![]() ![]() That drive capability guarantees each TTL output the capability to drive 10 TTL inputs with a guaranteed noise margin of 400mV or greater. TTL outputs will be 400mV or less when low (and sinking 16mA or less), and 2.4V or more when high (and sourcing 400uA or less). You need open collector outputs or similar to do wired-logic. Can you please explain me this Thanks a lot. Why is that so I read a book and it says that the current flowing will increase and also the output will not stay low if one of the outputs of the totem pole is low. If you connect such outputs together there will be a short circuit if they don't drive the same value. In totem pole output wired-logic connection is not allowed. TTL inputs are defined as 'low' if they are less than or equal to 800mV and 'high' if they are greater than or equal to 2V. A totem-pole output can drive the output both high and low. Without the collector resistor it would behave more like a few ohms (1.6K divided by the current gain of Qp). When Qs is 'off', the base of Qp is pulled to Vcc by Rc, so the effective pullup resistance is limited by the collector resistance Rcp - so it's about 120\$\Omega\$, meaning that for a 50pF load, the time constant is about 6ns. Totem pole outputs like this one use an active pullup, which is Qp, Rc and Rcp. ![]() If two totem pole outputs are connected, a shortcut occurs if they carry different digital signals (0/1 or 1/0), it means If one output voltage is high and the other low, the 'pullup' transistor of the first gate fights the 'pulldown' transistor of the other. ![]() It's a component part of the (active) pullup circuit, but it's not a 'pullup resistor'. Two totem pole outputs should never be tied together. The primary purpose of the 120\$\Omega\$ resistor is to reduce the current spikes when the output switches (when Qo and Qp are both on simultaneously for a brief moment). ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |