Detcon RXT-320 - Manual (Page 24)

RXT-320 Wireless Modbus ™
RXT-320 Wireless IM Rev. 2.1 Page 20 of 27
4.1.2 Modbus™ Broadcast Requests
The Modbus™ broadcast request was introduced in the RXT-320 to support commands to be executed across
all RXT-320 transceivers simultaneously. More specifically this is used to place all RXT-320s in a low power
sleep state to conserve battery life. If none of the RXT-320s are battery powered there is no need to issue a
sleep command.
According to the Modbus™ specification a broadcast request is defined as a request with the Address Field set
to 0 and is used for writing only. There is no response returned as with normal Modbus™ transactions since
all devices on the system are accessed at the same time. Both Function Codes 06 and 16 (writes) are supported
with a broadcast request on the RXT-320.
Since there is no response on the broadcast request, there is a requirement to wait for a period of time before
sending out the next request. This allows all slave devices to process the command before the next request is
received. This is defined as the “turnaround delay”. This turnaround delay should be approximately 150
milliseconds for broadcast requests sent to the RXT-320. Controllers in general can meet this timing because
it takes longer than this to generate the next request so it may be unnecessary to make any adjustments to meet
this timing.
NOTE: Controllers that generate Modbus™ broadcast requests to the RXT-320 must ensure a
turnaround delay of 150ms is met otherwise the next request can cause the broadcast request to
be overwritten and ignored.
Broadcast requests are not confined to sleep commands but can be used where any normal write command is
used. Broadcast requests received by the RXT-320 are also passed on to the local Modbus™ interface to allow
a controller to write to slave devices on that interface.
4.2 Modbus™ Register Map & Description
When the RXT-320 is assigned a Modbus™ address, the following registers become available to the controller
for access. All RXT-320 transceivers implement this register set. If the RXT-320 is configured as an Alarm
Station (address = 80h-8Fh or E0h-EFh) then a secondary set of registers is made available for controlling
Alarm Outputs (Table 8). Both register maps are shown below. Some registers are Read Only (R) and others
are Read/Write (R/W) as shown by the Access column. This equates to specific function codes where Read is
function code 03 and Write is function code 06 or 16.
NOTE: A write to a Read Only register is allowed and returns a response but it does not
change the value of the register. In some devices this would return an exception code.
The following sections will describe each register or register set for the register maps.