<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
</head>
<body bgcolor="#FFFFFF" text="#000000">
Hardware flow control is tangentially related to my feature request
#777. Hardware flow control uses the RTS signal line for flow
control; modern amateur radio cables use RTS (and/or DTR) for
power:<br>
<br>
In classical hardware flow control (classically a terminal connected
to a modem):<br>
<ol>
<li>When the DTE (Data Terminal Equipment: eg a terminal) is
powered on and ready to receive data, it asserts DTR (Data
Terminal Ready).</li>
<li>When the DCE (Data Communications Equipment: eg, a modem, or
other device communicating with the terminal) is ready, it
asserts DSR (Data Set Ready).</li>
<li>In an actual modem configuration, when the modem has
established a connection with a remote modem, it asserts CD
(Carrier Detect). Similarly, when an idle modem receives a
phone call attempt, it asserts RI (Ring Indicate). Depending
upon the modem configuration and what is connected to it, the
terminal may assert DTR, signalling the modem to answer the
ringing phone line (and subsequently asserts CD). While this is
interesting, it is normally not a factor in the case at hand;
I've only included it for context.</li>
<li>Normally a terminal that is ready to send asserts RTS (Request
To Send). This usually stays high for the duration of the
connection (an exception would be half-duplex modems, which is
not pertinent here).</li>
<li>Typically, modems could receive data from a terminal at a much
higher rate than the modem could send to the remote modem, and
as a result, had an internal buffer. The default state of a
ready modem was to assert CTS (clear to send). However, if the
buffer got too close to being full, a modem would drop CTS until
it had transmitted (emptied) most of the transmit buffer.</li>
</ol>
Hardware flow control is #4 and #5 above. In the modern computer
world, where the two devices are local (or simulated as same) and
there is no modem, the above changes slightly:<br>
<br>
<ol>
<li>DTR and DSR are asserted when the respective devices are
functionally ready, and stay in that state until the
communications session is over. Eg: A printer (DCE) asserts
DSR when its power-on sequence in complete, until power-off. A
software communications program (DTE) typically asserts DTR when
the program opens the serial connection, until it closes the
connection.</li>
<li>RTS and CTS are used for flow control, just as above.</li>
</ol>
<p><br>
When <b>SOFTWARE</b> flow control is used:<br>
</p>
<ol>
<li>RTS is asserted by the DTE at the same time as it asserts DTR,
and CTS is asserted by the DCE at the same time as it asserts
DSR.</li>
<li>One or both of the devices sends X-off and X-on bytes to tell
the other device to suspend and resume transmission,
respectively.</li>
</ol>
<p><br>
Sometimes two DTE devices need to communicate, and a wiring change
is made in the normally straight-through cable, so that what one
DTE device thinks is DTR/RTS/TX, the other device sees as
DSR/CTS/RX. Such is the "null modem" cable. Some manufacturers
of devices don't understand this or related wiring issues, leading
to the need for "gender changers", but this is not pertinent here.<br>
</p>
<p>One important feature of all of the above, is that each such
device acquires its own power.<br>
</p>
<p>However, with most Amateur Radios, the radio cable usually does
not bring anything except "ground" and the "TX" and "RX" data
lines. This is the case for all Icom radios that I am aware of,
as well as and most handhelds from other manufacturers. My Yaesu
FT-897D is an exception; the serial cable provides power to its
internal serial converter.<br>
</p>
<p>For some Icom radios (eg, the ID-880H and IC-2820H) that use the
Icom OPC-1529R cable, it's a simple three-wire cable providing
ground, TX, and RX. Presumably these are at RS-232C acceptable
levels (that's a whole different discussion). Other cables (eg,
the Yaesu one for the Yaesu FT-897D) have a little circuit board
in the RS-232C/DB-9 shell that does level conversion (and needs
power, except for the few radios where the cable provides the
power). Still other cables, like the ones that split out TX and
RX signals from a single wire, have additional circuitry for that
purpose.<br>
</p>
<p>Both of the latter cases must get their power from the DTR and/or
RTS lines from the connected computer. These signal lines were
never intended to provide power, but since modern circuits draw so
little power, using DTR and/or RTS for power doesn't create an
issue if done correctly.<br>
</p>
<p>As I noted in the feature request, some cables want both DTR and
RTS on, and use ground for the voltage reference base. Others
want RTS low and DTR high, and some want the reverse.<br>
</p>
<p><br>
</p>
<p><br>
<br>
</p>
</body>
</html>