, 0x0d, 0x12, 0xfe, 0xe0> (checksum 0xe0)
[00546260ms,aux] Nexstar+ HC -> Focuser, cmd FUCUS_GET_FW_VER: AUX packet <0x3b, 0x03, 0x0d, 0x12, 0xfe, 0xe0> (checksum 0xe0)
[00546388ms,aux] Nexstar+ HC -> Focuser, cmd FUCUS_GET_FW_VER: AUX packet <0x3b, 0x03, 0x0d, 0x12, 0xfe, 0xe0> (checksum 0xe0)
[00546516ms,aux] Nexstar+ HC -> Dew Heater Controller, cmd DEWHEATER_GET_FW_VER: AUX packet <0x3b, 0x03, 0x0d, 0xbb, 0xfe, 0x37> (checksum 0x37)
[00546790ms,aux] Nexstar+ HC -> Dew Heater Controller, cmd DEWHEATER_GET_FW_VER: AUX packet <0x3b, 0x03, 0x0d, 0xbb, 0xfe, 0x37> (checksum 0x37)
[00547143ms,aux] Nexstar+ HC -> Dew Heater Controller, cmd DEWHEATER_GET_FW_VER: AUX packet <0x3b, 0x03, 0x0d, 0xbb, 0xfe, 0x37> (checksum 0x37)
[00547496ms,aux] Nexstar+ HC -> Dew Heater Controller, cmd DEWHEATER_GET_FW_VER: AUX packet <0x3b, 0x03, 0x0d, 0xbb, 0xfe, 0x37> (checksum 0x37)
[00547849ms,aux] Nexstar+ HC -> Dew Heater Controller, cmd DEWHEATER_GET_FW_VER: AUX packet <0x3b, 0x03, 0x0d, 0xbb, 0xfe, 0x37> (checksum 0x37)
[00548232ms,aux] Nexstar+ HC -> AZM MC, cmd MTR_SET_POSITION: AUX packet <0x3b, 0x06, 0x0d, 0x10, 0x04, (0x40 0x00 0x00), 0x99> (checksum 0x99)
[00548370ms,aux] AZM MC -> Nexstar+ HC, cmd MTR_SET_POSITION: AUX packet <0x3b, 0x03, 0x10, 0x0d, 0x04, 0xdc> (checksum 0xdc)
[00548527ms,aux] Nexstar+ HC -> ALT MC, cmd MTR_SET_POSITION: AUX packet <0x3b, 0x06, 0x0d, 0x11, 0x04, (0x40 0x00 0x00), 0x98> (checksum 0x98)
[00548665ms,aux] ALT MC -> Nexstar+ HC, cmd MTR_SET_POSITION: AUX packet <0x3b, 0x03, 0x11, 0x0d, 0x04, 0xdb> (checksum 0xdb)
[00548822ms,aux] Nexstar+ HC -> AZM MC, cmd MTR_SET_CORDWRAP_POSITION: AUX packet <0x3b, 0x06, 0x0d, 0x10, 0x3a, (0xc0 0x00 0x00), 0xe3> (checksum 0xe3)
[00548964ms,aux] AZM MC -> Nexstar+ HC, cmd MTR_SET_CORDWRAP_POSITION: AUX packet <0x3b, 0x03, 0x10, 0x0d, 0x3a, 0xa6> (checksum 0xa6)
[00549097ms,aux] Nexstar+ HC -> AZM MC, cmd MTR_CORDWRAP_ON: AUX packet <0x3b, 0x03, 0x0d, 0x10, 0x38, 0xa8> (checksum 0xa8)
[00549224ms,aux] AZM MC -> Nexstar+ HC, cmd MTR_CORDWRAP_ON: AUX packet <0x3b, 0x03, 0x10, 0x0d, 0x38, 0xa8> (checksum 0xa8)
[00549381ms,aux] Nexstar+ HC -> ALT MC, cmd MTR_SET_CORDWRAP_POSITION: AUX packet <0x3b, 0x06, 0x0d, 0x11, 0x3a, (0xc0 0x00 0x00), 0xe2> (checksum 0xe2)
[00549523ms,aux] ALT MC -> Nexstar+ HC, cmd MTR_SET_CORDWRAP_POSITION: AUX packet <0x3b, 0x03, 0x11, 0x0d, 0x3a, 0xa5> (checksum 0xa5)
