User's Manual

TS4000 Radio Modem User’s Manual Serial Port 11
The most common form of hardware flow control, and the one used by most full
duplex wired (as opposed to wireless) modems, is RTS/CTS. With RTS/CTS
flow control, RTS provides flow control for the DTE and CTS provides flow control
for the DCE. One problem with RTS/CTS flow control is that for many half duplex
modems (most wireless modems) the RTS signal is used to frame transmit data
going from the DTE to the DCE. This use of RTS conflicts with using RTS for
flow control of data to the DTE.
An alternative form of hardware flow control is DTR/DSR. With DTR/DSR flow
control, DTR provides the flow control for the DTE and DSR provides the flow
control for the DCE.
Software Flow Control
Software flow control uses characters sent over the data lines to control data flow.
These characters are sent along with the normal flow of data between the DTE
and DCE. There is typically one character that is used to stop the flow of data
and a different character to restart data flow. Software flow control can use any
characters to start and stop flow. However the most common characters used
are the ASCII XON (starts flow) and XOFF (stops flow) characters. Because
these are the most common characters used, software flow control is often
referred to as XON/XOFF flow control. The ASCII XON character is the decimal
character 17 (0x11 hex) and is also known as DC1 or Ctrl-Q. The ASCII XOFF
character is the decimal character 19 (0x13 hex) and is also known as DC3 or
Ctrl-S (See Appendix B - ASCII Character Set).
A problem with software flow control is that the normal data passed over the
communications link cannot include the flow control characters. If it does, the
flow of data will be incorrectly stopped or started. This limits the characters that
can be used by the host application and also prevents the sending of binary (all
character numbers) data.
The TS4000 serial ports are setup as DCEs (Data Communication Equipment).
The TS4000 with the standard case uses two 9 pin subminiature D connectors
with female pins for the serial ports. The TS4000 with the watertight case uses a
19 pin environmentally sealed LEMO connector (see Appendix A - Serial Port).
Serial port 1 can be configured for either RS-232 or TTL signal levels. To change
the signal levels, the modem must be opened and the four jumper plugs next to
the serial port connector set to the desired position (see Appendix A - Serial Port,
Appendix F - Internal Jumper Block).
Serial port 2 is always set for RS-232 signal levels.
The serial ports can be setup to provide different internal electrical connections to
the DTR, DSR and RI pins. To change the pin connections, the modem must be
opened and the jumper plugs next to the serial port connector set to the desired
position (see Appendix F - Internal Jumper Block).
The RI (Ring Indicator) pin is pin 9 of a standard 9 pin subminiature D connector
and is an output for DCEs (the TS4000). The TS4000 has no internal RI signal
and therefore the RI pin is normally left unconnected.
Alternate Connection: RI for Modem Power
As an alternative, the DTR pin can be connected as a power pin into the TS4000.
This is non-standard use of this pin and therefore care should be taken when
Serial Port
Connector
Signal Levels
Signal Options
RI Pin Signal Options