3 ms·
As a historical note, there are several approaches for hooking multiple devices to a bus before TRI-STATE® logic. Early IBM mainframes (such as the IBM 1401) us
by kens 6y ago
As a historical note, there are several approaches for hooking multiple devices to a bus before TRI-STATE® logic. Early IBM mainframes (such as the IBM 1401) used AND-OR gates for the bus. Each signal onto the bus was gated by an AND gate to control who was talking to the bus, and the AND gates were connected by wired-OR.
Another common approach was the open-collector bus, for example in the Xerox Alto. The gates connected to the bus could pull the bus low but not high. Pull-up resistors pulled the bus high. In some cases, multiple devices would deliberately talk to the bus at the same time, OR-ing together their values. The downside of an open-collector bus is it's relatively slow to transition high, since you have a low-current resistor pulling it high. Tri-state drivers, on the other hand, can provide a solid current both high and low.
(TRI-STATE is a registered trademark of National Semiconductor, interestingly enough.)
- dragontamer 6y ago> open-collector bus Ah right, and that's still used by I2C if I remember correctly (a low speed bus that's surprisingly common). I didn't know about IBM's AND-OR bus before. But now that I think of it, the open-collector bus is the exact opposite of AND-OR. Open-collector is common today because modern chips (CMOS) can sink more current than they can source. I don't know if TTL (or whatever IBM Mainframes used back then) had different characteristics which made the positive-variation more useful.
- stephen_g 6y agoNational Semi merged into TI in 2011, so I assume the TRI-STATE trademark is owned by them now. I would say the word ‘tristate’ has basically become just about completely genericised by this point though.