MLM quick start

Subsurface modem (SSM)

• The serial data interface in the subsurface modem (SSM) encompasses a fully RS-232 compliant electrical interface in the receive direction ((i.e. the data being transmitted FROM an instrument TO the modem). 
• Serial data transmission signals from the SSM to a connected instrument have a voltage swing from 0 to 3V. This is compatible with most serial data receivers.
• The cable connection from the SSM to the connected instrument should be as short as possible to minimize capacitance.  The typical maximum length (typical cable) would be about 1 meter. 

The acceptable voltage level for the SSM is 8-22V (external) and 2-8V (battery).

The default baudrate of the SSM is 192000kbaud.

OEM SSM configuration

All OEM subsurface modems are configured to be shipped with the parameter LIDEN=0.  LIDEN=1 when the modem is connected to an RBR instrument.

Connector type: MCBH-6-FS (RS-232)

Receive line

The serial data receive line on the subsurface modem (SSM) is fully compliant with RS-232 signalling levels.

When interfacing to conventional digital logic, valid signal swings range from 0-VCC where VCC ranges from a minimum of 3V to a maximum of 25V.

Note that the receive input always has an input resistance of 5K ± 2K to 0V (“Ground”)

Transmit line

The serial data transmit output line is a  0-3V logic swing digital output . The electrical interface is CMOS logic gate (74AUP1G240) with external ESD protection. It is not intended to drive a long cable. The driver characteristics should be taken into account when connecting to the OEM equipment.

Note that if the connected instrument has an RS-232 interface, virtually all RS-232 interface chips will accept 0-3V logic as valid levels.

Head-end modem (HEM)

The HEM serial data electrical interface is fully compliant with RS-232 signalling in both the transmit and receive directions.

The acceptable voltage level for the HEM is 9.5V to 22V.

The default baudrate of the HEM is 115000baud.

Connector to mooring line

Benchtop testing and a “simulated mooring line”

When performing bench top testing, the user would normally thread a wire loop through the toroidal transformer mooring line couplers.
It is important to put a resistance in series with the wire loop that simulates conduction through the mooring line.

The value of this resistance should be in the range 20 to 100ohms.

System timeouts

The default system timeouts are defined with automated computer driven communications in mind. If one is sending commands manually to the system via an ASCII terminal or terminal program, it is advisable to increase the:

• HEM “sleep timeout” (ZTO, page 21 of the OEM command reference)

• SSM “sleep timeout”. This may not be as important as the HEM ZTO
• Open channel timeout. (MDOCDUR, page 26 of the OEM command  reference)

The “ minimum idle interval ” is the time between the closing down of the (mooring line) modem channel and the initiation of the next link between the HEM and SSM(s). More specifically, this time starts when ALL traffic on the mooring line link has gone to idle (i.e. there is NO signalling on the mooring line) AND there is no activity on the serial port of an SSM. An SSM cannot be reseized until the idle interval has passed. This time is fixed and based in hardware.
The user should wait at least 8 seconds between the absence of any data traffic on the mooring line link and the (re) establishment of a connection between the HEM and SSM(s)
This applies after any given action including discovery, seizing and communicating with a specific SSM or group of SSMs.

Serial port connected to an RBR logger... or not?

See LIDEN, page 25 of the OEM command reference.

Mooring line data transmission

The inter-modem protocol over the mooring line/seawater link is a packetized protocol with error detection.

Key features and benefits include

• Configurable choices for ACK/NACK protocol (page 31 and 32 of the OEM  command reference)
• Configurable choices for retransmit on error protocol (page 26 of the OEM  command reference)
• Configurable choices for deciding when the data in the serial transmit buffer  begins to transmitted over the inter-modem link (page 31 and 32 of the OEM command reference)

RBR instrument users

If your system is going to take advantage of the autonomous polling feature of the HEM (MLM-1000 system is used link to RBR data loggers) the most efficient way to configure the system for polling is to set it up “on the bench” and utilize the MLM interface in RBR Ruskin. That software has simple “configure to poll” features on the SSM pages as well as a simple user interface to configure the desired polling interval.

Waking up the modems via serial port activity

HEM

The transmission of any single character will from a host to the HEM while it is in the “sleep” state will cause it to “wake up”.
If a string of characters is transmitted to the HEM while it is asleep, the first character will trigger the HEM wake up. This first character will not be correctly received (as a character) by the HEM – it’s purpose is solely to wake up the unit. Any subsequent characters in a string will be correctly received.

SSM

The SSM has an “escape sequence” that is utilized during data transmission to enter command mode while the modem is in transparent mode. This same “escape sequence” is used to wake the modem up via the serial port.
For Version 1.0 firmware:

• The default escape character is HEX 01 (CTRL a)

• Thus, waking up the SSM from the serial port (using default values) would be

  CTRL-a-a-a
  For version 1.3 firmware:

• The default escape character is HEX 01 (CTRL a)

• The default escape sequence is also CTRL-a-a-a, however there is a  temporal aspect to the escape sequence. The escape sequence characters are preceded and followed by a guard period during which the data channel must be idle.