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Path: utzoo!linus!decvax!minow
From: minow@decvax.UUCP (Martin Minow)
Newsgroups: net.dcom
Subject: Flow Control -- revisited (again)
Message-ID: <269@decvax.UUCP>
Date: Wed, 9-Nov-83 13:31:09 EST
Article-I.D.: decvax.269
Posted: Wed Nov  9 13:31:09 1983
Date-Received: Thu, 10-Nov-83 10:26:40 EST
Organization: DEC UNIX Engineering Group
Lines: 43

Here is the communication line strategy we are using in
DECtalk.

The communications line input buffer (before any command
parsing) is 256 + 64 bytes long.

When 64 bytes have been stored in the buffer, the terminal
device handler sends XOFF to the host.  There is then room
for about 1/4 second of data at the maximum line data rate
of 9600 Baud.

When there are fewer than 16 bytes left in the buffer, the
device handler sends XON to the host.

The NUL character (Ascii 00) is discarded immediately on input;
it is never stored in the buffer.

Sending XOFF/XON is done outside the normal output buffering
mechanism -- if they are to be sent, they will be the next
character written to the device UART.

Running along side of all of this is a finite state machine
that watches the modem control inputs (CTS, DCD, DSR) and generates
the appropriate modem control output signals (RTS, DTR). This machine
is normally driven by change of state interrupts from the UART;
however, it is also driven by timers in some cases (to handle the
2 second minimum DSR true/DSR false/DSR true requirement of modems
in the UK, and the 500 ms. and 30 sec. disconnect watchdogs required
to prevent screwing up modems in Germany. A combination of change
of state interrupts and a timer handle the 2 second DCD negated
before disconnect requirement.

Modem control signals (DTR, DSR) are NEVER used for flow control,
because this will hang up any device that properly implements the
CCITT modem control standards.  Sadly, many devices in the United
States do not work properly.


Martin Minow		Dave Conroy
decvax!minow		decvax!dgc

PS: JSOL, feel free to reprint this in TELECOM.