results rather than numeric results are preferred. Numeric result codes were originally intended to make it easier for software to control the modem, but there are two primary reasons they should not be used:Software can be confused by a command echo. For example, if the following command were sent with echo on (E1) and numeric results (V0) on:                       AT ... S9=20 <CR>The resulting data, echoed by the modem, would be followed by the numeric result code zero, meaning OK:                      AT ... S9=20<CR>0 <CR>Software may become confused by seeing a 0<CR> result which is actually part of the command echo, then another 0<CR> which is the numeric result. A program can become unsynchronized with the command processor in the modem.Turning off echo mode (E0) in the initial setup string would solve this problem; however, do not end that command with any digits (simply E).Another shortcoming of numeric results is that the software must anticipate all possible responses. This requires updating controller software whenever new result codes are added. For example, suppose a CONNECT 115200 result were added with a numeric value of 31. If verbose results were used instead, and the controller directed to interpret the number after the CONNECT result as simply the connection speed in bits per second, no changes to the driver are necessitated by the new result code. If, however, numeric result codes were used, the result code 31 must be added to the table, and the controller modified to interpret it appropriately.As characters are received, they should be processed through a state machine providing the functionality of the following one. This state machine recognizes strings surrounded by <CR><LF> characters and stores the string in a character array. <CR><LF> are defined by S3 and S4.Sample State MachineInitialize with: state = 1 ;ch = <next character from the input>switch( state ){     case 1:     /*-- Scanning for leading CR --*/               if( ch == CR ) state = 2 ;               i = 0 ;               break ;     case 2:     /*-- Scanning for leading LF --*/               if( ch == LF ) state = 3 ;               else if( ch == CR ) state = 2 ;               else state = 1 ;               break ;     case 3:     /*-- Buffer result, watch for trailing CR --*/               if( ch == CR ) state = 4 ;               else buf[ i++ ] = ch ;               if( i > LIMIT ) state = 1 ;               break ;     case 4:     /*-- Scanning for trailing LF --*/               if( ch == LF ) state = 5 ;               else if( ch == CR ) state = 2 ;               else state = 1 ;               break ;}if( state == 5 ){     buf[ i ] = 0 ; /* Null terminate buffer */     <process result in 'buf'>     state = 1 ;}This state machine can be imbedded within a loop that reads all received data one character at a time, checks for a timeout, and also checks for user abort. Once a result is recognized, that loop can be exited or continued if additional results are expected.Once a result code string is returned, it can be compared against the known result code strings. Some strings may incorporate wild-card suffixes. For example CONNECT followed by any numeric value indicates a successful connection at the indicated transmission rate. Even if a result such as CONNECT 38400 is not anticipated, if the controller has been coded for wild-card recognition, the controller will be capable of interpreting such responses correctly. This practice also facilitates interpretation of connection failed messages that are preceded by NO followed by any other character string such as DIALTONE, CARRIER, or ANSWER.************************************************************************D.3  Modem PreparationOnce the modem has been identified, the controller can continue to program any registers or user-defined values into the modem necessary prior to initiating the connection process. Typically, the setup operation is separated from the connection processing because it is performed independently of whether the call establishment will be in the originating or answering mode.Setup commands can be issued at the highest transmission rate the modem supports as determined from the identification process or it may be fixed at a certain value if the modem is not identified.++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++D.3.1  ResetBefore issuing any other commands to the modem, it is advisable to issue a Z or &F command to the modem before the identification or setup process. No specific response should be anticipated. The modem may be set up to return numeric, or no result codes. If a reset will be used, the following points should be considered:*  Even if a recognizable result within 2.6 seconds, the program should continue. (Some modems do a lengthy reset process before responding with a result; others may be in Q1 or V0 mode.)*  Following an OK result, an additional 600ms delay should be imposed. Some modems will respond with an OK then do lengthy reset processing, in which case they are unable to accept additional commands.After the modem is reset, the first setup string (e.g., verbose rather than numeric result codes) should be issued, then the identification command.++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++D.3.2  SetupSoftware should generally provide some modem setup. However, the software can be written to rely on modem configuration via a stored profile recalled on reset, or  by DIP switches set depending on the product. In this case, any unique settings must have been set up prior to running the software, and all the program does is send the Z command to recall the desired profile. Even more basically, software can assume the modem is in the power-up state. However, unless the software will be used within a very predictable environment, these assumptions may result in failures with the controller software.Some commands will always be overridden by the controller in order to ensure its proper functioning. Other command options should either default to the factory setting, or simply act as the "transfer agent" for the commands specified by the user. Menus and dialogs can be provided to prompt the user for specific activities; the program can then interpret these requests and configure the modem accordingly, as Hayes Smartcom products do, or provide the user opportunity to enter AT command strings.Commands frequently set by a modem controller:E0         Turn off echo mode to avoid having command echoes pass            through the result code scanner.