20 ESP-4 MI Serial Hub Installer/User Guide
The data being received from the serial port is scanned for the string that activates command mode
(+++). When this string is detected, the hub will reply OK to the device and will enter
command mode.
The following rules must be followed to enter command mode.
• There must be a one-second guard time (no data traffic) before sending the +++.
• There must not be a delay longer than one second between each +.
• There must be another one-second guard time after the last +.
When a port enters command mode, the remote ESP-4 MI hub will not be aware of this state
change. The remote hub will obviously not be receiving any data from the TCP connection, but if it
receives serial data from its attached device, it will attempt to send it to the ESP-4 MI hub that is
not listening because it is in command mode. In this case, the data will be buffered until the port on
the intended recipient ESP-4 MI hub returns to data mode.
Response codes
The ESP-4 MI hub will respond to an AT command from the serial device with OK or ERROR plus
the status of the TCP connection to the remote endpoint.
Table 3.7 lists the possible response values; that is, when response codes are enabled and the
applicable format (numeric or text) is enabled.
Outgoing calls
When the Allow Outgoing Calls option is enabled and the ESP-4 MI hub is idle, input will be
parsed for valid ATDT strings and a TCP connection will be attempted to the remote end. If
another call is active, any connection request will be ignored, even if the Escape to Command
Mode string (+++) is detected.
Table 3.7: Modem Command Response Codes
Numeric Codes (ATV1 or ATV2) Text Code (ATV0 or ATV3)
0OK
2Ring
3 No Carrier
4Error
5 Connect
6 No Dialtone
7Busy
8 No Answer
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