Swarm M138 - Command Timing; Command Responses; Boot-Up Sequence; GPS Date;Time Reference

Command timing
Once the $ is received, all subsequent characters must occur within 250 milliseconds of the
previous character. If the inter-character delay exceeds 250 ms, the command will be silently
discarded, and the receiving state machine will consume and ignore any characters received
until the next $.
Command responses
An OK response confirms that the input parameters have been updated in response to a
command. An ERR response indicates that additional or invalid characters were included
between the two-character command designator and the * of the command.
Boot-up sequence
The user application should ignore any characters received during startup until the following
NMEA sentence has been received:
$M138 BOOT,RUNNING*49
The bootloader may output non-NMEA formatted messages during this time. These messages
include, but are not limited to: status messages, firmware update progress messages, and error
messages. These messages should be ignored and are for Swarm debugging purposes only.
GPS Date/Time Reference
The Modem will enter its GPS acquisition state once the boot-up sequence is complete. Please
refer to Figure 11 for estimates of how long the Modem will take to acquire a valid date/time
reference. The user application should not send any transmit data ($TD) commands until the
following NMEA sentence has been received:
$M138 DATETIME*35
November 2021 Swarm M138 Modem Manual - Rev 1.00 33/77