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Once the RF Terminal is Relay-ready, it can use the Relay instead of the Base Station to communicate. If a RF Terminal tries
to transmit 10 times to a Base Station without a response, it broadcasts a “who can hear me” message. If both the Base Station
and the Relay hear the message, whoever answers back to the RF Terminal first becomes the point of contact for that RF
Terminal.
Once a RF Terminal has established communication with a Relay, it addresses that particular Relay until another communication
failure (10 transmissions with no response) occurs. If a Base Station is within hearing distance of the RF Terminal, it will ignore
messages meant for the Relay.
When a Relay receives data from a RF Terminal, it then transmits that data to the Base Station over RS-422 twisted-pair cable.
The Base Station in turn transmits data (via cable) for that RF Terminal to the Relay, for subsequent broadcast to the RF Termi-
nal.
Relays are “dumb”. Relays do not know whether a transmission was received by the Base Station or not, so it is up to the RF
Terminal to retransmit its data if it does not receive a message from the Host Computer (via the Relay). The Relay can recognize
data from the Terminal though and if it receives 10 retransmissions from the RF Terminal, the Relay assumes that the Base Sta-
tion cannot hear it and broadcasts the message:
RELAY
n
CANNOT BE
HEARD BY THE BASE
NOTIFY SUPERVISOR
PRESS ANY KEY
At this point, the RF Terminal puts out the “who can hear me” message. The
RELAY n CANNOT BE HEARD
message usually
indicates a cabling problem and should be checked out immediately.
Sometimes a Relay gets a response from the Base Station that is partial data or garbage. The Terminal retransmits its data
since it has not received a new prompt. If this occurs ten times, the RF Terminal broadcasts, “who can hear me”. At this point
the Relay is still functioning and answers the RF Terminal’s call. Should the Relay respond to the RF Terminal first, the
whole sequence starts again. If the Relay
again
gets “garbage” messages from the Base and the Terminal re-transmits 10
times, then the Relay concludes that there is something wrong and broadcasts the RELAY CANNOT BE HEARD message.
This situation indicates that you may have an electrical “noise” problem – check your cabling as well as any electrical
equipment that is in the area.
Determining coverage areas for Base Stations and Relays
As we said before, it is almost impossible to predict the effective RF communications range in a given environment. The typical
area of coverage is a 3,000 – 10,000 ft. radius.
After a Site Test, if you have determined that you will need to add Relays to cover the area you want to operate in, you will need
to determine where to place your Relay in relation to your Base Station. To effectively cover an area, there must be overlap be-
tween the area covered by the Base Station and the area covered
by the Relay. The example on the right shows what can happen
with no area overlap:
K
ee
p
in
m
in
d
th
at
the maximum length of wire that can run between a Base and a Re-
lay or from Relay to Relay is 4000 feet. In most cases the range of
the LT7001 to the Base will exceed this amount unless you are op-
erating in a very challenging environment.
As you can see, the only area adequately covered is in a path where
the two circles touch. The “dead space” is completely without cov-
Summary of Contents for 7100 RF Terminal Series
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