User manual

905U-G Wireless Gateway User Manual
Page 25 ©September 2004
For I/O registers which are mapped to a remote output or another 905G, the comms fail
status is set if the 905G does not receive an acknowledgment for a message being sent to that
remote output. The comms fail status resets when a successful transmission occurs.
For I/O registers which have been mapped , from a remote input or another 905G, a comms
fail time period may be configured. If a radio message for this I/O register has not been
received within this time, then this registers comms fail status is set. The comms fail status
will reset when a message is received for this register. If the comms fail time is configured as
zero, then the comms fail status will never be activated.
The communications failure status is bit 15 of the status register for each I/O point. If the
host device reads a register as a digital or binary value, then the 905G returns bit 15 of the
register (0 or 1) - this is the comms fail bit of a status register.
It is important to use the comms fail status in the overall system design, as any system can
fail.
The 905G also provides an additional comms failure feature to stop the 905G transmitting
output messages to an individual remote address if the 905G already knows that this remote
address is in communication failure. This prevents the 905G from congesting the radio
channel with a lot of unnecessary transmissions (and re-transmissions). This function is
called "Don’t Send if In Comm Fail" and is configurable by the user for each individual
remote address. The 905G retains a "remote address comms fail" status for the remote
addresses configured for this function. If any output with this remote address goes into
communications failure, then the remote address comms fail status is set ("on" or 1) - every
time an input with this remote address receives a radio message, then the remote address
comms fail status is reset ("off" or 0). While the remote address comms fail status is set, the
905G disables any output messages being sent to this remote address.
When this feature is configured, all output transmissions are stopped if communications with
a remote module fails for a short period. They will start again when an input message from
this module is received. If the 905G determines that a output message should be sent to an
output which is disabled because of this feature, then the output message will not be sent and
the comms fail status of that output is set ("on" or 1).
If it is desired to use this function with a remote 905U module, but there are no inputs from
this module being used, then it is easy to configure an unused input or an internal input
(mains fail or low battery voltage etc). It is the comms fail status for the input, which is used,
not the input itself.
2.6.1 Monitoring Communications Failure
The host device can monitor the communications status of an I/O point by reading the status
register for this point as a binary/discrete register. Modbus, and many other protocols, will
convert a 16 bit register value to a binary/discrete value by returning the most significant bit
- for the status register, this corresponds to the comms status bit.
For example, to monitor the comms status of I/O register 1045, perform a binary/discrete
read on register 6045 (the status register for 1045). A value of “1” will be returned if this I/O
point is in comms fail, and a “0” returned if the status is normal.
If it is desired to monitor the comms status of all I/O points, it is more efficient to only
monitor the comms status of one I/O point at each remote module (if this point is in comms
fail, then all points at the remote module will be in comms fail). If this point is an input,
then the comms fail time for this input can be made short, to give an early warning of a