Microsoft KB Archive/94297

= Hardware Handshake for 3Server COM Ports =

Article ID: 94297

Article Last Modified on 10/31/2006



This article was previously published under Q94297



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

Keywords: KB94297

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