Date Code 20010518 Settings 6-13
SEL-2020 Instruction Manual
You can enable XON/XOFF data flow control on an SEL-2020 port with the SET P command by
setting XON_XOFF = Y. With XON/XOFF data flow control enabled, the SEL-2020 monitors
the volume of data in its received data buffer on that port and transmits an XON (hexadecimal
code 11) character when that port’s buffer drops below one-quarter (25%) full. The SEL-2020
transmits an XOFF (hexadecimal code 13) character when that port’s buffer is over three-fourths
(75%) full. A device connected to the SEL-2020 port with XON/XOFF software data flow
control enabled should terminate message transmission at the end of the message in progress
when an XOFF character is received from the SEL-2020 and should resume transmission when
an XON character is received.
Likewise, you can use XON/XOFF to control the SEL-2020 message and data transmission.
When the SEL-2020 receives an XOFF character when it is transmitting a message, the
SEL-2020 pauses transmission until it receives an XON character. If the SEL-2020 is not sending
a message or data when it receives XOFF, the SEL-2020 does not send any new transmission
until it receives an XON character from the other device.
Set RTS_CTS = N for any SEL-2020 to SEL device connection. If RTS_CTS = N, the SEL-2020
RTS output will always be asserted, and the CTS input status will have no effect on
communication.
Consult the instruction manual or contact the device vendor to determine the proper flow control
technique for each non-SEL device. If you select RTS/CTS hardware data flow control, make
sure that the cable you are using to connect the device to the SEL-2020 is wired for RTS/CTS.
When RTS/CTS hardware flow control is required, use SET P to set RTS_CTS = Y. Automatic
communication sources with RTS/CTS hardware data flow control enabled must stop message
transmission immediately when the SEL-2020 deasserts RTS so they do not overwrite the
SEL-2020 buffer on that port. Likewise, if RTS_CTS = Y, the SEL-2020 does not send message
or data characters until the CTS input is asserted.
Modem Operation
There are a number of issues to consider if you are using a modem. In particular, flow control
and disconnect sequences need to be considered.
When a modem connection is made, it is possible for the phone line connection to be at a lower
baud rate than the modem to SEL-2020 connection. This can lead to a loss of characters during
large data transfers, because the SEL-2020 can overrun the modem. This can be prevented by
setting the SEL-2020 to the lowest likely connection rate, typically 2400 baud. Alternatively,
you can enable RTS/CTS or XON/XOFF flow control between the SEL-2020 and the modem.
To use hardware flow control, set RTS_CTS = Y on the modem port. Only use this setting if the
RTS and CTS control lines are wired between the SEL-2020 and the modem. (For the internal
modem, this connection is already made.) The modem should default to use hardware flow
control.
Alternatively, to use software flow control, set XON_XOFF=Y on the modem port and modify
the MSTR setting to enable XON/XOFF handshaking within the modem. You will need to look
in your modem data sheet to determine the appropriate code for your modem.
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