ITU G1

Overview

The Harvest Industrial Telemetry Unit (ITU) is a GSM GPRS telemetry modem and data logger with multiple analog and digital inputs/outputs.

It is based on our standard SPE, but it has many added features:

• Up to 12 inputs (4 digital, 8 digital or analog)
• Analog inputs can be 4–20mA, 0–2.5V, 0–5V, 0–10V
• Two Dallas 1-wire bus inputs
• Two relay outputs
• Switched 2.5V output reference
• Switched voltage output (digitally variable 0–12V)
• AT commands for reporting and logging
• Stores 6000 log records in flash memory

Other Information

Much of the ITU's technical information is the same as the SPE's. See SPE Commands and ITU Physical Characteristics documents.

Dimensions

Plastic Box (ABS)
	Width = 171mm Height = 121mm Depth = 79mm Mass = 490g
Diecast Aluminium Box
	Width = 171mm Height = 121mm Depth = 79mm Mass = 895g
Diecast Aluminium Box with Mounting Plate
	Width = 197mm Height = 121mm Depth = 85mm Mass = 1135g

Top of page

External Connectors

Connector Types

• Data: Female 9-pin D-connector
• Antenna: SMA Jack (female, 50 Ohm ).
• Debug: Hirose type DF1EC-3P-2.5DSA
• Power: Pluggable rising clamp style screw terminal. Wire capacity up to 2.0mm diameter. (Dinkle style 5ESDV)
• Inputs/Outputs: Rising clamp style screw terminal. Wire capacity up to 2.0mm diameter. (Dinkle style EK500V)
• Wind sensor: Vertical RJ11 style modular telephone jack. Six way body with four contacts fitted.

Pinouts

Data:

1. Carrier Detect
2. Receive Data
3. Transmit Data
4. Data Terminal Ready
5. Signal Ground
6. Data Set Ready
7. Ready To Send
8. Clear To Send
9. Ring Indicator

Debug (external):

1. Transmit Data (from ITU)
2. Not connected
3. Ground

The external debug connector transmits a zero to 5V negative logic signal that is generally compatible with RS232 receivers. This signal is sourced from a CMOS level translator powered from the ITU midboard +5V supply. The internal debug connector is part of the SPE itself. The SPE debug interface uses a open drain N-MOSFET and a pull up resistor is connected from pin 2 to the unregulated SPE power supply. This allows a simple interface that offers typically zero to 12V signals and draws only leakage current under normal conditions. It is necessary to join pins one and two in the DE9 adaptor cable and the SPE power consumption will increase when the cable is plugged in. With pins one and two joined the same cable can be used with both the internal and external debug connectors.

Wind sensor:

1. No contact
2. Direction excitation
3. Direction wiper
4. Common (ground)
5. Speed pulses
6. No contact

Top of page

I/O Interface

Digital Inputs

These are the four digital inputs. Each input has a 10k resistor into the base of a BC847 NPN transistor. A jumper selectable 100k pull up is available for use with contact type inputs (switches and relays). For a voltage input (no pull up selected) the recommended minimum high level input voltage is 800mV and the maximum safe input is 50V. The recommended maximum low level input voltage is 400mV and the minimum safe input is -6V. When pull up is selected the maximum low level voltage (switch closed) is 400mV. This is equivalent to a maximum pull down resistance is 3k. Minimum high level (switch open) leakage resistance is 7k (800mV at the input). Input capacitance is 10nF.

Digital input two is used for the wind speed if an anemometer is fitted. Digital input four is used by the front panel test button so consideration needs to be given to this when wanting to use this input for another purpose. Note that the test button provides a momentary closure to ground so needs to be used in conjunction with pull up.

Analog Inputs

The eight analog inputs can be jumper configured for full scale input ranges of zero to 2.5V, 5.0V, 10.0V or 25mA. The 25mA range being intended for 4-20mA applications. For the three voltage ranges the input impedance is 4k/V (i.e. 10k, 20k and 40k respectively). The 25mA range has an input impedance of 100 Ohms so this results in a voltage burden of 2V at 20mA.

The following table lists the input limits for each of the input ranges. The first set of limits is for out of range conditions that may occur during operation. The second, absolute, set of limits should never be exceeded or else the ITU may suffer permanent damage. It's important to notice that the 2.5V and 4-20mA ranges can be damaged by less than the 12V power supply voltage. Extra care is needed when using these ranges and especially the 2.5V range. When using the 4-20mA range it might be a good idea to add a resistor in the circuit to limit the fault current should the transmitter (sensor) be shorted.

Analog Inputs as Digital

The eight analog inputs can each be configured as digital inputs on and input by input basis. These become digital inputs five through twelve. As digital inputs they have internal pull up so will read high unless pulled low. The pulling low can be done most simply by a contact closure to ground. If it is desired to use a signal that is switched between open and a voltage (say 12V) then this can be used by connecting a pull down resistor across the input. Up to a maximum of 29k could be used for the pull down so something like 10k would be a suitable choice. The mimimum high level input voltage in this case should be 4V.

Analog input two is used for wind direction if this is fitted.

Analog 8/Solar Panel

Analog input eight may be jumper selected to be either analog input eight, just like the other seven analog inputs, or the input for the 6V/1.2W solar panel used on Harvest solar weather stations. When used as a solar panel input the 12V SLA battery should be connected to the DC supply terminals. Do not use this input for solar panels of other than 6V or of higher output power.

Wind Sensor

The RJ11 style modular jack for the wind sensor provides a convient connection for the wind sensor used on Harvest weather stations. All the connections on this connector are in parallel with screw terminals connections so the srcew terminals may be used as an alternative. The direction excitation is the switched 2.5V reference. The direction wiper goes to analog input two and the speed pulses go to digital input two.

Sensor Inputs

The ITU has two inputs for Dallas 1-Wire bus sensors. Currently Harvest offers temperature and humidity sensors that may be connected to these inputs.

Relay Outputs

Two normally open relay outputs are provided. These are rated at 1A and 30V.

Switched 2.5V Reference

The 2.5V reference output is a buffered version of the SPE internal reference. It is switched on under software control when needed to perform analog to digital conversions. Maximum error is quite high at 4% but since the same reference may be selected for analog to digital conversions this output should be used for excitation of ratiometric sensors. Maximum load should not exceed 15mA.

Switched 12V

The switched 12V output is a buffered DAC output. The DAC gets its reference from the ITU precision reference. This reference has an initial accuracy of 0.4% but the DAC output is degraded by the gain setting resistors in the buffer stage. Final accuracy is better than 1.5%. The buffered output is capable of delivering at least 1A but heat dissipation in the buffer can reduce this limit. The maximum current can be approximated by 1/(2*(Supply_Voltage-Output_Voltage)*Duty_Cycle). If the DAC is set for always on the duty cycle is 1 but if it's set to be on only with the ADC the duty cycle is (^HASD+0.45)/(^HASI*10).

Top of page

Power Supply

The Harvest SPE is designed to be powered from a nominal 12V DC supply but can be operated from a supply voltage of between 9 and 18V DC.

Caution: The Harvest SPE must only be powered from an approved SELV power supply.

Due to the transmit bursts the ratio of peak to average current for any GSM phone is quite high. In testing we found the highest instantaneous peak current the ITU drew from a 12V supply was around 800mA.

A standard g20 SPE in low power mode with no terminal attached draws about 360µA from a 12V supply. This is linear regulated in the SPE so the current is fairly constant regardless of supply voltage. Adding the ITU board adds about another 200µA bringing the base current for the ITU to about 550µA. To this we have to add extra current drain depending on configuration and settings.

When designing a low power application is is important to consider the idle state of the inputs since this can have a big impact on the ITU power consumption. The table below shows the increase in supply current drain for different combinations of inputs, jumper settings and input states. The input circuits for the digital inputs and the analog inputs configured as digital are quite different and this results in significant differences in current drain.

Changes in the level of analog input signals have no significant impact on the ITU power draw however the power drawn by the sensors needs to be considered. Naturally the power draw by the sensors themselves needs to be taken into account but also the ITU analog input impedance should be considered. A sensor with a 0-10V output could have to supply up to 250µA to the ITU input. Bear in mind that the analog sampling delay and interval may reduce the average power consumption of the sensors but software limitations and the sampling requirements of some sensors may result in a other sensors being on for more time than desired.

• SPE alone, Low mode mode: 360µA
• ITU, No input jumpers, Low power mode: 550µA
• ITU, No input jumpers, g20 on: 2.3mA
• Idle, Terminal connected (4k load): 18mA
• GPRS session, no data transfer, no terminal: 26mA

The current draw with the GPRS session active is higher than might be expected because of the lighting of the connect LEDs.

Top of page

LED Indicators

The ITU has two LED indicators: a green "Connect" indicator and a red "Error" indicator. At a power on or reset both LEDs are lit briefly (for about 200ms).

Connect (Green) LED

The green "Connect" LED is lit whenever a PDP context is established. During establishment of such a PDP context the connect LED is flashed to indicate its progress. The flashing takes the form of a number of flashes at a rate of two per second followed by a off time and repeated every four seconds.

Once a PDP context established the connect LED is left on solid for the duration of PDP contect with the exception of the receive blinks. Whenever a packet of data is received from the network the connect LED is blinked off to indicate activitity.

Error (Red) LED

The red "Error" LED is flashed in a similar way to the connect LED but to indicate error conditions rather than progress.

Once a PDP context has been successfully established the red LED is used to indicate transmission of packets by blinking on.

Top of page