Re: Multiple devices using one GPS

John Pagett G4YTJ


A logic 1 on the input of an unpowered CMOS device will cause the input protection diode to conduct to the positive supply rail (Vdd), because the input voltage is higher than the local supply. Enough current - a few mA- can flow to power up the board in a parasitic way. I don't believe it's anything to worry about.


On 25 Mar 2018, at 11:42, Roger Hill <rhill@...> wrote:

Hi all.

I have one QLG1 feeding two devices, a U3S and a VFO/SIG gen.

I have fed the QLG1 with its own 5 volt supply, and so neither the U3S nor the VFO have the 5 volt line connected to the QLG1 (i.e in the row of 4 connections, the 5V line is not connected. The earth line IS connected.

The U3S and the VFO each have independently switched 5 volt supplies, but again the ground is common.

I should add that every thing works ok, BUT:

If I have the U3S turned on, and the VFO's 5 volt supply disconnected (or switched off), when the QLG1 is connected to both devices (3 lines, ground, PPS and serial), the VFO LCD lights up and pulses in time with the heartbeat. No writing on the LCD, but it's 'trying' to come on.

Disconnect the QLG1 from the VFO, and it all goes dead (as it should).

So it seems to me that the QLG1 is somehow turning on the VFO LCD, even though the 5 volt line is off.

How can I prevent this, please?



Roger Hill

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