GPS module strange TXD problem

[Roy-DH]

Howdy folks, my first post to this forum, tho' I have been reading for a while. I thought the eclectic collection of big brains might be able to help me.

I have a few gps modules, bought from Amazon because the price was right. Specifically -

https://www.amazon.ca/Geekstory-Antenna-Supports-GLONASS-Compatible/dp/B07ZR8R17Q/ref=sr_1_15?dchild=1&keywords=gps+module&qid=1614806415&sr=8-15

I am using these modules in a custom GPS-DO project. They work very well and give me the 1PPS signal I need for timing.

However, I could never get the TX data from the module to display.

I should mention that I am using the module as a stand alone device, with wires attached to the appropriate points on the board for 1PPS, TXD, VCC and GND.

I tried plugging the module, via it's USB connector, into a computer and running gps software.

The U-blox Connect program identifies the module as a M8/8 device and it seems to perform properly. No doubt it is a Chinese knock-off but since I don't need to write any data to the module it would suffice.

As soon as I selected the right comport in the software my scope showed me data on the TXD line and the software also showed all the expected NMEA data.

I found that when I closed the program the data would stop on the TXD line.

I experimented with various pull-up and pull-down schemes on the RXD line to try to trick the module into thinking the computer was there, but no joy. Since there are only two data lines from the computer to the module, TXD and RXD, I'm at a loss.

I have never seen this behavior in any other gps module I've used, including other M8 modules.

Any ideas would be appreciated.

regards, Roy