Reg-Serial connection

At the most basic level, say I want to transmit a number from 0 to 255; in binary this is 8 bits, e.g., 160 is 0xAA=10101010.

In a parallel connection, I just place 10101010 on a bunch of wires; all the signals are being sent "in parallel."

In a serial connection, I send 1, then 0, then 1, then 0, then 1, then 0, then

RS-232 and USB are serial connections, as is Firewire (IEEE-1394) and SATA. Traditional (Centronics) printer ports, SCSI, and IDE are parallel connections.

Everything else being equal, parallel connections are faster but also larger; a major motivation to use serial connections is the smaller number of wires or PCB traces needed if the serial connection can still be made "fast enough."

Serial data buses send data one bit at a time. Parallel buses send multiple bits simultaneously. The examples you give; RS232, USB are axamples of serial buses. The parallel printer port is an example of a parallel bus.

One question. Is it correct to describe a serial data connection as a "bus" ?

Steve

Sure, EDA conventions notwithstanding. What do you thing the second and third letters in 'USB' stand for?

Best regards, Spehro Pefhany

If it's a bus (it can connect >2 devices), like USB or RS422, then yes. I'm not sure if I'd call RS232 a bus.

Tim

Ah yes, that example was rather staring me in the face, here by the PC :-). Funny, I'd always thought of a bus as a parallel sort of thing, but I suppose they mostly were when I was younger.

Steve

I think the determining factor is if there are more than two devices on "the bus", that can share the data line by being selected/enabled individually. Just point A to point B, I wouldn't call a bus.

Cheers! Rich

And yet tend to refer to it as such when it's a microprocessor to memory parallel link, 2 "devices". Which is where I guess I got confused on the terminology. Hmm, but even though there are only 2 things on that "bus" the memory can be en/dis abled so is the ability to switch attached devices on/off the real definition of a bus architecture, rather than simply n>2?

Steve

I think you're mixing metaphors here. Even if you connect a uP to only one memory chip, there's still an "address bus" and a "data bus", but that's because they're parallel, and _could_ be expanded to accomodate more devices; each one has its own enable. This _can_ be done with a serial protocol, which would give you a "serial bus", but I think they're fairly well-defined in the specs somewhere. Have you tried a google on "serial bus"?

Good Luck! Rich

Ah, I see your point, thanks!

Steve