USB Guide Port Interface: GPUSB

The GPUSB is a USB-to-autoguider port interface made by Shoestring Astronomy, priced at $75.00 plus $14.00 shipping within the US. It allows a computer to send guiding commands to a telescope mount using the standard ST-4 style autoguide port. This compact and reliable interface is ideal for astrophotographers who want to autoguide their telescope during long-exposure imaging or simply improve tracking accuracy for better results.

The GPUSB works by emulating the directional button presses on a hand controller, sending correction signals to the mount’s RA (right ascension) and DEC (declination) motors. These guiding signals are transmitted through a standard RJ-12 connector, commonly known as the “autoguider port” or “ST-4 port,” making it compatible with a wide range of mounts that support this industry-standard input.

GPUSB (USB Guide Port Interface) overview and specifications

What the GPUSB does:

• Connects to a Windows PC via USB
• Allows autoguiding software to send correction pulses to the mount
• Interfaces with telescope mounts that use a standard ST-4 compatible autoguide port
• Does not transmit video or image data – it is only for guiding signal output
• Functions as an HID (Human Interface Device), so it does not require traditional drivers
   

Technical Details

• Computer Interface: USB 1.1 and USB 2.0 compatible
• Mount Interface: RJ-12 connector compatible with standard ST-4 autoguider ports
• Power: Draws power from the USB port
• Isolation: Includes optocouplers for electrical isolation between the PC and the mount
• Indicator: One LED to show when signals are being sent (active guiding pulses)
• No external power supply required
• Enclosure: Small rectangular USB dongle
• Cable: Comes with a short RJ-12 cable for connection to mount (length may vary depending on supplier)
   

The GPUSB is compatible with telescope mounts that use a passive ST-4 compatible autoguide port. These ports operate by grounding specific pins to activate guiding in four directions: right ascension positive (RA+), right ascension negative (RA–), declination positive (DEC+), and declination negative (DEC–). The GPUSB supports this wiring scheme and can interface with a range of mounts that adhere to the ST-4 standard. Some known compatible systems include the Meade LX200 Classic and LX200GPS, the Meade LX90 (if equipped with an autoguide port), Celestron’s Advanced GT, CG-5, CGEM, CGE, CPC, and NexStar series with an autoguide port, Orion Atlas EQ-G, Sky-Watcher EQ6 and HEQ5-Pro, and Losmandy mounts with the Gemini system such as the G-11 and GM-8 (version compatibility should be verified). The Takahashi EM-10 requires the GPCNV-TAK adapter, and the Vixen SkySensor 2000 requires the GPCNV-SS2K adapter to be used with the GPUSB. Mounts that lack an autoguide port may not be compatible unless modified or used with additional adapters.

The GPUSB is a guiding interface only; it does not have imaging capabilities, cannot connect directly to a guide camera, and does not provide GoTo control for the mount. It has no internal software or logic and relies completely on third-party autoguiding software for operation. Supported guiding applications include PHD2 Guiding, MaxIm DL, AstroArt, Guidemaster, and MetaGuide. It communicates using the HID (Human Interface Device) protocol, which is built into most modern Windows operating systems, meaning no special drivers are typically required. However, some systems may still prompt for permission to install HID-class devices, and if multiple USB guiding devices are used at once, software conflicts can occur.

The GPUSB works with Windows XP through Windows 11. It is not officially supported on macOS or Linux, and there is no compatibility with iOS or Android platforms. To install and set up the GPUSB, the user plugs it into a USB port, waits for automatic system recognition, and connects the RJ-12 cable between the GPUSB and the telescope’s autoguide port. The autoguiding software is then configured to use the GPUSB, and built-in calibration tools are used to set guide speed and direction. Most software will detect the GPUSB automatically without the need for manual configuration.

There are several limitations to the GPUSB. It only outputs simple on/off guide signals and does not support variable pulse durations or signal feedback. It can only operate with ST-4 style ports and does not function as a serial communication interface (e.g., it is not an ASCOM mount controller). In most cases, only one GPUSB can be used per computer unless specific custom configurations are made.

Included in the Box

• GPUSB interface unit
• RJ-12 autoguider cable
• Quick-start guide or reference card
• Packaging (varies depending on distributor)
   

Dimensions and Weight

• Dimensions: approx. 2.2 in x 1.1 in x 0.7 in
• Weight: approx. 1 oz
   

Maintenance and Reliability

There are no moving parts or user-serviceable components. It is a solid-state device. The opto-isolated design helps prevent damage from electrical surges or ground loops. No firmware updates are necessary or available. Long-term operation depends on normal USB handling and cable care.

GPUSB Operation, Compatibility, and Configuration Details

The GPUSB (Guide Port USB Interface) from Shoestring Astronomy enables a computer to send guiding pulses to telescope mounts that have an ST-4 style autoguide port. The interface is designed to be hardware-isolated and uses USB HID protocol, which simplifies driver management and supports a range of Windows operating systems.

Electrical Pinout and Autoguide Port Details

The GPUSB connects to the telescope mount using a 6-pin RJ-12 modular jack. This connector follows a de facto pinout that aligns with ST-4 compatibility. The device uses the following pin configuration (numbered left to right from the front of the jack):

• Pin 1: Open or sometimes powered (not connected internally)
• Pin 2: Common ground
• Pin 3: Right Ascension positive (RA+)
• Pin 4: Declination positive (Dec+)
• Pin 5: Declination negative (Dec–)
• Pin 6: Right Ascension negative (RA–)
   

Only pins 2 through 6 are used. The device does not connect to or depend on Pin 1. Most autoguide software allows user-configurable pin assignments, so the pin order is not rigid.

