In the first post in the series, we looked at the Garmin 18x LVC “puck”. We talked about a particularly insidious issue that affected [Andrew] – both of his GPS units. And we saw that Oscilloclock owners really need to be able to update the firmware in these units.

In Part 2, we went through the design of an Oscilloclock Garmin 18x USB Adapter, that would allow the GPS to connect to a PC where the Garmin software runs to upgrade the firmware.

Now we conclude the series, with a treatise on the construction of the Adapter. Enjoy!

The final design

Here’s the design we arrived at in the last post. Let’s go through the steps to build it!

Fish out Fake Chips

The key component required is a decent TTL serial to USB adapter with programmable inversion on the signal lines. But here we have to careful: many low-cost adapters out there are built around fake FTDI chips!

As mentioned before, we at Oscilloclock are pacifists. But if we were to wage war against anything, it would be fake components. They are unsafe, unreliable, unworkable, and entirely unethical. You get what you pay for, if you pay the right people. The people who design, manufacture, and support the real McCoy.

Besides ethics and reliability, there is also a practical reason we must avoid adapters based on fake FTDI chips – often the fake chips are not programmable. A true no-no. So watch out.

Program the inversion

FTDI provide a nifty utility called FT_Prog. Below shows the utility running on a PC with the adapter connected, and configuring to invert the transmit (TXD) and receive (RXD) signals.

Dividing the input signal

We need to figure out the most elegant way to install the voltage divider – the two resistors we described earlier that reduce the impact of noise.

The cleanest way seemed to be to install the 1.2k shunt resistor directly across the receive and ground pins in the adapter itself, as below.

What about the 270 ohm series resistor on the RXD line? Well, installing this inside the adapter unit itself would require cutting tracks on the PCB. And that would compromise our effort, reliability, and aesthetics goals! So instead, we’ll insert this into the cable later on.

Cable Connector Conundrum

Recall that [Andrew] has two Garmin 18x units – one fitted with a small GPS connector and the other with a large connector. Wiring up two independent cables would have been natural. However, the TTL Serial to USB adapter came with only one cable pre-fitted with the necessary “DuPont” (a.k.a. Qi or 2550) connector.

What’s the big deal? Surely we can just attach a Qi connector to another cable?

Ha! Connector tech is never that easy! It turns out that to make a perfect connection with Qi connectors, you need a special crimping tool. The Oscilloclock Lab does not have this tool. And we do NOT compromise on perfection! Given that this adapter is not the best reason to invest in an incredibly expensive tool, we decided to use the single pre-fitted cable and split out to two GPS connectors, with the larger one serving as the split point.

(In hindsight, we could have separately purchased another quality cable that was pre-fitted with the connector. Next time, folks!)

Wire up the cable

Our beloved ultra-quality Hirose connectors are a joy to look at, and a joy to use. But wiring the tiny smaller units up with high precision doesn’t exactly “spark joy”. Still, we persevere…

Now we need to install the 270 ohm series resistor. We simply cut the wire and splice it in.

A bit more heatshrink applied, and we’re done!

Closure at last

Using the 18x USB adapter, [Andrew] is at last able to upgrade his pucks and enjoy his clocks in their full glory with GPS-synchronized time and date once again!

Instructions for how to upgrade the software are posted on the Support – Garmin 18x page.

Did you enjoy this series? Stay tuned for more, as Oscillolife returns to nor….. Okay, not quite normal, but at least it returns!

In the previous post, we looked at the Garmin 18x LVC “puck”. We talked about a particularly insidious issue that affected [Andrew] – both of his GPS units. And we saw that Oscilloclock owners really need to be able to update the firmware in these units.

We introduced the Oscilloclock Garmin 18x USB Adapter, that allows an Oscilloclock owner to connect their puck to a PC to enable the firmware upgrade.

In this post, we’ll take a look at the design of the Oscilloclock Garmin 18x USB Adapter. It wasn’t GPS satellite launcher (a.k.a. ‘rocket’) science, but it certainly wasn’t as straightforward as it might seem!

The Garmin 18x LVC electrical interface

Referencing the manual, the Garmin 18x series comes in 3 basic interface variations:

• USB – USB 1.x interface, with a USB(-A) connector to plug into a PC
• PC – RS-232 serial interface*, with a DB9 connector to plug into a PC, and a massive cigarette lighter adapter plug to obtain power
• LVC – RS-232 serial interface*, with no connector – for wiring into a device

For our Oscilloclocks, we use the LVC variation and fit an attractive custom connector solution, avoiding the PC variation with its venerable, utilitarian, and aesthetically unpleasant DB-9 connector and cigarette lighter plug combo. (We may buck the trend one day and intentionally fit such sockets into that special retro clock build – who knows?!)

* Astute readers noticed the earlier asterisks. PC and LVC units are not quite true RS-232; their output voltage swings between 0V and +5V. Not so with devices having true RS-232 interfaces! A swing from -25V to +25V is legal and also lethal for any unsuspecting microcontroller. In the Oscilloclock design, we take advantage of Garmin’s voltage range cap to avoid having additional circuitry to adjust voltage levels.

Interfacing the 18x LVC to a PC

To upgrade the GPS firmware, the 18x LVC needs to connect nicely to a PC. But [Andrew] is an Oscilloclock Owner. He deserves more than just a good electrical connection. The interface also must be elegant and aesthetically pleasing, lightweight (for shipping), and easy to build. And – most of all – it has to be interesting enough to write a blog series about!

We can start with Figure 1 in the manual, which describes the most basic interface hook-up possible.

This interconnection option assumes two things: the PC has a DB-9 serial port, and there is a power source.

If we extend this option slightly, to take power from the PC’s USB port, we arrive at this: