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diff --git a/DOCS/vt102-ug/chapter6.html b/DOCS/vt102-ug/chapter6.html new file mode 100644 index 0000000..3c30015 --- /dev/null +++ b/DOCS/vt102-ug/chapter6.html @@ -0,0 +1,644 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> +<HTML> +<HEAD> +<TITLE>VT100.net: Digital VT102 User Guide</TITLE> +<LINK HREF="vt102-ug.css" TYPE="text/css" REL="stylesheet"> +</HEAD> +<BODY> +<DIV CLASS="navbar"><A HREF="http://vt100.net/"><IMG CLASS="button" SRC="vt100.net-logo.png" ALT="VT100.net" HEIGHT="16" WIDTH="102"></A> VT102 User Guide<TABLE WIDTH="100%"> +<COL SPAN="3" WIDTH="33%"> +<TBODY> +<TR> +<TD ALIGN="LEFT"><A HREF="chapter5.html">Chapter 5</A></TD> +<TD ALIGN="CENTER"><A HREF="contents.html">Contents</A></TD> +<TD ALIGN="RIGHT"><A HREF="chapter7.html">Chapter 7</A></TD> +</TR> +</TBODY> +</TABLE> +<HR></DIV> +<H1 ID="S6"><SPAN CLASS="chapnum">6</SPAN> Communication</H1> +<H2 ID="S6.1">General</H2> +<P>This chapter describes how the VT102 communicates with a computer +and optional serial printer. The chapter's main sections cover the two interfaces +used, modem and printer. The text provides a detailed description +of each connector and its signals. This chapter also describes the +terminal's communication features.</P> +<P CLASS="note">NOTE: See <A HREF="chapter8.html">Chapter 8</A> for a description of the 20 mA current loop interface.</P> +<H2 ID="S6.2">Connecting to the Computer</H2> +<P>The VT102 communication (modem) interface is a DB-25 male connector +mounted on the back of the terminal (<A HREF="figure6-1.html">Figure 6-1</A>). This interface meets +Electronic Industry Association (EIA) standards RS-423 and RS-232-C, +and International Telegraph and Telephone Consultative Committee +(CCITT) recommendations V.24 and V.28.</P> +<P>The terminal connects to a computer through a common carrier facility +(telephone line) or directly (<A HREF="figure6-2.html">Figure 6-2</A>). Using a telephone line requires +modems. Modems change characters transmitted between the terminal +and computer into signals transmitted over the telephone line. There are +several types of modems available. When using modems to connect the +terminal to the computer, you must select the following items.</P> +<DL> +<DT>A modem</DT> +<DD><P>The terminal modem must be compatible with the computer +modem. <A HREF="table6-1.html">Table 6-1</A> lists the possible modems for public switched telephone +lines. <A HREF="table6-2.html">Table 6-2</A> lists the possible modem configurations for +dedicated telephone lines.</P> +<P CLASS="note">NOTE: These tables list examples of possible modem configurations. +You can use equivalent modems supplied by other manufacturers.</P> +<P CLASS="note">DIGITAL DF02-AA and DF03-AA modems are equivalent to Bell 103J +and 212A. DF02 operates at 300 baud; DF03 operates and 300 or 1200 baud.</P></DD> +<DT>The terminal's internal communication switches</DT> +<DD><P>Compare the +modem's interface information to the interface information provided +under <A HREF="#S6.2.4">Modem Control</A> in this chapter. Determine which lines are not +used and disconnect these signals by using the internal communication switches.</P></DD> +<DT>The communication SET-UP features</DT> +<DD><P>You must select these features +to match the type of communication used by the computer. For a description +of communication features, see <A HREF="#S6.2.2">Modem Serial Characters</A>, +<A HREF="#S6.2.3">Break</A>, and <A HREF="#S6.2.4">Modem Control</A> in this chapter. See <A HREF="chapter3.html">Chapter 3</A> for more information +about SET-UP communication features.</P></DD> +</DL> +<H3 ID="S6.2.1">Communication Switch Settings</H3> +<P>The terminal has a set of ten internal communication switches. These +switches allow some communication signals to disconnect from the +modem connector.