There are a couple of birds I'd like to kill with one stone.

Up to now, the TX pin from the controller has been connected to the GPS module's RX line. This was originally done to facilitate having the controller send commands to the GPS receiver, but this has never proven necessary. Instead, that line has been used solely for diagnostic output, but when it's doing that, there's no way to independently talk to the GPS receiver.

At the same time, there have been a couple of times I've wanted to be able to plug subsidiary items into the diagnostic port without having to power them separately, but there's been no way to do that.

So I'm contemplating updating the boards to change the Mini-DIN-4 to a Mini-DIN-6. One of the extra pins would be connected to the controller's transmit pin, and the linkage between that pin and the GPS receiver's receive channel would be broken (with a solder jumper to allow it to be reconnected if this turns out badly). The other extra pin would be connected to the Vcc (+5V) rail. This means that the diagnostic connector would have 3 serial pins - two for the GPS and one output from the controller. It would also have the buffered PPS output and would now also have 5 volt power.

I can foresee using this to make a small daughterboard that would display the time and would parse the diagnostic output to glean performance information to display. I could also foresee a different board - a modified version of my LED GPS clock that would take power and GPS from the diagnostic connector.

The GPS receive pin would have a diode+pull-up level shifter to protect it from higher voltage inputs. I am as yet undecided whether or not to buffer the GPS serial output. It means changing the board layout (and buying a new stencil), but it would result in 5v TTL levels instead of 3.3v, and would protect the GPS-controller link from external interference.

The old MD-4:

1. PPS
2. TXD
3. RXD
4. GND

The new MD-6:

1. PPS
2. GPS TXD
3. GPS RXD
4. Diagnostic TXD
5. +5V
6. GND