Category : Communication (modem) tools and utilities
Archive   : LAN-DOC.ZIP
Filename : APPNOTES.003

LAN-LINK Application Notes Number 3.

By Joe Kasser G3ZCZ @ N4QQ
PO Box 3419
Silver Spring
Md 20918


Flow Control

This application note discuses the control of the flow of
characters between the Computer and the Terminal Node Controller
(TNC). The method of control of the flow of characters between
the TNC and the computer is called Flow Control. It is also used
to control the flow of characters to the computer's display.
Before looking at each of these flows, let's take a quick look at
the interface between the computer and the TNC.

The Interface.

The interface between the computer and the TNC consists of a
cable with connectors at each end. It is commonly called an RS-
232 cable, because the signals conform to the RS-232
specification (at least they are supposed to). The cable
contains one wire which carries data from the TNC to the
computer, one wire which carries data going the other way, and a
common or ground wire. This is the minimum cable. Without these
three wires, data cannot flow in both directions.

As long as the data entering either the computer or the TNC is
coming in slower than the device can process it, there is no
problem. However when the data comes in faster, it overflows and
characters can get lost. For this reason, almost every program
and TNC contains a data buffer to catch and store the characters
as they come in. This is well and good, but data still has a
habit of coming in too fast.

In order to stop the overflow from happening, flow control is
used. This is a mechanism used between the computer and the TNC
whereby one of the devices tells the other to stop sending when
it senses that its buffer is full, and then later, when its
program has depleted the buffer, tells the other device to start
sending again. This flow control can be used in both directions.

Hardware Flow Control

There are two types of flow control; hardware and software. The
RS-232 cable has special wires dedicated to hardware flow
control. Thus use of hardware flow control requires extra
conductors in the cable. In hardware flow control a 'Clear to
Send' voltage level is placed on the wire by the receiver. The
sender checks that level before sending any character, and only
sends characters as long as that signal is present. Should it
change state, the sender will wait until it reverts to the clear
to send state. Since the RS-232 cable is bidirectional, there
are two such lines, one used by each device.

The 'XFLOW' parameter is used to tell the TNC which flow control
approach to use. If XFLOW is ON, then software flow control is
used, if XFLOW if OFF, hardware flow control is in effect.

Software Flow Control

In software flow control, the signalling is done by two
characters reserved for that purpose. They are usually the
Control-S (^S) and Control-Q (^Q) characters. When the receiver
has enough data, it sends a ^S (Stop) character to the sender who
then stops transmitting and waits for a ^Q (start) character
before continuing. When the receiving device is ready for more
characters, it send a ^Q, and flow is restored.

Binary Data

Software flow control has the advantage of only needing the three
wires in the cable, but has problems when binary data is being
transmitted along that interface. The ^Q and ^S are binary
codes. As long as only ASCII text flows along the cable, binary
codes are never seen and the ^S/^Q flow control is usable. Once
binary characters flow, the receiving device cannot tell if a
received ^S is part of the data or is a flow control character.
Under these circumstances it stops receiving and waits for a ^Q,
which if binary data was flowing, may never come, and the program
seems to hang up.

The original TNC software designers recognized that such a
problem might occur, and put a filter parameter in the TNC
instructions to tell the TNC to filter the received characters in
the data stream and block various characters from being passed on
to the computer. Thus PK-232, TNC2 and MFJ1278 users could set
'MFILTER 19' to block the ^S character from being passed to the
PK-232 if it was received while monitoring an on-the-air packet.
KAM users could use the 'FILTER ON' command to achieve the same
effect.

Flow Control in LAN-LINK

All versions of LAN-LINK upto and including 1.55 used software
flow control. In the early days of packet radio there was little
if any binary data flowing on the LAN. A very few stations were
using YAPP to transfer binary files. Then along came NET/ROM
which signalled the state of the network to other nodes by means
of binary information. Stations using LAN-LINK hung up when the
binary information contained a ^S, if they had not set their TNC
to filter out the ^S before passing the received characters to
the computer. LAN-LINK version 1.56 and subsequent versions use
hardware flow control, hence they ignore the ^S and ^Q and don't
tend to hang up when monitoring packets containing binary data.

Display Flow Control

The TNC has the capability to echo back characters as they are
received on the RS-232 interface. The TNC usually has this
facility switched ON when powered up. This facility is
controlled by the 'ECHO' parameter. When 'Echo On' is set, the
TNC echoes back on the RS-232 interface any character received on
the interface. If it is receiving characters on the radio
interface at the same time, the characters will be sent down the
RA-232 line as they are received, namely mixed together. The
first dumb terminals and computer software used to interface the
TNC only had one screen or window to display. To stop the
characters being typed at the keyboard from appearing on the
screen mixed together with the characters coming in from the
radio, type-in flow control is used. The type-in flow control
parameter is 'FLOW'. If FLOW is ON, any character received on
the RS-232 interface (typed at the keyboard) will stop the TNC
from sending characters on the RS-232 interface to the computer
until you type an 'Enter character'. If you forget about the
FLOW parameter and it is ON, The TNC will not send any characters
to your screen until the TNC buffer fills up, and the system will
appear to have hung up. These days, with LAN-LINK and two
separate windows for incoming and outgoing characters, set 'FLOW
OFF'.

Summary

To make everything work properly with both flow control and type-
in flow control, make sure the following parameters are present
in your LAN-LINK.SYS file, AND THEY ARE LOADED INTO YOUR TNC.

FLOW OFF
XFLOW OFF
STOP 00
START 00
XON 00
XOFF 00

73 Joe G3ZCZ

LAN-LINK is available from the author. For an evaluation copy,
download it from Compuserve or send $5.00. On the other hand,
send $35.00 for a registered copy which will entitle you to at
least one free update. When you write in, say where you saw this
Application Note.