</P> +<P>These switch settings usually stay at their factory settings. However, you +must configure these switches for some modems. <A HREF="figure6-3.html">Figure 6-3</A> shows the +communication signals which have switches.</P> +<P CLASS="note">NOTE: The internal communication switches are ignored when using 20 +mA current loop communication.</P> +<H3 ID="S6.2.2">Modem Serial Characters</H3> +<P>The terminal communicates with the computer by transmitting serial characters. +Serial characters have a start bit, seven or eight data bits, an optional +parity bit, and one or two stop bits. <A HREF="figure6-4.html">Figure 6-4</A> shows the serial character +format.</P> +<P>The modem data/parity bits SET-UP feature selects the number of data +bits per character and type of parity. With 8-bit characters, the terminal +forces the last data bit to the space (0) condition, and ignores the last +data bit when received. Data bits are transmitted with the least significant +bit first. (See ANSI X3.15-1976 for details on serial character format).</P> +<P>The terminal uses the parity bit to detect transmission errors in both transmitted +and received characters. If you do not select parity, the terminal +cannot transmit or check parity bits.</P> +<P>If you select parity, the receive parity SET-UP feature determines if the +terminal checks or ignores the parity of received characters. However, +the terminal only checks received characters for odd or even parity, not +mark and space parity.</P> +<P>The stop bits SET-UP feature selects the number of stop bits (one or two) +in a character.</P> +<H3 ID="S6.2.3">Break</H3> +<P>Break is a space condition transmitted for 0.275 seconds ± 10 percent. +The break enable SET-UP feature enables or disables a break. When receiving +in half-duplex with supervisory control (HDX A), the terminal performs +a break by turning secondary request to send (SRTS) off for 0.275 +seconds ± 10 percent. The computer response to the break depends on +the computer and software.</P> +<P>If you hold down <STRONG>SHIFT</STRONG> and press <STRONG>BREAK</STRONG>, the terminal generates a disconnect. +This causes the terminal to turn off the data terminal ready +(DTR) and request to send (RTS) signals. The transmit data line (TXD) is +held in a mark condition. After 0.22 seconds, the terminal tests the condition +of data set ready (DSR). When DSR turns off or after 1.8 seconds, the +disconnect is complete. When the disconnect character enable SET-UP +feature is on, the terminal transmits the disconnect character before the +DTR and RTS signals turn off. The disconnect character is selected by the +turnaround/disconnect SET-UP feature.</P> +<H3 ID="S6.2.4">Modem Control</H3> +<P>The terminal can communicate in full- or half-duplex, selected by the +modem control SET-UP feature. This feature has five selections, three for +full-duplex and two for half-duplex. <A HREF="table6-3.html">Table 6-3</A> describes these selections +and their usual applications.</P> +<P>The modem control feature must be compatible with the computer and +modem. <A HREF="table6-4.html">Table 6-4</A> matches feature selections and possible modems. You +can use equivalent modems supplied by other manufacturers.</P> +<P CLASS="note">NOTE: In several countries outside North America, communications +equipment must be certified before being connected to modems provided +by the local telecommunications authority (for example, German FTZ, +Sweden PTT, and England BT). In those countries, contact your local +Field Service office for more information about certification. In many +cases, DIGITAL has already provided the needed information to the local +telecommunication authority.