KnowledgeBase Archive

An Archive of Early Microsoft KnowledgeBase Articles

View on GitHub

Q94297: Hardware Handshake for 3Server COM Ports

Article: Q94297
Product(s): Microsoft LAN Manager
Version(s): 
Operating System(s): 
Keyword(s): 
Last Modified: 30-JUL-2001

SUMMARY
=======

Getting a printer to talk to a server serial port isn't always as easy as it
seems. Communication between a serial output device such as a printer (Data
Communications Equipment, or DCE) and a server (Data Termination Equipment, or
DTE) is called handshaking. This handshaking is needed for the printer to tell
the server that its buffer is full and can't accept any more data. The two most
common types of handshaking used are XON/XOFF and hardware.

MORE INFORMATION
================

XON/XOFF is software handshaking. Handshaking is accomplished by the exchange of
control codes between the printer and the server via the transmit and receive
lines, pins 2 and 3 respectively (RS232-C). When the printer's data buffer is
full, it sends an XOFF telling the server's print processor to pause output.
When the buffer has available space for more data, an XON character is sent and
the server continues to send until the next XOFF. For example, when you type a
long text document to your screen, the data scrolls off the screen too rapidly
for you to read. When you press CTRL+S, you are sending an XOFF control
character. XOFF tells the processor to pause the screen output until you've
finished reading. Output continues when the processor receives a CTRL+Q (XON).
The printer talks to the server in the same manner. This XON/XOFF handshaking
continues until the server has sent the entire spool file.

XON/XOFF handshaking is adequate for most serial communication needs and can
easily be set up on the server by selecting the correct parameters on the mode
and 3mode lines. A sample XON/XOFF mode statement is shown further below in
example 1.

Hardware handshaking uses signal voltage levels instead of control code to
communicate between DTE and DCE devices. There are several signal options that
can be used. The most common signals used are Data Terminal Ready (DTR) pin 20
on the printer, which is connected to Clear To Send (CTS) pin 5 and Data Set
Ready (DSR) pin 6 on the server. When the printer data buffer fills, the voltage
level on pin 20, DTR, changes from high to low. This tells the server to stop
sending data. When the buffer has available space for more data, the DTR voltage
level toggles high and the server continues to send until the next DTR voltage
drop.

Hardware handshaking is the default setting (see example 2 below). The minimum
parameters needed for hardware handshaking are shown in example 3. Note that DTR
can be set to either ON or HS with the same outcome.

Sample COM Port Configurations (excerpt from C:\LANMAN\NETPROG\SETMODE.CMD)
---------------------------------------------------------------------------

Example 1: XON/XOFF

  c:\os2\mode
  COM1:9600,N,8,1,TO=OFF,XON=ON,IDSR=OFF,ODSR=OFF,OCTS=OFF,DTR=OFF,RTS=OFF
  >> startup.log 2>>&1
  c:\os2\mode COM1: >> startup.log 2>>&1
  c:\3mode COM1;9600;n;8;1;0; >> c:\startup.log 2>>&1

Example 2: Hardware (default settings)

  c:\os2\mode
  COM1:9600,N,8,1,TO=OFF,XON=OFF,IDSR=ON,ODSR=ON,OCTS=ON,DTR=ON,RTS=ON
  >> startup.log 2>>&1
  c:\os2\mode COM1: >> startup.log 2>>&1
  c:\3mode COM1;9600;n;8;1;1; >> c:\startup.log 2>>&1

Example 3: Hardware (minimal settings)

  c:\os2\mode
  COM1:9600,N,8,1,TO=OFF,XON=OFF,IDSR=OFF,ODSR=ON,OCTS=OFF,DTR=ON(hs),
  RTS=OFF >> startup.log 2>>&1
  c:\os2\mode COM1: >> startup.log 2>>&1
  c:\3mode COM1:9600;n;8;1;1; >> c:\startup.log 2>>&1

Testing was done with the following equipment and settings:

3Com 3S500 server with LAN Manager 2.1, both COM 1 and COM 3 tested E-Z Interface
IBM XT - HP printer cable
HP 7550a plotter:

     Data flow = Remote standalone
     Handshake = Hardware
     Bypass = Off
     Duplex = Full
     Parity = 8 Bit, off
     Baud rate = 9600

Additional query words:

======================================================================
Keywords          :  

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.