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APTB Version 3.6

The APTB3 is the newest product in GSI’s line of multi-purpose tank transmitter cards. It is backward compatible with the previous MACB and APTB products but utilizes enhanced microprocessor circuit board technology to provide a number of new features. It also provides all the functionality of the earlier GSI 1901 and 2001 transmitter cards and is mechanically and electrically compatible with those earlier products. A Bluetooth Low-Energy device was added with version APTB3.5. It is available on all APTB3 models from version APTB3.5 and up. The BLE device enables the user to use a smartphone app (Blue HHT) instead of a physical hand-held terminal to configure the board. BlueHHT is available from the Apple App Store for iPhone and iPad devices with iOS 13.0 or later. APTB2 and APTB3 cards can be upgraded for Bluetooth by adding a GSI 1590 BlueComm module to the transmitter to eliminate the need for using a physical HHT device. The 1590 BlueComm module plugs into the same RJ12 connector that is normally used for a physical HHT device. Support for Capacitive Encoders was added with board version APTB3.6. APTB3 boards with this feature can utilize either legacy brush encoders or capacitive encoders. APTB3.6 boards must utilize firmware version 220406 or later when using capacitive encoders. Like its predecessors, the APTB3 utilizes plug-in Personality Modules (PMs) to provide electrical interfaces, power regulation, and surge protection to connect the APTB3 to a wide variety of communication networks. All PMs supported by the legacy MACB and APTB modules can also be utilized on the APTB3. The basic APTB3 uses the same circuit board as its APTB2 predecessor, but has more memory, a more powerful microcontroller and a number of additional features. The APTB3 can be used as a direct replacement for any existing APTB2 applications. The APTB3.5 and later boards are physically different from the APTB2 and APTB3 boards because they have additional circuit components that were not on those earlier versions.

Some of the major APTB3 enhancements include:
• Live configuration and diagnostic capability utilizing a Hand-Held terminal or computer without disconnecting the board from the system
• Bluetooth capable (APTB3.5 and up)
• Supports a wide variety of absolute level encoders utilizing either legacy brush and code disk encoders (APTB3 and up) or capacitive encoders (APTB3.6 and up)
• LED indicators for communication and operation status

Expanded analog input capability, providing such capabilities as:
• Support for averaging or multi-point RTD temperature inputs
• Ability to measure product or water level from 4-20 mA instruments
• Ability to measure parameters such as pressure, temperature, density, etc. from 4-20 mA instruments
• Ability to compute gross and net volumes if equipped with density input and configured with strapping tables and various tank and product parameters
• Optional configurable master or slave communication port with the ability to acquire data from secondary gauging devices and merge that data into the APTB3 calculations and data tables
• On-board sensor for monitoring ambient temperature within the transmitter housing – including low/high historical recording
• Broader range of protocol support including MODBUS-TCP
• Support for additional PMs that were not previously available with the earlier products including an Ethernet PM
• Non-volatile memory utilizing flash and EEPROM technology for program and configuration settings
• In-situ firmware upgrade capability when new features are released
• Capability of powering the board directly from an external power supply for special applications and for testing
• No DIP or Rotary switches
• Utilizes resettable fuses

Technical Details


The APTB3 is a 5” diameter circular circuit board with the same footprint and hole pattern as its predecessor MACB and APTB products. In general, plug-in Personality Modules (PMs) are used to provide electrical interface, power regulation, and surge suppression to connect the APTB3 to a variety of field networks. All protocol and data processing is performed on the main APTB3 as specified by individual application configuration settings. The configuration settings can be viewed and changed with a plug-in Hand-Held Terminal or with the GSI Config and GSI Test programs running on a laptop computer.


