Opening Station Control left the compass blank for a second or two while the
amplifiers, relay boards and power supply filled at once.
Nothing was slow. The status bar keeps the shared heading loop running, so when
the compass mounts there is already a tick pending — up to the full three-second
idle interval away — and the new subscriber simply waited out the rest of it.
The heading was known the whole time and had nowhere to be read from.
The last heading is now kept and handed to whoever subscribes next. That matters
more on the Alpha SPID this was reported on than it would elsewhere: every poll
is an open, a read at 600 baud and a close, so even fetching immediately on
mount would not have been immediate.
In a microtask, so a subscriber is never called back before subscribeRotorHeading
has returned to it.
Asked whether the faster poll applies to all the rotator backends. It does —
GetRotatorHeading is one binding over PstRotator, Rotator Genius, GS-232/ARCO,
DCU-1 and SPID alike, and it reads only the ACTIVE rotor, so two towers do not
double it. But counting what that costs turned up two things worth fixing.
Every backend builds a fresh client per call — spid.New, gs232.NewSerial,
dcu1.NewSerial, rotgenius.New — so one poll is one OPEN and CLOSE of a serial
port or a TCP connection, not a read on a link already up. And the status bar
and the Station Control compass each ran their own interval against that same
binding, so with the tab open the controller was asked twice over. At the 700 ms
I had just set, that was nearly three port opens a second on a 600-baud line.
Both now share one loop, and it adapts: 500 ms while the position is changing,
3 s once it has been still for six seconds. These controllers do not report
"moving" — a SPID answers a position and nothing else — so it is inferred from
the position itself, and held briefly after the last change so the tail of a
movement stays smooth. Commanding a move, a stop, or switching rotor polls at
once, so the needle starts sweeping on the click.
A slow controller cannot stack requests behind itself either: at 600 baud a SPID
reply takes a fifth of a second on the wire alone, and a port still open from
the last poll cannot be opened again.
rouggy