User's Manual

4-5
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 Terminal.
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 re-
transmissions from the RF Terminal, the Relay assumes that the Base Station
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. After
the Relay transmits the RELAY CANNOT BE HEARD message, it will no
longer answer the “who can hear me” request from the RF Terminal.
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 and no
longer functions. 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
400-1000 ft. radius.