</P> +<H3 ID="S6.2.5">Full-Duplex Communication</H3> +<P>Full-duplex communication lets you transmit and receive characters in +both directions at the same time. The terminal has three types of +full-duplex communication available: without modem control, with modem +control, and asymmetric full-duplex. All modem control selections except +full-duplex with no modem control (FDX A) use modem control signals during +communication. Modem control signals verify that the terminal and +computer are connected before communicating.</P> +<H4 ID="S6.2.5.1">Full-Duplex with No Modem Control (FDX A)</H4> +<P>This selection lets the +terminal communicate with the computer regardless of the received +modem control signals. The terminal assumes it is connected to the computer. +<A HREF="table6-5.html">Table 6-5</A> lists the signals used in FDX A. The terminal uses only +the connector signals listed.</P> +<P CLASS="note">NOTE: FDX A is used with the 20 mA current loop option (VT1XX-CA).</P> +<H4 ID="S6.2.5.2">FDX A Connection</H4> +<P>The terminal assumes it is connected to the computer +when the DTR signal is on. Communication is allowed regardless of +the condition of other modem control lines. DTR is on at all times except +when the terminal is off-line or performing a long break disconnect.</P> +<P>The terminal prepares to connect to the computer immediately after DTR +turns off. The terminal prepares to communicate by:</P> +<UL> +<LI>Erasing the keyboard buffer and turning off KBD LOCKED (if a keyboard +locked condition occurred)</LI> +<LI>Stopping print operations and ignoring new print operation requests</LI> +<LI>Using the character set selected by the US/UK character set SET-UP +feature. (Computer-selected character sets are not used).</LI> +<LI>Assuming XON if you selected auto XON/XOFF. (Transmission is allowed +in both directions).</LI> +</UL> +<H4 ID="S6.2.5.3">FDX A Disconnection</H4> +<P>The terminal disconnects (hangs up) the communication +(phone) line by turning DTR off. A disconnect occurs when the +terminal:</P> +<UL> +<LI>Switches off-line</LI> +<LI>Performs a recall, reset, or general default</LI> +<LI>Transmits a long break disconnect using <STRONG>SHIFT</STRONG> and <STRONG>BREAK</STRONG>. (Transmits +a disconnect character if the disconnect character enable SET-UP +feature is on).</LI> +<LI>Receives a disconnect character with the disconnect character enable +SET-UP feature on.</LI> +</UL> +<H4 ID="S6.2.5.4">Full-Duplex with Modem Controls (FDX B)</H4> +<P>This selection allows communication +when the terminal receives the correct modem control signals. +These signals verify the connection between the terminal and computer +before and during communication. The terminal cannot communicate without +this connection. <A HREF="table6-6.html">Table 6-6</A> lists the signals used in FDX B. The terminal +uses only the connector signals listed.</P> +<H4 ID="S6.2.5.5">FDX B Connection</H4> +<P>Before communicating, the terminal verifies the connection +to the computer by using modem control signals. The clear to +send (CTS), receive line signal detector (RLSD) and data set ready (DSR) +signals must be on, or communication is not permitted.</P> +<P>The terminal prepares to connect to the computer when DSR turns on. +The terminal prepares to communicate by:</P> +<UL> +<LI>Erasing the keyboard buffer and turning off KBD LOCKED (if a keyboard +locked condition occurred)</LI> +<LI>Stopping print operations and ignoring new print operation requests</LI> +<LI>Using the character set selected by the US/UK character set SET-UP +feature. (Computer-selected character sets are not used).</LI> +<LI>Selecting numeric keypad mode. (The keypad generates numeric +characters. The cursor keys transmit cursor movement commands).</LI> +<LI>Selecting replace mode. (Turning off insert mode. INSERT is off and +received characters appear at cursor position, replacing old display +character).</LI> +<LI>Assuming XON if you selected auto XON/XOFF. (Transmission is allowed +in both directions).