[00549656ms,aux] Nexstar+ HC -> ALT MC, cmd MTR_CORDWRAP_ON: AUX packet <0x3b, 0x03, 0x0d, 0x11, 0x38, 0xa7> (checksum 0xa7)
[00549783ms,aux] ALT MC -> Nexstar+ HC, cmd MTR_CORDWRAP_ON: AUX packet <0x3b, 0x03, 0x11, 0x0d, 0x38, 0xa7> (checksum 0xa7)
[00616314ms,aux] Nexstar+ HC -> AZM MC, cmd MTR_SET_POSITION: AUX packet <0x3b, 0x06, 0x0d, 0x10, 0x04, (0x40 0x00 0x00), 0x99> (checksum 0x99)
[00616451ms,aux] AZM MC -> Nexstar+ HC, cmd MTR_SET_POSITION: AUX packet <0x3b, 0x03, 0x10, 0x0d, 0x04, 0xdc> (checksum 0xdc)
[00616609ms,aux] Nexstar+ HC -> ALT MC, cmd MTR_SET_POSITION: AUX packet <0x3b, 0x06, 0x0d, 0x11, 0x04, (0x40 0x00 0x00), 0x98> (checksum 0x98)
[00616746ms,aux] ALT MC -> Nexstar+ HC, cmd MTR_SET_POSITION: AUX packet <0x3b, 0x03, 0x11, 0x0d, 0x04, 0xdb> (checksum 0xdb)
[00616904ms,aux] Nexstar+ HC -> AZM MC, cmd MTR_SET_CORDWRAP_POSITION: AUX packet <0x3b, 0x06, 0x0d, 0x10, 0x3a, (0xc0 0x00 0x00), 0xe3> (checksum 0xe3)
[00617046ms,aux] AZM MC -> Nexstar+ HC, cmd MTR_SET_CORDWRAP_POSITION: AUX packet <0x3b, 0x03, 0x10, 0x0d, 0x3a, 0xa6> (checksum 0xa6)
[00617178ms,aux] Nexstar+ HC -> AZM MC, cmd MTR_CORDWRAP_ON: AUX packet <0x3b, 0x03, 0x0d, 0x10, 0x38, 0xa8> (checksum 0xa8)
[00617306ms,aux] AZM MC -> Nexstar+ HC, cmd MTR_CORDWRAP_ON: AUX packet <0x3b, 0x03, 0x10, 0x0d, 0x38, 0xa8> (checksum 0xa8)
[00617463ms,aux] Nexstar+ HC -> ALT MC, cmd MTR_SET_CORDWRAP_POSITION: AUX packet <0x3b, 0x06, 0x0d, 0x11, 0x3a, (0xc0 0x00 0x00), 0xe2> (checksum 0xe2)
[00617605ms,aux] ALT MC -> Nexstar+ HC, cmd MTR_SET_CORDWRAP_POSITION: AUX packet <0x3b, 0x03, 0x11, 0x0d, 0x3a, 0xa5> (checksum 0xa5)
[00617737ms,aux] Nexstar+ HC -> ALT MC, cmd MTR_CORDWRAP_ON: AUX packet <0x3b, 0x03, 0x0d, 0x11, 0x38, 0xa7> (checksum 0xa7)
[00617864ms,aux] ALT MC -> Nexstar+ HC, cmd MTR_CORDWRAP_ON: AUX packet <0x3b, 0x03, 0x11, 0x0d, 0x38, 0xa7> (checksum 0xa7)
[00618021ms,aux] Nexstar+ HC -> AZM MC, cmd MTR_PTRACK: AUX packet <0x3b, 0x06, 0x0d, 0x10, 0x06, (0x00 0x00 0x00), 0xd7> (checksum 0xd7)
[00618156ms,aux] AZM MC -> Nexstar+ HC, cmd MTR_PTRACK: AUX packet <0x3b, 0x03, 0x10, 0x0d, 0x06, 0xda> (checksum 0xda)
[00618310ms,aux] Nexstar+ HC -> ALT MC, cmd MTR_PTRACK: AUX packet <0x3b, 0x06, 0x0d, 0x11, 0x06, (0x00 0x00 0x00), 0xd6> (checksum 0xd6)
[00618444ms,aux] ALT MC -> Nexstar+ HC, cmd MTR_PTRACK: AUX packet <0x3b, 0x03, 0x11, 0x0d, 0x06, 0xd9> (checksum 0xd9)