&F         Recall the factory profile.Q1         Enable result codes to ensure that commands are being            processed, and to synchronize with the modem command            processor (except for synchronous communications where result            codes may cause the DTE confusion).V0 or V1   Use either numeric result codes or verbose (recommended)            result codes.S0=0       Disable auto-answer during the setup process to avoid            inadvertent disruption by an incoming call.H0         Ensure modem is on hook before continuing to the answer or            originate process.S12=10     Set the escape sequence guard time to 200ms to hasten the            escape for hang-up process. Also reduces the probability of            inadvertent user escapes.S2=*       Change the escape sequence character for two reasons: To            avoid inadvertent user escapes, and to provide different            escape sequence characters for answer and originate ends.            This prevents inadvertent escaping when data is echoed.S4=*       Modify the linefeed character to make the <CR><LF>NO            CARRIER<CR><LF> result code more unique if you scan for it           to detect carrier loss.S95=60     Enables the result codes which will provide the maximum            amount of information about the connection when it is            established.Two typical setup sequences using these recommendations are shown below:Recommendation for software design based on using pre-existing user settings:              AT E Q V S0=0 H S12=10 S2=28 S4=31 S95=60 <CR>Recommendation for software design based on starting from a known factory setting:               AT &F S2=28 S4=31 S12=10 S95=60Note: Where the zero suffix is used, it may be omitted from a command. Spaces are shown above for readability, but the use of spaces between commands is not recommended. Once this setup command has been sent, and the OK response returned, the controller can continue to the originate or answer processing.If user-programmed settings are included in additional setup strings, or the user is permitted to enter AT setup strings, the software should anticipate ERROR result codes. If an ERROR is returned in response to such a command, the result does not have to be reported to the user, but the controller should not be prevented from continuing in either case. Many times a connection can be made even though some setting is in error or is inappropriate for the class of modem being addressed.++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++D.3.3  Establishing the Desired Connection and Fallback Strategy        (S36 Developers' Tips)S36 determines which fallback action will occur if the protocols and procedures set by S46 and S48 do not produce a LAP-based (LAPB) error-controlled connection.The optimum fallback control strategy depends on which Hayes error-control modem you have. The fallback behavior is determined by S36, S46, S48, and &Q5.There have been two improvements in the V-series, OPTIMA and ACCURA EC modems that affect S36: The first added &Q6 Asynchronous Speed Buffering(ASB); ASB provides the same fixed speed DTE interface and local flow control as in the error control mode but without the error control or remote flow control. The second improvement added V.42 with its Alternate Protocol (MNP).Previously, S36 only consisted of one bit, Bit 0, which permitted the user to decide whether the modem would hang up or not if it did not succeed in negotiating an error-controlled connection. The factory default for S36 was 1, which meant fall back to direct async rather than hang up if an error control protocol is not established in &Q5 mode.When &Q6 (ASB) was added, a fall back option for &Q5 mode, which can be controlled with S36, was also added. If S36 Bit 1 is set, and bit 0 is also set, the modem will fall back to ASB instead of direct async if no error control protocol is established in &Q5 mode. The factory default for S36 remained 1.When V.42 was added, it added another fall back option, MNP, to S36. If S36 Bit 2 is set, then MNP will be attempted if the primary protocol selected in S48 is not established. Bit 2 is evaluated before Bits 0, then Bit 1. If MNP is established as the protocol, then Bits 0 and 1 are ignored. The factory default became 5 and later was changed to 7 to take more advantage of ASB.Now that we know that there are three versions of S36, we may need a way to tell which version our software is controlling. First, verify that it is a Hayes error-control modem. This is done by issuing an I4 command and looking for the existence of a "b-string". A b-string begins with a lower case letter "b" and is followed by several upper case letters or numbers forming a hex value.The easiest way for software to identify whether a Hayes modem supports Asynchronous Speed Buffering (also known as ASB, Buffered Async, or Normal) is to issue the following AT command,  AT &Q6 &Q5 <CR>. If the result code is OK then &Q6 is supported, and ASB is also supported as a fall back. If the result code is ERROR, then &Q6 is not supported. If this is the case, then software should be prepared to fall back to a direct async connection.The easiest way for software to identify whether a Hayes modem supports V.42 (and MNP) is to decode the first character after the leading "b" in the b-string of the I4 ID command response. The characters following the "b" in that line are "ASCII-hex" (0-9, A-F), which decode into 4 bits of ID code (3-0).  If Bit 3 is set, then MNP is supported; Bit 2 is V.42.Armed with the knowledge of which S36 bits are supported, the software may now safely configure the modem and properly anticipate the fall-back action.It should be safe to set S36 bit 2 to enable V.42 (and MNP) even if those protocols are not supported in the modem.If the modem does not support ASB, then software should be prepared to follow the CONNECT XXXXX speed result code and change the DTE port speed to match the indicated line speed of the direct connection when no error controlled connection was negotiated.If software will not change port speeds in response to the CONNECT message, then when software has identified a 'pre-ASB' modem, it should set S36 to 0 or 4 so that if no protocol is negotiated, the modem will hang up.The following table shows the order in which the bits of S36 are evaluated: (Remember, these steps only occur after the S46/S48 selections have failed to make a LAPBased error controlled connection in &Q5 mode.)