Ion chamber timeout

[canismarko]

This PR adds a proper timeout calculation to the IonChamber.trigger() method. Previously, the default timeout was used regardless of the settings on the scaler. Now, the new method IonChamber.default_timeout() is used to determine the best timeout based on the scaler configuration.

Things to do before merging:

• add tests
• write docs
• update iconfig_testing.toml
• flake8, black, and isort

[canismarko]

Fixes #304

[yannachen]

What is our clock_frequency? Is it a constant value depending on the scaler hardware? Or a dynamic value? When is the value changed?

[canismarko]

Good question.

What is our clock_frequency?

It's the value circled in red in the image below. There's a dedicated clock that the scaler uses to know how much time has passed. In the below example, the clock is set to 9.6e6 Hz. When we ask the scaler to count to 300 seconds, then it uses this clock and will count for 300 * 9.6e6 = 2880000000 clock cycles, by making this the preset value for channel 1 on the scaler.

Is it a constant value depending on the scaler hardware? Or a dynamic value?

Whether it's changeable depends on where it comes from. The Struck 3820 VME scaler has an internal clock that is always 50 MHz. The scaler's internal registers are 32 bits wide, so the biggest number that it can keep track of is 2**32 == 4294967296. This means that if the clock is 50 MHz, the longest the scaler can count for is 2**32 / 5e7 = 85.9 seconds. This is too short for us and since we can't change the internal registers then we need a slower clock.

When is the value changed?

We're moving towards the MCS counter to get away from VME, and these scalers don't have an internal clock. Instead they have 4 frequency generators whose frequency can be set in EPICS, and so use one of these as an external clock. We can set it to as low a frequency as needed to count for the length of time we need.

It's probably not changed very often, but it can be changed when needed for long scans. Eventually I envision Haven's IonChamber device being able to set the clock frequency dynamically based on the preset time requested for the measurement.