# DF-4750 — nmdm TIOCCDTR uses inverted mask: keeps DTR, wipes all other modem bits

## Build (guest, any user)
```sh
cc -O -Wall -o /tmp/nmdm_probe nmdm_probe.c
```

## Run
Root once: `kldload nmdm` (creates /dev/nmdm0A,B at 0666).
Then as any unprivileged user: `/tmp/nmdm_probe`.

## Expected output (buggy kernel)
```
TIOCMSET(LE|DTR|RTS)         = 0
TIOCMGET after SET           = 0x0007 (LE=1 DTR=1 RTS=1)
TIOCCDTR                     = 0
TIOCMGET after TIOCCDTR      = 0x0002 -> DTR=SET   <-- BUG: must be cleared RTS=clear <-- BUG: must stay set
```
`TIOCCDTR` must clear DTR only; instead DTR stays set and every other bit
(LE, RTS, ...) is cleared, because nmdmioctl() line 565 does
`ourpart->modemsignals &= TIOCM_DTR;` (missing `~`).

## After fix
`TIOCMGET after TIOCCDTR = 0x0005 -> DTR=clear [correct] RTS=SET [correct]`
(see fix_run.log — same module A/B as DF-4749).