The Alinco DR-735T is a very capable, dual band, dual VFO, full duplex, cross-band repeat capable radio, with an industry standard mini DIN TNC connector on the back for working packet. While I was reading about the capabilities of this radio in its manual, I noticed that it can received 9600bps packet radio, but can only transmit up to 4800bps. The reason why was not intuitively obvious to me upon first glance at the radio’s specifications, so I went deeper to investigate. This took me to the service manual, which I will break down here below.

The External Connector

The mini DIN connector on the back has a fairly standard pinout as shown below. Note it says 4800bps max for the DATA IN. I want to find out why!

Mini DIN Pinout

In the service manual, I start by locating pin 1 on the DIN connector.

External Mini DIN on Schematic

Internal Circuitry

Next I trace this to a pair of 2-1 muxes.

These muxes are operated together (that is, their control signal is the same signal). From my investigation, this is the mux pair that is activated when selecting the TNC operation for the radio. The right-most mux, IC114, selects between MIC IN audio from the mic port, through a series of filters, or the TNC port we’re interested in. The left-most mux, IC112, selects which TX generator chip we’re going to be using. The path I’ve highlighted in red is the TNC audio input path I’m trying to trace.

Transmission Modulation

Next the signal travels to the MICIN pin on a chip with designator IC109, and part number BK4811B.

BK4811B Generator IC

This is where the magic of modulation happens, and where I ultimately found the reason for only supporting up to 4800bps. Scouring the web for a datasheet for this chip, I found a PDF I could download. Checking the relevant sections of the chip’s documentation, I found the culprit, screenshot below:

The reason for 9600bps limitation

The audio path into the chip is shown in the blocks at the bottom. The bullet points 6 & 7 tell the details of why 9600bps isn’t usable. The low-pass and high-pass blocks cannot be bypassed on this chip, and the maximum achievable width is ~2.7k, which is not enough for 9600bps. You can turn off the compander and pre-emphasis, but there’s no way to expand the LPF->HPF network bandwidhth. To run 1200-4800bps you really only need ~1kHz, but 9600bps needs to have much more.

Wrapping Up

Now that I am satisfied that the radio can *not* in fact TX 9600bps (but can receive it, perhaps another analysis in another post to come), I will get on with getting DireWolf setup for APRS, and FlDigi for other fun digital modes.