The devices (pump controllers) are named
TT01, TT02, TT03, TT04, NT07 and NT10, respectively.
These were attached to COM1, COM2, COM3,
COM4, COM7 and COM10 (only in the interests of matching the
number scheme, above).
When I installed the software, the first
five worked perfectly.
But the last one was unable to connect.
On a hunch, we re-mapped the last comport
to COM8:, in the operating system.
Now all 6 work perfectly.
I wondered if there is a
single-numeric-digit constraint, on the serial port number?