Serial Port Extender
A Harvest Serial Port Extender (SPE) is a GSM GPRS telemetry modem
used at a remote location to connect between a remote device (with a
serial port) and a central computer.
Remote equipment can be controlled from anywhere in the
world, as if it were connected directly to the host-end computer.
Harvest can customise the SPE's hardware and software to suit
The SPE is far more than just a GPRS telemetry modem with
an IP stack built in. It has many functions which ensure that the
remote device is always able to communicate with the host
system, even after power failures, network outages and
indeterminate network states.
GPRS is a great tool for telemetry, but it takes a lot of
work to develop a robust system. It took three months to build
a prototype SPE and 18 months to get it to the point of
being a fully robust, reliable remote terminal. If you don't
want to be visiting remote sites just to reset your GPRS
modems—build in reliability with the Harvest Serial
These units are used in large numbers by Horizons Regional Council, NIWA, New Zealand Met Service and Environment Southland.
Go to our Getting Started
guide to view details on how to set up an SPE for the first
An SPE can be used for telemetry, security systems, data
loggers, hydrology equipment, active signs, and any remote
equipment with a serial data port. Some of our special
solutions and applications are:
- Solar-Powered. We sell a weather-proof,
solar powered SPE for use at remote sites. It can be used for
weather stations, frost alarms, and any remote logging
Weather Station. We sell complete remote
which send their data to our web site from anywhere in the
world. The weather stations can be solar or battery powered,
and they can monitor air and soil temperatures, rainfall, wind
speed and direction, humidity, battery voltage, water flow,
soil moisture, and more.
- Frost Alarm. The weather stations can be
configured as Frost Alarms
for vineyard and orchard owners. Owners can set up alarms for
almost any input, and have alarms sent as text messages,
voice messages, or emails.
- Industrial. A rugged IP65 version of the
SPE is available for use in industrial environments.
- Loggers. Special versions are available
to communicate with Campbell Scientific data loggers and
Unidata (Star) loggers. These units communicate while
preserving the battery-saving features built into the loggers.
- Rail Crossing Monitor. We make an SPE-based
Rail Crossing Monitor that keeps track of
various functions at rail level crossings.
- Vending Machines. Remote monitoring of
vending machines provides up-to-date sales and vending
information. Contact us for
Below are some basic specifications for our SPE. See our
Technical Information page for
- Motorola g24 GPRS modules are mounted on
a carrier board together with a UDP/IP packet assembler and
disassembler (PAD). The g24 is a dual-band module, and versions
are available for Europe and Asia-Pacific as well as for North
- UDP protocol is used for minimum latency
and data overhead.
- Automatically ping at a user-defined
interval to ensure network connection.
- Lots of I/O. Four counter or digital inputs,
eight analog or digital inputs, two Dallas one-wire inputs, and
three voltage or contact closure outputs.
- Baud rate configurable to match speed of
remote device (DTE). An RS-232 DE9 connector connects the SPE
to the device.
- Versatile configuration. Easily change
IP settings, PAD forwarding character and timeout, automatic
connection settings, event-triggered calling and logging, etc.
All settings are non-volatile.
- Remote code download. Ability to download
new software to the SPE via its GPRS link.
- Power supply can be from 5–30V DC
unregulated (or we can ship with a 110/230V AC supply).
- Dimensions are 161mm x 81mm x 37mm for a
standard SPE. Optionally supplied with stubby antenna.
Typical Host-End Connection Methods
- Internet Hosting. This is the lowest cost
and most internationally portable solution. Remote units will
have dynamic IP addresses, so an auxiliary identification method
is required (such as the Harvest
Serial Number). This approach requires that the field units
begin communication with the host rather than waiting for a
- VPN via frame relay or DSL. This is a more
secure option. With a VPN (Virtual Private Network), the carrier
provides fixed, private IP addresses for the field units. Data
is delivered via the VPN over a frame relay or DSL circuit. With
fixed IP addresses polling is made posible.
Host-end software can talk with the SPE via the familiar
Windows Socket API (or any UDP/IP interface). This has the
advantages of using existing driver software and the abiliby
to support a large and dynamic number of simultaneous
sessions. Our UDPTerm test software (see
uses this method.
As an alternative a virtual COM port driver can be used,
but this will impose a fixed limit on the number of
Industrial Telemetry Unit (ITU)
We now have a separate page for our
Industrial Telemetry Unit.