Worth Data RF Terminal 7000 - Manual (Page 37)

coverage in more than one location that is difficult to cover with a single base. An example of this would be
inside two or more metal buildings where a base would be in one building and a relay in the other. Relays
work like a remote antenna, passing data to the Base Station via cable instead of radio frequency. Base
Stations are used as Relay Stations by changing the jumpers inside. See Appendix A for details.
Relay Stations are attached to the Base Station using a cable that connects from the Base’s RELAY port to
the Relays’ RELAY port. When you order a Relay Station, you receive a 3-foot test cable with it. Although
Relay Stations will increase your range of operation, they will also add about ½ second to the response
time.
How Relay Stations work…
It helps to know how Relay Stations work before you add them to your system. Although Relays
increase your operational range, they also slow the response time of your system. In order to use Relays,
the Terminal must be configured to acknowledge that Relays are present. This is done using the Relay
Existence setup parameter. By default, the RF Terminal is not configured to look for Relays. 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 retransmissions
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.