soft_uart Transmission ends marking OR spacing

[skiddaw]

I am trying to transmit 110 baud to an actual 20mA current loop Teletype using GPIO-2 (SDA) as the Tx line. I can see the data on an oscilloscope but the transmission often ends in a spacing state (0= no current) instead of marking (1=20mA).
Since finding this on the original I loaded soft_uart_hippy but with similar results except when I tried changing the baud to 2400 the inversion appears to have taken over. At 300 baud the final state is random again.
Test line: echo "U" > /dev/ttySOFT0
Raspbian 9.8 stretch
kernel 4.14.98+
model B Rev 2
firmware Feb 12 2019 10:42:42
stty -F /dev/ttySOFT0 110 cs8
sudo insmod soft_uart.ko gpio_tx=2 gpio_rx=3