</LI> +</UL> +<H4 ID="S6.2.5.6">FDX B Disconnection</H4> +<P>The terminal hangs up the communication phone +line by turning DTR off. A disconnect occurs when the terminal:</P> +<UL> +<LI>Switches off-line</LI> +<LI>Performs a recall, reset, or general default</LI> +<LI>Receives a disconnect character with the disconnect character enable +SET-UP feature on</LI> +<LI>Transmits a long break disconnect using <STRONG>SHIFT</STRONG> and <STRONG>BREAK</STRONG>. (Transmits +a disconnect character if the disconnect character enable SET-UP +feature is on).</LI> +<LI>Loses DSR during communication</LI> +<LI>During a connection, does not receive RLSD within 30 seconds after +DSR</LI> +<LI>After a connection, loses RLSD for a greater time than allowed by the +disconnect delay SET-UP feature.</LI> +</UL> +<H4 ID="S6.2.5.7">Asymmetric Full-Duplex (FDX C)</H4> +<P>This is full-duplex communication using +a half-duplex modem with a secondary channel. The terminal receives +characters on the primary channel at 600 or 1200 baud. It transmits characters +on the secondary channel at 75 baud. You use the internal communication +switches to configure the terminal to use a secondary channel. +<A HREF="table6-7.html">Table 6-7</A> lists the signals used in FDX C. The terminal uses only the connector +signals listed.</P> +<H4 ID="S6.2.5.8">FDX C Connection</H4> +<P>Before communicating, the terminal verifies the connection +to the computer by using modem control signals. The secondary +clear to send (SCTS), RLSD, and DSR signals must be on, or communication +is not permitted.</P> +<P>The terminal prepares to connect to the computer when DSR turns on. +The terminal prepares to communicate by:</P> +<UL> +<LI>Erasing the keyboard buffer and turning off KBD LOCKED (if a keyboard +locked condition occurred)</LI> +<LI>Stopping print operations and ignoring new print operation requests</LI> +<LI>Using the character set selected by the US/UK character set SET-UP +feature. (Computer-selected character sets are not used).</LI> +<LI>Selecting numeric keypad mode. (The keypad generates numeric +characters. The cursor keys transmit cursor movement commands).</LI> +<LI>Selecting replace mode. (Turning off insert mode. INSERT is off and +all received characters appear at cursor position, replacing old display +character).</LI> +<LI>Assuming XON if you selected auto XON/XOFF. (Transmission is allowed +in both directions).</LI> +</UL> +<H4 ID="S6.2.5.9">FDX C Disconnection</H4> +<P>The terminal hangs up phone line by turning DTR +off. A disconnect occurs when the terminal:</P> +<UL> +<LI>Switches off-line</LI> +<LI>Performs a recall, reset, or general default</LI> +<LI>Receives a disconnect character with the disconnect character enable +SET-UP feature on</LI> +<LI>Transmits a long break disconnect using <STRONG>SHIFT</STRONG> and <STRONG>BREAK</STRONG>. (Transmits +a disconnect character if the disconnect character enable SET-UP +feature is on).</LI> +<LI>Loses DSR during communication</LI> +<LI>During a connection, does not receive RLSD within 30 seconds after +DSR</LI> +<LI>After a connection, loses RLSD for a greater time than allowed by the +disconnect delay SET-UP feature.</LI> +</UL> +<H3 ID="S6.2.6">Half-Duplex Communication</H3> +<P>Half-duplex lets you transmit and receive communication characters in +both directions, one direction at a time. Therefore, you need a method to +control the direction of the communication line. The line direction determines +whether the terminal transmits or receives characters. Every time +the transmitting device wants to receive, the line must be turned around. +The terminal uses two types of half-duplex communication to control line +direction, supervisory control or coded control.</P> +<P CLASS="note">NOTE: In half-duplex, characters typed on the keyboard do not appear +immediately on the screen unless you selected local echo.