# Why do people use AT commands in serial communication?

I need to know why people in embedded systems use AT commands?
When I have asked people say that it is a standard.

So my question is: What does "AT" means? Why do people keep saying it's a standard?

• +++ATH0 anyone? – kinokijuf Feb 7 '12 at 19:18
• @kinokijuf what do u mean ? – xsari3x Feb 7 '12 at 19:34
• why people in embedded systems use AT commands - there is nothing embedded-specific about your question or the use of AT on a serial link. It could be that you've seen this on an embedded system, but its origin is explained below and is not specific to embedded systems. (Be careful not to paint things with too broad of a brush.) – Radian Feb 8 '12 at 17:43
• The AT command was even implemented in something called modemu: a Unix program that creates a master/slave pseudo-tty device, and simulates a modem which actually goes over telnet. You "dial" a host with ATD<hostname>. The funny thing is that program came out almost exactly at a time when I needed it, around 1996: version 0.0.1. I have not needed it since. And it is still 0.0.1! I used it in conjunction with minicom to do zmodem transfers over telnet to remote hosts that could only be reached that way. – Kaz Apr 18 '13 at 23:23
• @kinokijuf Please don't put that in comments. You just made StackOverflow hang up on me! :-) – Curt J. Sampson May 31 '19 at 0:12

One seldom-appreciated detail about "AT" commands is that many modems would start out in "auto-baud/auto-parity" mode. Initially, the modem would start out not trying to actually decode any serial data, but would simply watch for a consecutive low pulse and high pulse whose widths matched the same valid bit period (e.g. 3.333ms for 300 baud, 833us for 1200 baud, etc.). Upon finding that, they would see if the next low pulse was five times that width. If so, they would watch for either another high-low-high or else for at least 1.5 bit times of high. Finding either of those would indicate that the modem had just seen a 0x41 or 0xC1 (i.e. "A") of the identified baud rate. It would further indicate either the attached computer was using either 8-N-1 or 7-E-1, or that it was using either 7-N-1 or 7-O-1. In either case, it would look for the next character to be either 0x54 or 0xD4 (i.e. "T"). That would allow the modem to further categorize the character length and parity settings.

Note that everything received before the "AT" would be ignored. If echo was turned on, the data would be echoed back to the attached computer simply by mirroring all line transitions without any serial decoding. If a computer sent data prior to the "AT" at e.g. 247 baud, it would be echoed back at that speed.

Nowadays, a few devices use an initial "A" for auto-baud-rate detection, but otherwise the fact that commands start with "AT" is basically a historical curiosity.

• ehm ... what do you mean "7-N-1" ?? – xsari3x Feb 7 '12 at 19:32
• Seven data bits, no parity, one stop bit. Allows data to be sent 11% faster than 8-N-1, if one isn't going to be sending any data with the high bit set. – supercat Feb 7 '12 at 20:04
• Except that auto-bauding was (and is) typically done on the +++ part of a +++AT or +++<guard time>AT command. See en.wikipedia.org/wiki/Time_Independent_Escape_Sequence – david Apr 19 '13 at 4:48
• @david: I don't see any mention of auto-bauding in the time-independent escape sequence article, nor have I ever seen a modem accept +++ at any baud rate other than the one it was using for communication. The characters 0x9E 0x86 sent back-to-back at 2400-8-N-1 (or ^N^F at 2400-7-O-1) would produce exactly the same line transitions as a "+" character at 1200 baud, so I would regard "+" as an odd choice for a baud-training character. – supercat Apr 19 '13 at 15:23
• Wikipedia(!) reference was just for people who don't know about +++. Hardware I use trains on 'U', software trains on custom strings. Do you know of a standard? All connect strings used to start +++, so auto-bauding was complete before any other communication. – david Apr 22 '13 at 1:18

It refers to the Hayes command set which has been the standard for a long time for issuing commands to modems (and other equipment) over a serial line.

Instead of the commands and data having two separate lines, only one line is used and to switch into command mode from data a certain sequence is sent, e.g. +++ followed by a set length pause. Then the next data is seen as a command by the receiving equipment.
The reason to use something like this is the fact that it avoids the need for another pair of lines, which in many cases are simply not available, especially in small embedded systems.

Have a look at the Wiki page and the links at the bottom - there is plenty of detail there.

There are all sorts of extensions to the original AT set though, so I wouldn't bank on everything that mentions AT to actually use all the original Hayes commands. For example I have a bluetooth serial chip here which IIRC uses it's own AT type set.
I'm no expert on it though, I just remember hacking around with commands in ye olde days of dial up and BBS.

The "AT" command set was to solve a problem of needing out of band control information over the same byte-stream channel arbitrary data was sent. This was a common problem of modems, back when they were external boxes connected to computers via a serial cable.

Hayes was a manufacturer of such modems, and gained a lot of early popularity. Their solution for the out of band problem was to send the modem mostly two letter ASCII control commands with a special sequence to put it in data pass thru mode. To reduce the likelyhood of random stuff looking like commands, their command sequences all started with the AT command, which stood for "attention".

Hayes gained so much market share that other modem manufacturers had to implement the same command set to be compatible. That way customers could use their modems without having to re-write software, which was already set up to drive a Hayes modem.

Nowadays, this scheme is rarely used but of course something that was so pervasive sticks around in dark corners even today.

• I liked this historical note about Hayes & how it forced it own standard , concatenating your answer with the above two answer is more than enough :) – xsari3x Feb 7 '12 at 19:34
• Incidentally, many newer modems no longer seem to require that the characters "AT" be uppercase, which increases the frequency with which a disconnect in the middle of sending a text file will cause errant modem behavior. – supercat Feb 7 '12 at 21:16
• The Band-Break control code is +++AT or +++<guard time>. AT is the first two characters of a standard control set, and stands for ATtention. – david Apr 19 '13 at 4:49

There is an especially good document that describes the history of the "AT" commands that can be found here:

http://nemesis.lonestar.org/reference/telecom/modems/at/history.html

It contains many pages of good "history" on how the protocol came about.

## why people in embedded systems use AT commands?

I'm not one of those "people in embedded systems" but I'd say that AT commands are still in use because they come from a well-defined low-overhead standard for in-line signaling.

What that means is that you can use the same communication channel both for signaling (AT commands to manage the communication) and data (actual data you want to send). The AT standard specifies how to differentiate between the two so you and your serial device don't get confused when talking to each other.

## What does "AT" means?

AT is for ATtention

## Why do people keep saying it's a standard?

Well, because it is. I'd say it is actually a mix of de-facto standardization and a couple of "real" standards and some recommendations.

• This seems to say nothing that was not already said in other answers years ago. If you're going to dredge up an old question, it really should be to say something important that has not been said before. – Chris Stratton Mar 20 '18 at 16:24