SECTION 4D - HAYES COMPATIBLE MODEM INTERFACE
MODEM COMMUNICATIONS PROTOCOL
Communications between the Model 16 and the Hayes compatible modem take
place via ASCII text strings. The Model 16 is setup from the factory and
equipped with default command strings that are compatible with the AT
command set used by Hayes compatible modems. The Model 16 actually uses
only two commands: one to initialize the modem, and one to hang up the
modem. The initialization string is programmed to place the modem into an
auto-answer mode and to use the full level of responses (X3 or X4 commands)
so that the Model 16 is informed of incoming calls and the baud rate that
the modem connects at. It also sets the speaker operation and volume as
well as some other parameters. The hangup string consists of some delays
followed by a sequence to get the modems attention followed by some more
delays and then a hangup command. The default initialization and hangup
strings are shown below along with their interpretation. Consult your
modems operational manual for more information on the Hayes AT command set.
When the Model 16 sends a command text string to the modem, it expects the
modem to respond with "OK" or the numeric code "0". If the modem responds
with the error code or if the modem fails to respond within 5 seconds, the
Model 16 will attempt to re-initialize the modem. To initialize the modem,
the Model 16 will first change the serial port baud rate to the programmed
rate since it may have been changed due to auto-baud rate detect from a
previous call. It then send the modem initialization string, again
expecting an "OK" from the modem.
When first powered on, if the Model 16 has been programmed for modem
operation, it will send the modem initialization string. It will then wait
up to 5 seconds for a response from the modem. If no response is seen, the
Model 16 will send the hangup string. This process will continue until the
Model 16 sees an "OK" from the modem or until local access is requested by
sending a break to the Model 16. When a Hayes compatible modem is connected
to the Model 16, it will respond to commands from the Model 16 with "OK" or
with a numeric code indicating receipt of the command.
Since the command text strings sent by the Model 16 are user programmable,
it is possible that they may contain incorrect information. If a command
string contains unknown commands, then the modem should respond with
"ERROR" or the equivalent numeric code "4". If this happens, then the Model
16 will send the modem hangup command followed by the initialization
string. It should be noted that a bad initialization string may cause an
endless loop of communications between the Model 16 and the modem. The
Model 16 trying to initialize the modem by sending the initialization
command string each time the modem responds with an error code. If this
happens, you will see constant receive and transmit activity on the modems
indicator lamps. You can test the modem by connecting it directly to a dumb
terminal or a PC running a terminal emulation program. Type the programmed
initialization string directly into the modem. If it responds with anything
other than "OK" or "0", then you must find the error and correct the
programming for the Model 16.
4D-4