Device Power and Connectivity

• Powered entirely via USB; no battery or external power is needed
• Uses HID (Human Interface Device) USB class, which is supported natively in most versions of Windows
• Red LED lights up when the unit is plugged in to indicate power
• LED can be toggled or changed to green depending on the software signal
• No internal battery or manual switch required
   

Electrical Isolation

The device includes optocouplers between the USB circuitry and the autoguider output lines. This isolation prevents ground loops and reduces the chance of electrical interference between the computer and telescope mount. This design helps maintain signal integrity in observatory environments where long cables or mixed electrical systems are present.

Operating System Support

• Windows 98 SE: Supported with manual installation steps
• Windows 2000, XP, Vista, 7, 8, 10, 11: Plug-and-play installation using built-in HID drivers
• macOS, Linux, mobile systems: Not officially supported
   

Device Installation

Under Windows 2000/XP and later:

1. Plug into USB port; red LED should light.
2. Windows will automatically install the HID driver.
3. A “New Hardware Found” balloon appears.
4. Once complete, the GPUSB is ready to use.
    

Under Windows 98 SE:

1. Plug in the device; red LED lights up.
2. "Add New Hardware Wizard" appears.
3. Select “Search for the best driver.”
4. Uncheck all search locations.
5. Point to C:\WINDOWS\INF\HIDDEV.INF if needed.
6. Insert the Windows CD-ROM when prompted.
7. Complete the wizard.
    

Verification Utility – GPUSBCheck

Shoestring Astronomy provides a diagnostic utility called GPUSBCheck, which verifies the GPUSB is working correctly before integrating it into an autoguiding system.

GPUSBCheck allows the following:

• Toggle LED from red to green
• Simulate movement commands in all four directions (RA+/– and DEC+/–)
• Visually and audibly confirm the mount's response
• Check hardware function independently from third-party software
   

The utility is useful for troubleshooting mount responsiveness or confirming physical connection and wiring.

Compatibility Check Procedure

To verify that your mount responds to GPUSB control:

1. Connect GPUSB to the computer’s USB port.
2. Connect an RJ-12 cable from GPUSB to the mount’s autoguider port.
3. Launch GPUSBCheck.
4. Press direction buttons and observe:
   1. Audible motor movement
   2. Visual change in telescope orientation (use a distant object if indoors)
   3. Response in both directions per axis
5. If no movement occurs:
   1. Confirm cable is wired straight-through (pin 1 to pin 1)
   2. Check for mount settings like "terrestrial mode," which can disable guiding inputs
       

Shoestring Astronomy recommends testing the adapter before field use, to avoid debugging under nighttime conditions.

Cable Requirements

• Uses standard RJ-12 cable (6P6C)
• Must be straight-through wiring, not reversed
• Optional splitter available for using hand controller and GPUSB simultaneously
   

Software Integration

GPUSB is supported by multiple third-party guiding programs. These can access the device in one of two ways:

1. Direct GPUSB support
2. ASCOM Telescope Driver (for programs that use the ASCOM platform)
    

Steps for ASCOM Integration:

1. Install ASCOM platform
2. Download and install GPUSB ASCOM driver from Shoestring Astronomy’s site
3. Within autoguide software, select GPUSB as the ASCOM telescope driver
    

When configured via ASCOM, the software treats the GPUSB like a basic telescope mount capable of guiding movement only (no GoTo, slewing, or tracking control).

Device Behavior and Signal Output

• Each directional input is activated by grounding a specific control pin
• Active commands mimic button presses on manual hand controllers
• Signal duration and timing are controlled entirely by the autoguiding software
• No pulse width modulation or signal shaping is performed by the hardware
• GPUSB has no internal logic or feedback – it is passive between the software and the mount
   

Limitations and Scope

• Designed only for mounts with ST-4 compatible RJ-12 ports
• Does not work with serial-controlled or GoTo mounts without a guide port
• No image or video signal handling
• No native support for camera integration
• No Wi-Fi or Bluetooth capability
• Cannot daisy-chain with other USB devices on the same port without a hub
   

Warranty Information

• 1-year limited warranty from the original purchase date
• Covers manufacturing and workmanship defects
• Does not cover:
  • Normal wear
  • Damage from misuse, abuse, or modification
• Shipping damage (customer must file claim with carrier)
• Returns accepted within 30 days if product is in mint condition
• Original packaging and all accessories required for returns
   

Physical Overview

• Enclosure: plastic case with molded USB plug and RJ-12 jack
• LED indicator for activity status (red/green)
• No user-accessible firmware or reset mechanism
• Rugged enough for transport; should be stored in dry, dust-free environments
   

Conclusion

The GPUSB is a purpose-built device for users who need to send guiding pulses from a computer to a telescope mount. It connects via USB and outputs signals through an ST-4 compatible port, controlled by guiding software. It does not perform imaging, does not connect to cameras, and does not communicate GoTo commands. It works with many popular mounts and supports most Windows-based autoguiding applications.

There are no advanced configuration options or firmware changes. It performs a specific role in an astrophotography workflow and does so by relaying guiding signals from a PC to a mount, using a simple, hardware-isolated interface.