While this is an old subject in terms of technology, I find that every few years I end up describing it to a new group. Surprisingly to me, every time people seem to appreciate the information, and I think it’s just not covered often in broader imaging trainings. So today I’m writing about a favorite subject of mine – TTL control. To do this I’ll be referencing some fun technology – namely:
- The newly announced Excelitas Tetrem LED Light source
- Our Starlyte Nebula software (Now supporting the Tetrem)
- The Triggerscope Mini
- The PCO Edge Camera
TL:DR
The Problem
For illustration of the device control, I’ve captured a few real example frames from my oscilloscope, but also added some simulated oscilloscope examples that I hope will better depict the work behind the scenes for these devices.
I think it’s most important to frame this in a question,
When my software controls devices, how fast do they respond?
Assume we click a shutter button in any software. This can be for a laser, an LED, even an old mechanical shutter. In every case a bunch of things must happen for the end device to actually change it’s state. For that state (let’s say, “OFF” to change to “ON”,
- the software must accept the command
- and issue control signals to the device connection.
- That connection talks to the device controller (normally, a microcontroller)
- The microcontroller reads the command and does what it’s told
- The state changes.
Now, along the way, each device has a communication bus, and each bus has a traffic control system to insure multiple communications don’t conflict. As a result, the timing from the start of the button click to the end is variable – sometimes, up to 20 milliseconds!
Now, because the software is typically programmed to “confirm” each thing it does, it doesn’t just shout orders into the dark. Instead, the software waits for a confirmation, or acknowledgment, of the command being received, and oftentimes it will confirm the new state is active.
Consider this means a new set of jobs which must be completed.
- The device controller changes state
- The device controller then replies, or reports state
- The communication connection on the PC receives the reply
- The software reads the reply, and confirms it’s correct
- The software continues to the next job.
And so again, we can see a stack-up of not only delay, but a variable amount of delay.
If we visualize this in a graphic example like the one below, we can see the software code is sent to an LED microcontroller, the command is processed, and the led illuminates. However, we can also see there is a lot of room for improvement!
LED Control via Software (PC → LED Controller → GPIO)
A host PC issues serial commands. The LED Controller parses each command and drives a GPIO pin. The oscilloscope captures the resulting digital waveform on PA5.
Another way to look at this control is what happens with a typical complex instrument. Below is a video where I control 3 devices. The PCO Camera, a Zaber filter turret, and the Tetrem. Notice how long it takes just to get 3 time points for 2 channels. The exposure is short, that's not the issue - the issue here is of course, the devices need to move, but also each device must receive + process + move + report positions, and this takes a long time.
TTL Control -> Old + Fast = Awesome
So what is TTL? Basically, nothing more than an on/off switch! Just like a light switch allows an electrical signal to feed a light, TTL uses a voltage to send a signal to another device. You can find a thorough history on this communication method here if interested.
What we care about is that we have an on/off control system. Because it's nothing more than a voltage, no crazy translations need to happen for things to immediately react. If a TTL control line is on, or "High", then the device should be doing whatever ON is. Many devices have this input, but few people even know about it! For example, most Focus controllers, XY Stages, Peristaltic Pumps, Perfusion systems, and many other devices like lasers, shutters, and actuators accept a TTL input for control.
TTL Logic typically comes in 2 flavors. Older systems will use 5V for "on" and 0V for "off". The device being controlled might use thresholds or a midpoint. Some devices, assuming you give them at least 2.5V, will go active. Others want at least 4V or 4.5V before they enable. Newer devices often use 3.3v control. This is the same concept as 5V, just with lower voltages. One word of caution here - before connecting things up, it's good to confirm the logic of your devices. Generally, the amount of power these use is quite low, and signal inputs won't break things - but in the case of an exposed microcontroller pin, you can definitely cook a 3.3v input with a 5V line!
TTL Blanking - A partial Solution
So what can be done about this overhead? First, we can consider that the only time we really need the illumination on, is when the camera is truly collecting data. One way to do this is to use the I/O control cables found on most scientific cameras. In my case I routed the expose signal from the edge camera into the Tetrem on a single channel, and then, told the tetrem in my software to use "TTL Control". With these changes, the LED source now waits for a TTL input to go high, before it enables it's emission.