[00618589ms,aux] Nexstar+ HC -> AZM MC, cmd MTR_PTRACK: AUX packet <0x3b, 0x05, 0x0d, 0x10, 0x06, (0xff 0xff), 0xda> (checksum 0xda)
[00618720ms,aux] AZM MC -> Nexstar+ HC, cmd MTR_PTRACK: AUX packet <0x3b, 0x03, 0x10, 0x0d, 0x06, 0xda> (checksum 0xda)

The communication between INDI Celestron AUX driver and the USB ⇆ AUX relay looks as follows:

[00123927ms,usb] INDI Celestron AUX -> AZM MC, cmd MTR_GET_VERSION: AUX packet <0x3b, 0x03, 0x20, 0x10, 0xfe, 0xcf> (checksum 0xcf)
[00124099ms,aux] AZM MC -> INDI Celestron AUX, cmd NXS_GET_FW_VER: AUX packet <0x3b, 0x07, 0x10, 0x20, 0xfe, (0x07 0x0f 0x20 0x4e), 0x47> (checksum 0x47)
[00124203ms,usb] INDI Celestron AUX -> ALT MC, cmd MTR_GET_VERSION: AUX packet <0x3b, 0x03, 0x20, 0x11, 0xfe, 0xce> (checksum 0xce)
[00124374ms,aux] ALT MC -> INDI Celestron AUX, cmd NXS_GET_FW_VER: AUX packet <0x3b, 0x07, 0x11, 0x20, 0xfe, (0x07 0x0f 0x20 0x4e), 0x46> (checksum 0x46)
[00124478ms,usb] INDI Celestron AUX -> AZM MC, cmd MTR_GET_AUTOGUIDE_RATE: AUX packet <0x3b, 0x03, 0x20, 0x10, 0x47, 0x86> (checksum 0x86)
[00124623ms,aux] AZM MC -> INDI Celestron AUX, cmd MTR_GET_AUTOGUIDE_RATE: AUX packet <0x3b, 0x04, 0x10, 0x20, 0x47, (0x33), 0x52> (checksum 0x52)
[00124723ms,usb] INDI Celestron AUX -> ALT MC, cmd MTR_GET_AUTOGUIDE_RATE: AUX packet <0x3b, 0x03, 0x20, 0x11, 0x47, 0x85> (checksum 0x85)
[00124868ms,aux] ALT MC -> INDI Celestron AUX, cmd MTR_GET_AUTOGUIDE_RATE: AUX packet <0x3b, 0x04, 0x11, 0x20, 0x47, (0xcc), 0xb8> (checksum 0xb8)
[00124969ms,usb] INDI Celestron AUX -> AZM MC, cmd MTR_IS_CORDWRAP_ON: AUX packet <0x3b, 0x03, 0x20, 0x10, 0x3b, 0x92> (checksum 0x92)
[00125112ms,aux] AZM MC -> INDI Celestron AUX, cmd MTR_IS_CORDWRAP_ON: AUX packet <0x3b, 0x04, 0x10, 0x20, 0x3b, (0x00), 0x91> (checksum 0x91)
[00125210ms,usb] INDI Celestron AUX -> AZM MC, cmd MTR_GET_CORDWRAP_POSITION: AUX packet <0x3b, 0x03, 0x20, 0x10, 0x3c, 0x91> (checksum 0x91)
[00125376ms,aux] AZM MC -> INDI Celestron AUX, cmd MTR_GET_CORDWRAP_POSITION: AUX packet <0x3b, 0x06, 0x10, 0x20, 0x3c, (0x00 0x00 0x00), 0x8e> (checksum 0x8e)
[00125484ms,usb] INDI Celestron AUX -> AZM MC, cmd MTR_GET_MODEL: AUX packet <0x3b, 0x03, 0x20, 0x10, 0x05, 0xc8> (checksum 0xc8)
[00125634ms,aux] AZM MC -> INDI Celestron AUX, cmd MTR_GET_MODEL: AUX packet <0x3b, 0x05, 0x10, 0x20, 0x05, (0x14 0x85), 0x2d> (checksum 0x2d)
