DIY Alinco ERW-7 Programming Lead

An Alinco DR-735T was purchased recently, and so came the need to program it via the freely available Alinco Clone Utility software.

I wasn’t about to pay $69 for a simple programming lead, and found this schematic floating around on the internet that uses an FTDI USB serial adapter, something I already have.

Note that the schematic above incorrectly suggests the connector is 4 pole.

The connector to suit the DR-735T is a 3.5mm 3 pole plug, ie; a standard 3.5mm stereo TRS plug.
(Tip – Ring – Sleeve)

Simple enough, a 10k pullup resistor, and a 1N4148 silicon diode.

Let’s make one!

The resistor and diode are soldered onto a 0.1″ header socket so the FTDI adapter can be unplugged and still be used with breadboards.

Signal line is connected to the ring, ground is connected to the sleeve, tip is not connected.

To avoid any potential shorts inside the plug, the solder tab to the tip was cut off, as indicated by the arrow.

I’m sure this circuit might have worked with the original author’s Alinco (model unknown), but it certainly doesn’t work with a DR-735T.

Why?

Lets’ take a look at the DR-735T schematic.

There is already a 10k pull-up resistor inside, R475.

With two 10k pull-ups in parallel, we effectively have 5k pulling the signal line up to +5V.

That along with the Vf (forward voltage drop) of a 1N4148 being 0.6 – 0.7V, and a logic “low” on that signal line is, at best, 0.6V.
The voltage for this “low” signal is too high for the radio’s MCU to recognise this as a logic “low”.

The solution?

Get rid of the external 10k from the lead, and use a Schottky diode instead, such as a BAT85.
A Schottky diode has a much lower Vf, typically 0.2 – 0.3V.

This means a logic low will be around ~0.25V and not ~0.65V.

Here is my circuit that “just works”.