why totem pole outputs cannot be connected together

This is known as a wire-or arrangement (a bit confusing in a way - if any window input is out of range the output will go low).Īs the output transition time would be slow, there can be an issue for digital logic, but that can be solved with a single schmitt trigger. This admittedly slower, but it is possible there may be numerous such window comparators, and the use of a single resistor to signal a fault when any output goes low greatly simplifies a design for a global fault trap. Here the output is fed to a gate to decode the output because standard totem pole drivers cannot be tied together as there would be output pin contention however usually power sensing is a relatively slow event (there are always exceptions), so it would be possible to replace a gate with a resistor if these were open collector: Simulate this circuit – Schematic created using CircuitLab If totem pole drivers were used, there would need to be some sort of arbitration to select which device can transmit (and there would need to be a way to ensure the output of devices that should not drive the bus were high impedance), but that is overkill for a slow speed bus.Ĭonsider a window comparator made of two comparators with totem pole outputs: In I2C and derivatives, all devices are connected on the bidirectional SDA line. I intend to try 4.7K but since these are SMD 1206 array resistors, I have ordered another set at 4.7k to try out.There are numerous occasions where an open collector (or open drain) solution is appropriate. In other words, since the PCF only has a very low current source capability (100uA), it is up to the external resistor to source the necessary current to 'establish' a proper logic 0 and 10K might be too high. Now this does make sense! I had the same thought as well. Perhaps your 10K pullups could be a bit stronger (i.e. Since according to datasheet the PCF can drive leds directly. Typically your indicator leds would be on the ULN2803As outputs. The required pullup resistors for SDA, SCL and INT are not shown. There are current limiting resistors for these leds eventhough they are not show here. Also, the LED current limiting resistors (not shown) would need to limit the current to within 10mA.

The circuit for these LEDs isn't shown, but it implies that you're trying to source LED current when you should be sinking LED current instead.

P6 and P7 on the PCF8574T are net named as +LED1 and +LED2.

Why totem pole outputs cannot be connected together