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PDM Installation & Configuration: Wired Inputs

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Wired Inputs


00:00 - A couple of times so far in the course, we've talked about how the PMU decouples input commands from the actual switching on and off of connected devices, letting us now define all these relationships in software instead of them being hard wired into the harness build itself.
00:16 In another course module we'll show how an integrated keypad can be used to send commands to the PMU via CAN but it's still very common to directly wire switches and analog sensors to the PMU and use these inputs as commands.
00:31 We'll go through the most common wired inputs you're likely to encounter and discuss how they should be connected.
00:37 The first of these is a digital on/off switch.
00:40 Most PMUs have dedicated switch input and ground level pins.
00:44 Digital switches should be wired between these two pins such that when the switch is off an internal pull up resistor in the PMU will cause the signal on the input to be high.
00:55 And when the switch is on it connects the switch input pin to the PMU's ground level pin and that level will be read as low.
01:03 Ensuring we wire these switches to the PMU's dedicated ground pin for this purpose avoids any issues caused by ground offsets within the vehicle.
01:11 A slight modification to this is a momentary switch.
01:14 This is a switch that has a default state, normally off or open and it only connects its contacts when the switch is pressed.
01:22 As soon as it's released, it returns to its default state.
01:25 THis might be exactly the type of input we want with a window washer pump being a pretty good example of where this would be appropriate.
01:32 But it's also possible to configure the PMU to read this input as latching.
01:37 When a momentary switch input is configured as latching, a single press on the switch will latch that input on with another single press of that switch releasing the latching and reading it as off.
01:48 This is purely done in software, the actual voltage levels on the input pin don't latch one way or the other so you'll need to configure the input to act this way.
01:57 Most PMUs will have a check box marked something like latch input on the screen for setting up a digital input pin.
02:03 Most PMUs require at least one switched input that tells them to turn on or wake up.
02:09 Sometimes a general purpose switched input pin can be used for this or the PMU may have a dedicated input for this called 12 volt switched or something similar.
02:18 Although the PMU has a permanent 12 volt connection to the battery positive or isolator output, they will by default be in asleep or power off state to avoid draining the battery.
02:31 They need a signal on this input pin to tell them to actually turn on.
02:34 This gives an interesting situation when we're using solely a CAN keypad for our inputs.
02:39 As this keypad itself needs power which is likely provided by the PMU but if the PMU is off and isn't powering the keypad, the keypad can't send a signal back to the PMU telling it to turn on.
02:53 So it's the chicken and egg scenario all over again.
02:55 If the use of a dedicated physical switch input isn't desirable in this situation, it's often possible to configure the PMU to be always on.
03:04 So that as soon as it receives power on its main input stud, it'll turn on and deliver power as programmed, The main battery isolator switch is then used as the master switch to turn the system on and off.
03:19 It's also very common for PMUs to be able to read analog input signals.
03:22 One common use case for this is to read the position of a rotary knob switch that is set up as a variable resistor divider.
03:29 The way these work is that each position of the switch connects the input across a different resistor divider configuration, giving a different voltage level for each switch position.
03:41 Now if you're interested in how to build one of these switches, we've got an in depth webinar on the topic in our gold member's webinar archive.
03:47 It's also possible to wire analog sensors like temperature and pressure sensors to these inputs as well.
03:53 More commonly these inputs would be read by the ECU but if every input channel on the ECU is already spoken for and the particular sensor doesn't need to be read at a particularly high frequency, such as most temperature inputs, the PMU can be an excellent entry point for this signal into the system then transmitting it out to other devices via CAN.
04:13 A PMU capable of reading an analog input will have a dedicated sensor supply pin and it's crucial this and the matching ground pins are used to power the sensor.
04:23 This once again ensures the correct voltage levels around the vehicle are being compared ensuring accurate readings.
04:30 Another reason we might wire an analog input directly to a PMU is if that data is completely critical to the PMU operation and it needs to have it before other devices that might otherwise read that sensor have been turned on.
04:44 An example of this might be a compressed air supply system on a race car.
04:49 This compressed air might be used for quite a few different purposes around the vehicle, from gear shifting to throttle blip to clutch actuation.
04:56 These functions are possibly controlled by various different ECUs within the vehicle and designating the control of the air compressor to a particular one of these might not make sense.
05:06 Instead we hand control of the air compressor over to the PMU and wire the pressure sensor directly to it.
05:13 The PMU is then configured to turn on the air compressor when the system pressure dips below a certain level.
05:19 Say 100 psi and off once the system is topped back up to say 115 psi.
05:25 The other devices in the system can then rely on the system pressure being within that window.
05:32 That being said, the PMU will likely be communicating the system pressure out to these other devices so although they don't have direct control over it, they can at least monitor it.
05:41 In this module we've discussed the direct wiring of inputs to the PMU.
05:45 Most PMUs can accept both digital and analog inputs and we follow the same wiring rules that we do when wiring inputs like this to an ECU.
05:54 We wire digital inputs switches between the input pin and the dedicated ground pin on the PMU to eliminate ground offset issues.
06:02 Momentary digital inputs may be read as they're physically pressed or configured as latching inputs in the software.
06:10 The PMU can act as another entry point for analog sensor information into the overall system with the PMU reading the sensor value and transmitting it to other devices via CAN or it might make sense to have the PMU directly reading an analog sensor to generate a command for controlling one of its outputs.
06:27 In either case, the sensor is powered by the dedicated 5 volt sensor supply and sensor ground pins of the PMU, once again avoiding any ground offset issues.

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