AMASS Data Technologies Inc.

PCOM-SDI:

Pliant RS232 to SDI-12 Master Interface

Pliant Technology Specialists

Pliant : readily yielding to influence

PCOM-SDI:RS232 to SDI-12 interface

PCOM-SDI: Pliant RS232 to SDI-12 Master Interface

The AMASSER PCOM-SDI provides an interface between RS232 and SDI-12 devices. In its standard configuration it serves as an interface between a RS232 host such as a computer or programmable controller, and a SDI-12 slave such as a sensor. Alternatively, it can also be configured so that the SDI-12 device is the master. The standard configuration is depicted below.

Supporting up to 10 SDI-12 slaves (sensors), the PCOM-SDI enables its user to operate in SDI-12 transparent mode thus bestowing the complex protocol restraints of SDI-12 upon the unit. The host need only send ASCII messages to the AMASSER PCOM-SDI via its RS232 port to obtain data from or setup the SDI-12 sensors on the bus. These messages can be any SDI-12 command supported by the sensors. Upon receiving a command from the RS-232 host, the PCOM-SDI then forwards it to the sensor and performs all the necessary communications and timings to obtain the final response from the sensor as prescribed by the SDI-12 protocol, including the D command, retries, timeouts, etc… The PCOM-SDI then transmits the response from the SDI-12 sensor to the host via the RS232 interface.

The host PC or PLC requires a protocol-driver that sends and receives ASCII messages that are transferred in the RS232 data stream and uses relaxed timing and no break handling. The "!" character that terminates all SDI-12 commands serves as the delimiter and triggers the PCOM-SDI into action. The host must wait for a response prior to issuing a new command to the PCOM-SDI.

Firmware Support Providing:

Hardware Features:

Host Protocol

All messages that are sent and received from the host are ASCII strings transferred in the RS232 data stream with relaxed timing and no break handling. For example, if the AMASS Data PSE-SDI shaft encoder is connected to the SDI-12 bus at address '0', the proper command to obtain a water level reading is "0M0!". To get the reading, therefore, the host need only send the ASCII string "0M0!". The AMASSER PCOM-SDI would then obtain the reading from the sensor in the manner prescribed by the SDI-12 protocol and return it to the host on the RS232 line as an ASCII string in the form: "a+<Response String><CR><LF>".

Host command: aM0! or aM1! or aM2!, etc..
PCOM-SDI response to host: a+<value #1>+<value #2>+…<CR><LF>

where a: Address of the sensor on the SDI-12 bus
<CR>: Carriage Return character
<LF>: Line Feed character

Alternatively, the PCOM-SDI can be configured in the Manual-Retrieve mode using the 'S6' command.  In this mode, the PCOM-SDI transmits its response from the sensor to the host without attempting retrieval of the data with the D command.  The host must initiate the retrieval by transmitting the D command as follows:

ex:
Host command: aM0!
PCOM-SDI response to host: a0051<CR><LF>
Host command: aD0!
PCOM-SDI response to host: a+<value><CR><LF>
 

SDI-12 Protocol

When the host sends a command to the AMASSER PCOM-SDI to initiate a measurement it becomes a Master SDI-12 controller. The SDI-12 slaves, that is the sensors, are then accessed as prescribed by the SDI-12 protocol.
In order to communicate with a sensor, the controller sends a "break" signal, which consists of sending a spacing signal (5 V) for more than 12 millisec. This will wake all sensors connected to the line. The AMASSER PCOM-SDI then sends the command that was submitted by the host onto the SDI-12 bus. The format of each byte of data is 1 start bit, 7 data bits with the least significant bit first, 1 parity bit with even parity, and 1 stop bit. The first significant byte of any message is the address of the sensor; all sensors with different addresses return to the "sleep" state and ignore the rest of the command. The last character of a command is always "!", and the controller turns off its transmitter within 7.5 millisec after sending the "!". The sensor responds by setting the data line to marking (0 V) for 8.33 millisec, and then begins its response. The first bit of the response must be sent within 15 millisec of receiving the last byte of the command as prescribed in the SDI-12 protocol.
The response from the sensor contains information about a) the time that must elapse before the reading will be complete and ready for retrieval and b) the number of values returned by the sensor as a result of the command. The AMASSER PCOM-SDI then sends the "Send Data" (aD!) command to retrieve the data which it then returns to the host on the RS232 line.

For details on the SDI-12 protocol visit http://www.sdi-12.org.

Specifications

Processor - Atmel 89S8252 @ 3.6864 MHz.
Internal RAM - 256 bytes RAM
Internal Code Space - 8 Kbytes Flash

Input

ASCII strings via RS232 data line (standard configuration)
Can also be configured for SDI-12 master

Output

SDI-12 protocol (standard configuration)

Can also be configured for RS232 slaves

Power Supply

Power supplied by a 8 to 30 VDC source.

Connectors

Physical Characteristics

Height - 32 mm. (1.25 in.)
Width - 83 mm. (3.25 in.)
Depth - 160 mm. (6.25 in.)
 

Environmental Characteristics

Operating : -40 to +55 C
Storage : -60 to +100C
Humidity : <= 100% non-condensing

Commands

  1. Any valid SDI-12 commands
  2. Identification command AI!
  3. Configure for Auto-Retrieval mode


The above information is believed to be true at the time of printing. AMASS Data Technologies Inc. reserves the right to modify specifications without notice. All trademarks are owned by their respective companies.

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