UM-0085-B09
DT80 Range User Manual
Page 194
RG
Software Flow Control
In this mode, the receiver controls the flow of characters by transmitting
•
the
XOFF
character (ASCII 19) to stop the sender from sending further characters
•
the
XON
character (ASCII 17) to allow the sender to resume sending characters.
If the DT80 receives an
XOFF
character, it will stop transmission within two character periods. If no
XON
is received
within 60 seconds (see
PARAMETERS
P26
) the DT80 will resume transmitting anyway.
Hardware Flow Control
With hardware flow control, the transmission of characters is managed by the
RTS
(Request To Send) and
CTS
(Clear
To Send) signals. For a DTE device such as a computer or the DT80:
•
the
RTS
signal is an output; the device activates this signal while it is able to receive data
•
the
CTS
signal is an input; the device will only transmit if this signal is active
So if a computer and the DT80 are set up to communicate using hardware flow control, then the RTS and CTS lines
must be "crossed over". Thus when one end deactivates its RTS signal then the other will see its CTS signal deactivate,
causing it to stop transmitting.
The DT80 communications cable (product code IBM-6) has the
RTS
/
CTS
lines connected in this crossover manner –
see
Host Port null modem cable (P392)
.
Note:
for a DCE device (e.g. a modem), the RTS and CTS functions are swapped (as are the
TxD
and
RxD
pins). This allows a
"straight through" cable to be used when connecting a DT80 to a modem.
Hardware flow control is inherently more reliable than software flow control, because the flow control state (send/don't
send) is continuously indicated by the hardware signals. Software flow control can get into difficulties if line noise causes
an
XON
or
XOFF
character to be lost, for example.
Hardware flow control is therefore the preferred method. It is not, however, the default because it can only be used if the
cable is wired appropriately and the host computer is configured to use hardware flow control. (In this sense software
flow control is a little more "forgiving").
If the RS232 cable is accidentally disconnected, the DT80 will no longer see
CTS
active, so it will stop transmitting. If,
however, this condition persists for more than 60 seconds (see
PARAMETERS
P26
), the DT80 will conclude that
the host computer is no longer connected and will stop trying to transmit, until
CTS
again becomes active.
No Flow Control
The DT80 can also be set to use no flow control, in which case there is no control of the sender by the receiver. Use this
setting with care, and only where there is no risk of the receiver being over-run by excess data from the sender,
otherwise data loss will occur.
Sleep Mode
If the DT80 is in low power sleep mode, it can be woken by sending a character to the RS232 port (either host port or
serial sensor port). Note, however, that the character that was sent will be discarded, if other characters are sent
immediately afterwards they may be discarded, too.
It is recommended that the DT80 be woken by sending a CR character, then waiting at least 500ms before sending any
commands. (
Note:
the "Wakeup Required" option in
DeTransfer
can be used to automatically prefix all commands by the
abovementioned wakeup sequence
)
Only the RS232 ports can be used to wake the DT80 in this way, because when the DT80 goes to sleep any Ethernet or
USB connections are terminated.
This also means that the RS232 port can be used to monitor real time data returns where the DT80 is configured to go to
sleep between scheduled scans – something which is not possible with the USB or Ethernet port.