Data Communications Networking Devices - 4th Ed.pdf

4.3 INTELLIGENT MODEMS ________________________________________________________ 437normally use unique commands and S register settings to access and control thosepreviously mentioned features.Modem registersA third key to the degree of compatibility between non-Hayes and HayesSmartmodems is the number, use and programmability of registers contained inthe modem. Hayes Smartmodems contain a series of programmable registers thatgovern the function of the modem and the operation of some of the commands inthe modem's command set. Table 4.24 lists the functions of the ®rst 13 registersbuilt into the Hayes Smartmodem 2400, to include the default value of each registerand the range of settings permitted. These registers are known as S registers, sincethey are set with the S command in the Hayes command set. In addition, thecurrent value of each register can be read under program control, permittingsoftware developers to market communications programs that permit the user toeasily modify the default values of the modem's S registers.Table 4.24Sregister control parametersRegister Function Default value RangeS0 Ring to answer on0±255S1 Counts number of rings 0 0±255S2 Escape code character ASC11 43 ASC11 0±127S3 Carriage return character ASC11 13 ASC11 0±127S4 Line feed character ASC11 10 ASC11 0±127S5 Backspace character ASC11 8 ASC11 0±127S6 Dial tone wait time s) 2 2±255S7 Carrier wait time s) 30 1±255S8 Pause time caused by comma s) 2 0±255S9 Carrier detect response time 1/10s) 6 1±255S10 Time delay between loss of carrier 7 1±255and hang-up 1/10s)S11 Touch-tone duration and spacing time 70 50±255ms)S12 Escape sequence guard time in units 50 0±255of 20 ms Usually depends on a switch settingTo understand the utility of the ability to read and reset the values of themodem's S registers, consider the time period a Smartmodem waits for a dial toneprior to going off-hook and dialing a telephone number. Since the dial tone waittime is controlled by the S6 register, a program offering the user the ability tochange this wait time might ®rst read and display the setting of this register duringthe program's initialization. The reading of the S6 register would be accomplishedby the program sending the following command to the modem:AT S6?