Specifications

AN93
26 Rev. 0.8
frame is terminated with an <EM> <flag> or <EM>
<err>. The host should also expect to occasionally see
the <EM> <mark> indicator if the sending modem
experienced a transmitter underrun or overrun problem.
In general, the RTS flow control is not used. However, if
it is used, and if RTS is negated for too long of a time,
eventually, the receive buffers will overflow. This is
called a receiver overrun, and the modem sends an
<EM> <rover> indicator. A receiver overrun is
considered to be a catastrophic failure and the host is
expected to terminate the session. Host software must
be designed so that an <EM> <rover> indicator does
not occur.
It is expected that the <EM> <rover> indicator be
encountered during system debug, and designing the
system software properly to avoid having these
indicators occur should be the design goal.
Please note that there is an option available in the
U87[10]. The reason for this option is to determine what
the modem sends to the DTE when the modem
receives back-to-back occurrences of the special
characters 0x19, 0x99, 0x11 and 0x13 at the DCE.
As an example, if the following string is received at the
DCE:
<0x19> <0x19> <0x11> <0x11>
If U87[10] = 0, this is what the host software will receive
at the DTE:
<0x19> <0x5D> <0x19> <0xA2>
If U87[10] = 1, this is what the host software will receive
at the DTE:
<0x19> <0x5C> <0x19> <0x5C> <0x19> <0xA0>
<0x19> <0xA0>
The choice of how to program U87[10] is based on
whether or not it is desired to simplify the host receive
parsing algorithm, or to guarantee that the receive
throughput is not overly affected by the <EM>
<shielding>. At the very worse case, if there is a large
frame consisting only of special characters, the required
throughput at the DTE will have to be at least 2x that of
the DCE rate to account for the <EM> shielding
overhead.
There are two methods of ending a call. One way is to
use the <EM> <eot> command, followed by an ATH.
Note that sending the <EM> <eot> command will cause
the modem to go to command mode and stop the
transmitter, however, the modem does not go back on
hook until the ATH.
The other method is to use the <EM> <ecs> command
to escape to command mode, and then issue an ATH
command. The main difference being that the <EM>
<ecs> does not shut off the transmitter. The <EM>
<ecs> can also be followed by an ATO command if it is
desired that the connection be resumed.
AT Command Set
AT commands begin with the letters AT, end with a
carriage return, and are case insensitive. However,
case cannot be mixed in a single command. The only
exception to this format is the A/ command. This
command is neither preceded by AT nor followed by a
carriage return but re-executes the previous command
immediately when the “/” character is typed. Generally,
AT commands can be divided into two groups, control
commands and configuration commands. Control
commands, such as ATD, cause the modem to perform
an action (in this case, dialing). The value of this type of
command is changed at a particular time to perform a
particular action. For example, the command
“ATDT1234<CR>” causes the modem to go off-hook
and dial the number 1234 via DTMF. No change is
made to the modem settings during the execution of an
action command.
Configuration commands change modem
characteristics until they are modified or reversed by a
subsequent configuration command or the modem is
reset. Modem configuration status can be determined
with the use of ATY$, ATSn?, or AT:Raa commands
where Y is a group of AT command arguments, n is an
S-register number (decimal), and aa is the hexadecimal
address of a U-Register.
The AT commands for reading configuration status are
listed in Table 16. Each command is followed by a
carriage return.
Table 16. Configuration Status
Command Action
ATY$ settings Displays status of a group of
settings.
AT$ Basic AT command settings.
AT&$ AT& command settings.
AT%$ AT% command settings.
AT\$ AT\ command settings.
ATSn? Displays contents of S-Register n
ATS$ Displays contents of all S-Registers
AT:Raa Displays contents of U-Register aa
AT:R Displays the current contents of all
U-Registers.
AT+VCID? Displays caller ID setting.