Personality Modules

Personality Modules available for the APTB3 include the following:

• RS485 PM – Connects to a 2-wire RS485 data bus; requires 24VDC field power; supports MODBUS-RTU and a variety of ASCII protocols; the RS485 PM can optionally be powered from 48VDC field power.
• Ethernet PM – Provides 10/100 BASE-TX copper connectivity via an RJ45 jack; requires 24VDC field power; supports MODBUS-TCP, MODBUS-RTU, and a variety of ASCII protocols; external copper-to fiber optic converters are also available.
• Serial Fiber PM – Provides high speed, fiber optic connectivity for serial data communication; requires 24VDC field power; supports MODBUS-RTU and a variety of ASCII protocols.
• Radio PM – Provides wireless connectivity for serial data communication; requires 24VDC field power; supports MODBUS-RTU and a variety of ASCII protocols. It is available with either a point-to-point broadband radio or a mesh radio.
• Mark-Space PM – Connects to legacy Varec Mark-Space field busses; requires 48VDC field power; supports the Varec Mark-Space 56-bit and 40-bit protocols.
• TP600 PM – Connects to legacy GPE Current Loop field busses; requires 48VAC field power; supports the various GPE 31422 and 31423 protocols.
• Tankway PM – Connects to legacy L&J Tankway field busses; requires 48V to 60VDC field power; supports the L&J 2-byte Tankway protocol.
• Analog Output PM – Provides two 4-20 mA analog outputs to transmit level and temperature to an analog-input device. Analog Output PMs may also be equipped with a serial communication interface to provide simultaneous digital and analog communication capability.


Power Requirements

The APTB3 normally obtains +5VDC provided by voltage regulators on the attached PM device. Alternatively it can be powered directly from a +12V to +36VDC power supply (nominal +24VDC) by changing a jumper setting on the board. When powered directly from a power supply, the APTB3 requires approximately 100 mA at 24VDC (approximately 2.5 Watts), not including the power requirements of whatever PM is utilized with the APTB3.


Operating Specifications

The APTB3 operating specifications are as follows:

Temperature Range -25 °F to +160 °F
Relative Humidity 5% to 90%, non-condensing
Temperature Measurement Range -82 °F to +388 °F (for Platinum RTDs)
Level Measurement Range 0 to 96 feet


Major Circuitry

The major circuitry provisions of the APTB3 are described in this section. Some are optional features that may not be installed on all boards.



The APTB3 microprocessor is an 8051 derivative with 128K of flash memory and 8K RAM. This memory is supplemented by an external 256Kbit EEPROM. The microprocessor contains a 9-channel 12-bit ADC, on-chip voltage reference source, and two UARTs. It has 32 configurable, general-purpose I/O pins, including support for SPI and SMB data busses.
The microprocessor’s flash memory can be updated via a JTAG programming device that connects to a computer’s USB port. GSI firmware also permits firmware updates via direct serial communication to the APTB3 board.
One analog input is dedicated to an on-chip temperature sensor that monitors the ambient temperature of the APTB3. This sensor measures the die temperature of the microprocessor chip, and the APTB3 compensates for the self-heating characteristics of the chip so the ambient temperature of the board can be determined. This provides a good approximation (within +/-5 ˚F) of the temperature inside the transmitter housing. The high and low values measured by this sensor are stored in non-volatile memory for historical reference.
The APTB3 application program is stored in the on-chip flash memory of the microprocessor. All configuration parameters and downloadable user data are stored in the EEPROM. The microprocessor includes both SPI and SMB interface capability. Both of these busses are routed to a dual inline header so they can be utilized to provide additional expansion capability via external modules connected to that header.


Analog Inputs

The APTB3 includes conditioning circuitry for 8 analog inputs that support either 4-20mA or 0-5VDC signal ranges. One of the analog inputs is dedicated for an RTD and the remaining 7 are routed to a connector for application use as required. One of these 7 may be used as for an optional second RTD input. The analog conditioning circuitry also permits any or all of the 7 inputs to be used as digital input signals. Digital inputs may either be passive (dry contact) or active (maximum input voltage = 5VDC). The 12-bit microprocessor ADC is over-sampled and filtered to provide pseudo 16-bit resolution. These algorithms plus the use of physical low-pass filters in the signal conditioning circuitry provide a high degree of stability for the analog inputs. In general, the user is free to utilize the 7 available analog inputs for any purpose. Each input can be scaled and calibrated to generate any meaningful engineering unit representation that the user’s application requires.
However, if it is desired to use some of the particular pre-defined APTB3 features, the user must only use the particular analog inputs that are allocated for those features.