So now, let's assume that when the camera is exposing, we can enable the LED line. The software can act as a logical "AND" if needed, but the line won't enable unless there is real collection of light by the camera. You can see an example of this below.
Software-Commanded vs. TTL-Triggered LED Control
A single event fires two paths simultaneously: the PC issues a serial command (software path), while a TTL line drives the controller directly (hardware path). The oscilloscope shows the resulting latency gap.
For the example, the yellow line is the camera output, or trigger output, the teal line is the LED B brightness result, and the purple line is the software-only reaction. In this example, we can see the delay (latency) of the response, yet in most software, the purple line will illuminate before the camera exposure. What this means for research is your cells are being exposed to excitation energy, and you aren't even collecting that light onto a sensor! So, we are in effect just cooking cells for fun. NOT GOOD!
Using the Triggerscope for State Control of Devices
This is great for a single LED, but how can we control more than 1 channel? For this, we would use a logic controller like the Triggerscope Mini! With the mini, the camera expose line is routed to it's TRIG input, and up to 4 output channels are connected to it's TTL control pins.
With this done, we can assign an active line using the triggerscope. This provides fast easy state switching, and can even be done over multiple devices. Doing this will mean, for this example, we won't be moving our filter cube device. Instead, we would use a multi-band dichroic. However we can switch the LED excitation, and that's how we will gain some speed here.
In the example above, we see a high speed switch. This is simply by telling the Triggerscope Mini to change channels, instead of the LED. But - there is still more to do to improve our speed!
Streaming for the Win
The above example uses the triggerscope via USB, and while we only have 1 device, we are still using a "set + capture + set + capture" type of sequence. What if we just told the camera, "run as FAST as you can." Then, we used the output signal from the camera to count frames, and we told the triggerscope, "every other frame, switch from LED 1 to LED 2, then back to 1". THIS is the power of "Streaming", or also called "Sequencing". With this capability, we pre-load stuff to do into the triggerscope. Then, we arm the triggerscope. With that done, our triggerscope is sitting, locked into a state, simply waiting to receive camera frame TTL inputs. Below is a graphical depiction of this technique.
Now, we tell the camera, "Give us 2 channels * 10 frames = 20 pictures, and do so as fast as possible". In effect, "give us video of 20 frames".
In this method, there is zero control overhead. The camera free-runs, the triggerscope switches, and the LED reacts. Below is a video example of this, note also there is a small in-view live feed from a web camera (for all these video samples), and you can see that, indeed, streaming is crazy fast! For the below example, we've tied the camera "transfer" line into the triggerscope. This way, the triggerscope recieves a brief pulse before capture occurs. We enable on the "low" side of the signal, and illuminate. You can see for the CMOS style readout on the left of the example, once readout is done, the channel switches.
Trigger Switch — Camera-Driven LED Sequencing via Triggerscope
Each time the camera completes a frame, it emits a TTL pulse. The Triggerscope (Advanced Research Consulting) uses that pulse to advance its state machine and re-route its outputs — alternating TTL1 and TTL2 to the LED Controller so LED1 and LED2 illuminate consecutive frames.
So what does the above look like in real life? Check out this example below! Note that I select in my "lambda" control box the "Stream" checkbox. Then once start capture is pressed, the triggerscope is armed, the camera is readied, and we snag 20 images in < 300ms!
Of course, we can extend this using the 5V DAC control line on the triggerscope for capture of Z as well as channel, and if using the more capable triggerscope 4B, we can control even more devices.
I hope this article has helped explain not only why I make devices such as the triggerscope, but also how it's important to use software that provides complete control for your light sources and other devices, as well as the ability to do things like this. My sincere thanks to Excelitas for lending me the Edge camera and Tetrem LED, both of which have worked flawlessly on my system. Nebula has drivers for both products fully integrated for streaming control if you have interest in using it.
-Austin
