4-4
Relay Existence
setup parameter. By default, the RF Terminal is
not
configured to look for Relays. This setup parameter can only be accessed
via the RF Terminal keypad - it cannot be configured using the bar coded
RF Terminal Setup Menu. See
Chapter 2
;
RF System Setup
for details.
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 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.
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.