These are as follows:
• AIN1 (pin 1 of J6) – when an analog input is used for Product Level measurement in lieu of an actual level encoder.
• AIN2 (pin 2 of J6) – when an analog input is used for Product Temperature measurement in lieu of an actual RTD input.
• AIN3 (pin 3 of J6) – when a water bottom sensor is utilized to provide BS&W information to the APTB3 for advanced tank gauging calculations.
This input is also used when it is desired to utilize the “auxiliary analog input” feature of the GPE TP600 protocol. In that case, this input may be used for any purpose that the user & application require.
• AIN4 (pin 4 of J6) – when a densitometer or other density measurement device is utilized to provide Product Density information to the APTB3 for advanced tank gauging calculations.
• AIN5 (pin 5 of J6) – when a pressure sensor is utilized to provide Vapor Pressure information to the APTB3 for advanced tank gauging calculations.
• AIN6 (pin 6 of J6) – when a bottom pressure sensor is utilized to provide to provide Product Mass measurement information to the APTB3 for advanced tank gauging calculations.
• AIN7 (pin 7 of J6) – if a second RTD input is to be connected to the APTB3, it will utilize AIN7. In that case, nothing may be connected to this pin. Note that this RTD must be a platinum device.
If a second RTD input is not required, then AIN7 is available for an analog input for any desired purpose. The data value in engineering units of all analog inputs are available in the APTB3 MODBUS register map. If an APTB3 application does not require the use of AIN1-AIN6 for a pre-assigned purpose, the user may then use those inputs for whatever other purpose may be desired and read the results of those inputs via the Analog Input data registers in the MODBUS map.


Temperature Monitoring

Temperature measurement is generally performed by connecting an RTD to the APTB3 board. The board utilizes a current transmitter chip with sensor excitation and linearization to produce a 4-20mA analog signal that is directly proportional to the RTD resistance. That 4-20mA signal is then measured and processed just like any other 4-20mA analog input. This chip is configured (via gain and linearization resistors) to monitor RTD resistances from 75 to 175 ohms. This corresponds to a –82 ˚F to +388 ˚F range for Platinum RTDs or a –40 ˚F to +440 ˚F range for Copper RTDs. These ranges presume Platinum RTDs with 100 ohm resistance at 32 ˚F and alpha of .00385 and Copper RTDs with 100 ohm resistance at 77 ˚F and alpha of .00427. The RTD resistance range of 75 to 175 ohms was chosen to achieve the highest possible temperature measurement resolution (approximately 0.1 ˚F) while accommodating a temperature range that is suitable for most applications.
Alternatively, the APTB3 can be configured and calibrated for an RTD resistance range of 75 to 218 ohms which corresponds to a 82 ˚F to +602 ˚F range for Platinum RTDs or a –40 ˚F to +457 ˚F range for Copper RTDs. The temperature measurement resolution of the APTB3 is reduced slightly to approximately 0.15 ˚F when this high-temperature range is selected. The APTB3 can also be manufactured with alternate gain and linearization resistors to provide different ranges for special applications. Contact GSI for details if your application requires wider or different ranges of temperature measurement or different RTD types. Two of these temperature sensing chips can be installed on the APTB3 and are normally used to monitor single 2-wire or 3-wire RTDs. The second RTD can be either a standard or high temperature range device, but it must be a platinum RTD. Alternatively, the board can communicate with an external temperature multiplexer to select an averaging or multi-point RTD inputs for either or both of the RTD input circuits.
In addition to RTD inputs, the APTB3 has the ability to measure temperatures from 4-20 mA instruments. Reference the previous Analog Input description for guidelines on using analog inputs for temperature measurement. It can also obtain temperature data from external serial communication devices.