[00125732ms,usb] INDI Celestron AUX -> Main Board, cmd MB_GET_FW_VER: AUX packet <0x3b, 0x03, 0x20, 0x01, 0xfe, 0xde> (checksum 0xde)
[00126828ms,usb] INDI Celestron AUX -> Nexstar HC, cmd NXS_GET_FW_VER: AUX packet <0x3b, 0x03, 0x20, 0x04, 0xfe, 0xdb> (checksum 0xdb)
[00127924ms,usb] INDI Celestron AUX -> Nexstar+ HC, cmd NXS_GET_FW_VER: AUX packet <0x3b, 0x03, 0x20, 0x0d, 0xfe, 0xd2> (checksum 0xd2)
[00129021ms,usb] INDI Celestron AUX -> AZM MC, cmd MTR_GET_VERSION: AUX packet <0x3b, 0x03, 0x20, 0x10, 0xfe, 0xcf> (checksum 0xcf)
[00129193ms,aux] AZM MC -> INDI Celestron AUX, cmd NXS_GET_FW_VER: AUX packet <0x3b, 0x07, 0x10, 0x20, 0xfe, (0x07 0x0f 0x20 0x4e), 0x47> (checksum 0x47)
[00129297ms,usb] INDI Celestron AUX -> ALT MC, cmd MTR_GET_VERSION: AUX packet <0x3b, 0x03, 0x20, 0x11, 0xfe, 0xce> (checksum 0xce)
[00129468ms,aux] ALT MC -> INDI Celestron AUX, cmd NXS_GET_FW_VER: AUX packet <0x3b, 0x07, 0x11, 0x20, 0xfe, (0x07 0x0f 0x20 0x4e), 0x46> (checksum 0x46)
[00129572ms,usb] INDI Celestron AUX -> GPS, cmd GPS_GET_FW_VER: AUX packet <0x3b, 0x03, 0x20, 0xb0, 0xfe, 0x2f> (checksum 0x2f)
[00130665ms,usb] INDI Celestron AUX -> Nexstar EVO WiFi, cmd WIFI_GET_FW_VER: AUX packet <0x3b, 0x03, 0x20, 0xb5, 0xfe, 0x2a> (checksum 0x2a)
[00131765ms,usb] INDI Celestron AUX -> Battery Power Cont, cmd BAT_GET_FW_VER: AUX packet <0x3b, 0x03, 0x20, 0xb6, 0xfe, 0x29> (checksum 0x29)
[00132867ms,usb] INDI Celestron AUX -> AZM MC, cmd MTR_GET_POSITION: AUX packet <0x3b, 0x03, 0x20, 0x10, 0x01, 0xcc> (checksum 0xcc)
[00133030ms,aux] AZM MC -> INDI Celestron AUX, cmd MTR_GET_POSITION: AUX packet <0x3b, 0x06, 0x10, 0x20, 0x01, (0xff 0xff 0xff), 0xcc> (checksum 0xcc)
[00133132ms,usb] INDI Celestron AUX -> ALT MC, cmd MTR_GET_POSITION: AUX packet <0x3b, 0x03, 0x20, 0x11, 0x01, 0xcb> (checksum 0xcb)
[00133294ms,aux] ALT MC -> INDI Celestron AUX, cmd MTR_GET_POSITION: AUX packet <0x3b, 0x06, 0x11, 0x20, 0x01, (0x00 0x00 0x00), 0xc8> (checksum 0xc8)
[00133397ms,usb] INDI Celestron AUX -> ALT MC, cmd MTR_AUX_GUIDE: AUX packet <0x3b, 0x05, 0x20, 0x11, 0x26, (0x50 0x14), 0x40> (checksum 0x40)
[00133534ms,aux] ALT MC -> INDI Celestron AUX, cmd MTR_AUX_GUIDE: AUX packet <0x3b, 0x03, 0x11, 0x20, 0x26, 0xa6> (checksum 0xa6)
[00134399ms,usb] INDI Celestron AUX -> AZM MC, cmd MTR_GET_POSITION: AUX packet <0x3b, 0x03, 0x20, 0x10, 0x01, 0xcc> (checksum 0xcc)