User manual

805U Radio Modem Module User Manual
Page 24 © April 2011
The time taken to transmit a message is :-
|------------HEADER-------------|--------------------DATA-----------|-----------|
lead-in system unit repeater dest. zero data error check
10msec address addr addrs's addr byte
No. of bytes 2 1 0 - 5 1 1 no. of data bytes 2
The time for each byte is 0.5msec @19200 bits/sec. If error checking is not configured at the
receiving unit, data will start to be output approx 1msec after the "zero" byte has been
received. If error checking is configured, data will be output approx 2msec after the end of the
message. For example, a message with 20 bytes of data transmitted at 19200 bits/sec with no
repeaters, will be output approx 24msec after the data is input, assuming that error checking
is configured.
There are several configurations for controlled mode:
3.6.1 Auto-Connect Controlled Mode
Auto-connect mode is similar to “fixed line” modem operation. The master unit wants to
connect to only one slave address. Normally the slave address is preconfigured. On power up,
the master unit continuously tries to connect to the slave address. The slave unit, on power
up, does not try to connect but waits for a connect message. Once a connection is made, the
DCD status at both ends is set and data can be transferred in either direction. If there is no
data transfer within the preconfigured “link check” time, the master will send a connect
message to check the communications link.
The connection will be broken if a communication failure occurs, or if one of the hosts issues
a “disconnect” AT command (although this would be unusual for this mode). When the
master is disconnected, it will continuously try to connect to its auto-connect address.
Up to five repeater addresses may be configured as part of the auto-connect address. That is,
up to 5 intermediate modules can act as repeaters between the master and slave modules.
3.6.2 Low Power Auto-Connect Mode
For special installations, the 805U may be operated in a low power condition where it
switches off its receiver - power consumption is reduced to approx 30% of normal. The low
power condition will occur if the 805U is configured for auto-connect mode AND if the low
power mode feature is configured. The DTR signal on the RS232 port will control switching
between low power and normal operation. When DTR is “low”, the module will change to
low power operation, reset any connected link and switch off its receiver. When DTR goes
“high”, the module will revert to normal operation and will auto-connect to its slave address.
The use of this low power operation may be applicable in remote locations where there is a
limited power supply such as solar panels. In this situation, the DTR signal from the host
device is used to “wake-up” the 805U unit. The 805U unit will then operate normally until
the DTR signal is reset by the host device.
3.6.3 Single-Connect Controlled Mode
Single-connect is similar to a dial-up modem. In this mode, the module will only make one
attempt to connect to a destination address. A host device will issue a connect AT command,
and the module will send a connect message to the destination address. If the destination
module responds, then the link is connected and the DCD status is set. If the destination