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Chapter 4:
Operational Theory
Before you jump in and start writing a complex host program, it might be nice to be familiar with the theory behind the operation of
your RF Terminal. The RF Terminal has three different modes of communication:
Two-Way Mode
- the host program transmits requests for data to the terminal via the Base Station. The RF Terminal transmits
a response back to the Base Station, which in turn sends the data on to the host program. This is a truly interactive mode allow-
ing you to create flexible programs for a variety of applications that are computer led and controlled.
One-Way Mode
- the RF Terminal transmits to the host with only confirmation from the Base Station. The host program receives
data from the Base Station as it would any other serial device. The host cannot send data to the terminal; it can only receive infor-
mation.
Site Survey Mode –
the Base Station and RF Terminal work together to evaluate the site and determine the best location for the
Base Station. The site survey evaluates the signal strength of a number of test packets that are exchanged between the Base and
Terminal. The higher the number, the more successful your communications will be from that area. This helps you to identify prob-
lem areas before you implement your RF Terminal system. At 30 ft., this is also the acid test for suspected bad radios in a base or in
a terminal.
Let’s start with a discussion of the basic theory behind a
Two-Way
RF Terminal system.
How the Two-Way RF System works
Basic RF System communications…
The RF system consists of three components –
Host Computer
,
Base Station
and
RF Terminal
. The
Base Station
connects to the
Host Computer
via the serial port. The application running on the
Host Computer
sends a data prompt to the com port where the
Base Station receives it. The
Base Station
then transmits the data prompt via radio frequency to the intended
RF Terminal
. The
RF Terminal
displays the data prompt on the display and waits for the operator to enter the requested data. Once the operator en-
ters his data, the
RF Terminal
transmits the data to the
Base Station,
which in turn passes it on to the
Host Computer
. The applica-
tion on the host computer processes the information and sends a new data prompt out to the
Base Station
and the whole process
begins again.
A little more in depth…
This RF system’s dialogue is Terminal initiated. The Terminal says, “I’m here, give me something to do. The Worth Data RF sys-
tem is different from other systems in that our RF Terminal does not constantly “listen” for a data prompt from the host. We de-
cided to use a different approach that would help to eliminate unnecessary radio traffic, conserve battery power, reduce the size of
the Terminal, and greatly simplify the operation.
Here is how it works:
Each RF Terminal has a unique
Terminal ID
. When the RF Terminal powers up, it asks if you want to
SIGN ON?
Pressing YES
at the
SIGN ON?
prompt causes the RF Terminal to transmit it’s
Terminal ID
and a byte of data
indicating to the Base Station
that it wants to sign on to the system.
When you press YES to the
SIGN ON
prompt on the RF Terminal, the Terminal will display the following message:
WAITING ON BASE TO ACKNOWLEDGE
This message is normal when first establishing communication and may occur occasionally during normal operation.
When the Base Station receives a
SIGN ON
message from a RF Terminal, the Base Station transmits the SIGN ON information
to the host computer. The host computer application can then do one of two things:
If it has something for the Terminal to do, it can send a prompt to the Base, which in turn transmits it to the Terminal. The RF
Terminal receives the prompt, waits for the operator to enter the requested data, and then transmits the data back to the Base
Station.
If the host program does nothing within an allotted time, the Terminal displays the message:
WAITING ON HOST PROMPT
Содержание 7100 RF Terminal Series
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