Astro tools, Electronics

Isolated eqmod adapter for the telescope control

There are popular and quite cheap amateur telescope mounts from different vendors: Sky-Watcher, Celestron and so on.
Most of that mounts contains motors and some controller schematic. User can control this devices in different ways. One way is to use mount’s handcontroller, other way is connect to computer and use some software to control and guide the telescope. Such connection can be made through special adapters (often called eqmod). Technically this adapter is just some kind ot serial converter, RS232 to UART (TTL) or USB to UART.
Here I’m describing a very reliable and simple adapter with galvanic isolation that I was developed and made.

So why don’t use adapters provided by mount’s manufacturers or some cheap chinese eqmod adapters?

Biggest problem with devices is a lack of galvanic isolation. Connecting mount through this adapters you doing a direct connection between computer GND and mount’s power supply. In case of failure with power supply or disconnecting wires your mount’s controller can blow up, and computer port also can be damaged.  I know a few cases.

Another problem with such adapters is a component base. In most cases manufacturer uses cheap converter IC like a pl2303 or ch340.
This chips is not perfect and may cause troubles on some operating systems.

This is a reasons why I made my own eqmod adapter.
My device is based on the FT232 chip which is a little bit expensive but more reliable. Galvanic isolation is based on ADUM12o1, which is a pretty good dual-channel digital isolators.

The ADuM1200/ADuM1201 chips are based on the Analog Devices iCoupler® technology – high speed CMOS and monolithic transformer technologies. These isolation components provide outstanding
performance characteristics superior to alternatives, such as optocouplers.

Device schematic:

Left part of the isolator chip is powered from the USB 5v line and right part is powered from the mount 12v source via AMS1117-5 voltage regulator.

On this schematic you can see some protection and filtering elements.
L1 is ferrite bead, which i get from the old motherboard (you can find this beads around USB ports). This part is needed to reduce EMI noise from the FT232R and associated circuitry being radiated down the USB cable to the USB host.

Resistor R3 and R4 is are matching elements and can improve stability when long USB cable is used.
Mount IO lines also contains protection elements: R1, R2 resistors and two zener diodes. This circuit can protect from the possible voltage surges due to inductance of the wires. This allows to use a quite long wire between adapter and mount.

All elements are mounted on compact two-sided board.

On the USB side I used type B socket, which is a very common and reliable.

Mount part is connected using 8p8c (Ethernet) socket. This connection is used because most of the mounts has a similar socket so you can use a regular Ethernet patch cord with any convenient length.

…most of the mounts, except Skywatcher EQ6.
This mount uses DB9 (well known as a “com port”) connector and maybe a little confusing, but this is TTL UART. I don’t know why Sky-Watcher did this.

To connect this mount you need to build special cable with schematic below.

To build this device I’ve ordered a 10 cheap boards with a swell red solder mask.

Here is finished eqmod adapters:

Most of the made devices I sold for the local astronomy community and astronomy amateurs.
All users are happy 🙂

Of course this device can be used not only with telescopes. This is a just good USB to UART adapter with isolation so it can be used anywhere with any devices (which can supply 6-12 volts).

Thanks for reading!

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