Brush Encoder Inputs

The APTB3 includes encoder-monitoring circuitry for both GSI and Varec mechanical encoders that utilize brushes and code disks. The direction of gauge rotation and the type of encoder (fractional, decimal, or metric) are configurable parameters.
When configured for Brush Encoder operation, the APTB3 continuously reads the encoder positions and computes the corresponding product level multiple times each second. This level data is then stored in the internal RAM memory of the microprocessor for immediate transmission to a Host system when requested. The level data can be provided in a variety of engineering unit and numerical data formats.
In addition to encoder inputs, the APTB3 has the ability to measure either water level or product level (or both) from 4-20 mA instruments.
The maximum level value with brush encoders is 95-11-15 or 95.99.
Capacitive Encoder Inputs:
All APTB3 boards from version APTB3.6 and up include encoder-monitoring circuitry for both brush encoders and capacitive encoders. Like the legacy brush and code disk encoders, the capacitive encoders are absolute encoders, meaning that any level changes that occur while power is removed from the board or encoders will be tracked and updated when power is applied. The direction of gauge rotation is a configurable parameter. No mechanical calibration is required with capacitive encoders. Calibration is performed by simply entering the current hand-line value via a hand-held terminal or Blue HHT app.
When configured for Capacitive Encoder operation, the APTB3 continuously reads the encoder positions and computes the corresponding product level multiple times each second. This level data is then stored in the internal RAM memory of the microprocessor for immediate transmission to a Host system when requested. Level data from capacitive encoders is always calculated in thousandths of feet, but is available to Host interfaces in a variety of engineering unit and numerical data formats.
The theoretical maximum level value with capacitive encoders is 99-11-15 (or 99.999 feet); however, the maximum practical value is 96 feet for compatibility with brush encoder applications. Values from 96.001 to 99.999 feet should be considered error readings or an indication that the encoders are not properly calibrated.


Communication Ports

The APTB3 contains two standard and one optional communication port. Port1 is routed to the Personality Module (PM) header and is used to provide Host Communication interfaces via the various plug-in PMs. This port can operate from 300 baud to 115200 baud. Port2 provides an RS232 interface to the HHT/Laptop Computer connector (J7). This port normally operates only at 9600 baud, except that it can be switched to 115200 baud for downloading new firmware to the APTB3 microprocessor. A BlueComm Module can also be connected to this port to provide Bluetooth capability for APTB2 or APTB3 cards that do not have a built-in Bluetooth module. Port3 is an optional communication port that utilizes an SPI UART chip. It is configurable for either Master or Slave operation, depending on application requirements. An on-board RS232 or RS485 interface is available for Port3. This port is also routed to the PM header to accommodate Personality Modules that might require more than one communication port. This port can operate from 1200 to 115200 baud. The APTB3 also supports bit-bang protocols such as Varec Mark-Space. In most cases, a Personality Module will only utilize either the standard Port1 or the custom bit-bang interface for Host Communication. However, APTB3 does have the ability to operate its standard Port1 simultaneously with bit-bang communication if an application requires this capability.


Switch Inputs

The APTB3 can monitor 4 dry-contact switch inputs in addition to its ability to detect digital inputs via the analog input circuitry. Optical isolators in the switch detection circuitry provide isolation between the board and the field.


Relay Outputs

Relay Outputs are an optional APTB3 feature and are only available if the APTB3 is ordered with this feature – it is not a field-installable feature. This option equips the APTB3 with two form-C relay outputs. All 3 terminations (NO, NC, and C) from the relays are routed to the relay output connector. The relay contacts are rated at 1A @30VDC or 0.5A @ 120VAC.
The relays can be switched on or off via MODBUS write commands. They will normally be in their de-energized state when the board is reset or powered on. It is also possible to configure the relay outputs for automatic activation based on product level, product temperature, or ambient temperature.

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