Worth Data RF Terminal 7000 - Manual (Page 38)

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 between
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:
Keep in mind that the maximum length of wire that can
run between a Base and a Relay 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 operating 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 coverage. Alternatively, locating the
Base Station and Relay as shown below results in better coverage:
To Site Survey a Relay, all other Relays and Base Stations must be turned off. This is the only way to know
for sure which Relay is responding. Alternatively, perform the Relays’ test out of range of the other Relays and
Base Stations.
Relay Installation
Relay Stations are connected to the Base by twisted-pair wire. See Appendix B: Adding Relays for the
pin outs and a testing plan.
Is radio traffic contention likely?
The radio traffic time is about 30ms per transaction. Radio time is not going to be a gating factor, even with
more terminals than the allowed 64 maximum.
The bottleneck could partially be the serial port baud rate in high volume applications. The default baud rate
is 9600 baud; you can increase this up to 115,200 baud, but the greater the baud rate, the less the RS-232