</P> +<H4 ID="S6.2.6.1">Half-Duplex with Supervisory Control (HDX A)</H4> +<P>This selection lets the computer control line turnarounds by using the secondary channel control +signals. The secondary channel does not transfer data. <A HREF="table6-8.html">Table 6-8</A> lists the +signals used in HDX A. The terminal uses only the connector signals listed.</P> +<H4 ID="S6.2.6.2">HDX A Connection</H4> +<P>Before communicating, the terminal verifies the connection +to the computer by using DSR. The terminal prepares to connect +to the computer when DSR turns on. The terminal prepares to communicate +by:</P> +<UL> +<LI>Erasing the keyboard buffer and turning off KBD LOCKED (if a keyboard +locked condition occurred)</LI> +<LI>Stopping print operations and ignoring new print operation requests</LI> +<LI>Using the character set selected by the US/UK character set SET-UP +feature. (Computer-selected character sets are not used).</LI> +<LI>Selecting numeric keypad mode. (The keypad generates numeric +characters. The cursor keys transmit cursor movement commands).</LI> +<LI>Selecting replace mode. (Turning off insert mode. INSERT is off and +all received characters appear at the cursor position, replacing old +display character).</LI> +<LI>Selecting the initial direction of transmission by using the initial direction +SET-UP feature.</LI> +</UL> +<H4 ID="S6.2.6.3">HDX A Character Transmission and Reception</H4> +<P>After the terminal connects +to the computer, the secondary channel modem control signals +control line turnaround. The terminal receives data as follows.</P> +<TABLE> +<COL SPAN="2" WIDTH="50%"> +<THEAD> +<TR> +<TH>Computer</TH> +<TH>Terminal</TH> +</TR> +</THEAD> +<TBODY> +<TR> +<TD>Computer turns secondary request to send (SRTS) off.</TD> +<TD></TD> +</TR> +<TR> +<TD></TD> +<TD>This turns secondary receive line signal detector (SRLSD) off at terminal.<BR> +Terminal turns request to send (RTS) off. (CTS turns off).</TD> +</TR> +<TR> +<TD>This turns RLSD off at computer.<BR> +Computer turns RTS on.</TD> +<TD></TD> +</TR> +<TR> +<TD></TD> +<TD>This turns RLSD on at terminal.<BR> +Terminal turns SRTS on.</TD> +</TR> +<TR> +<TD>This turns SRLSD on at computer.</TD> +<TD></TD> +</TR> +<TR> +<TD></TD> +<TD>Terminal receives characters transmitted by computer.</TD> +</TR> +</TBODY> +</TABLE> +<P>The terminal transmits data as follows.</P> +<TABLE> +<COL SPAN="2" WIDTH="50%"> +<THEAD> +<TR> +<TH>Computer</TH> +<TH>Terminal</TH> +</TR> +</THEAD> +<TBODY> +<TR> +<TD>Computer turns RTS off and SRTS on.</TD> +<TD></TD> +</TR> +<TR> +<TD></TD> +<TD>This turns RLSD off and turns SRLSD on at terminal.<BR> +Terminal then turns SRTS off and RTS on.</TD> +</TR> +<TR> +<TD>This turns SRLSD off and RLSD on at computer.</TD> +<TD></TD> +</TR> +<TR> +<TD></TD> +<TD>Terminal waits for CTS to turn on, allowing transmission of characters to computer.</TD> +</TR> +</TBODY> +</TABLE> +<P><A HREF="table6-9.html">Table 6-9</A> summarizes the modem control signal conditions when transmitting +and receiving characters.</P> +<H4 ID="S6.2.6.4">HDX A Disconnection</H4> +<P>The terminal hangs up the phone line by turning +DTR off. A disconnect occurs when the terminal:</P> +<UL> +<LI>Switches off-line</LI> +<LI>Performs a recall, reset, or general default</LI> +<LI>Receives a disconnect character with the disconnect character enable +SET-UP feature on</LI> +<LI>Transmits a long break disconnect using <STRONG>SHIFT</STRONG> and <STRONG>BREAK</STRONG>. (Transmits +a disconnect character if the disconnect character enable SET-UP +feature is on).</LI> +<LI>Loses DSR during communication</LI> +<LI>Line turnaround does not complete within 5 seconds.</LI> +</UL> +<H4 ID="S6.2.6.5">Half-Duplex with Coded Control (HDX B)</H4> +<P>This selection lets the transmitting +device control line turnarounds with a turnaround character specified +by the turnaround/disconnect character SET-UP feature. The turnaround +character indicates that a line should turn around. The +transmitting device uses modem control lines to perform line turnarounds. +<A HREF="table6-10.html">Table 6-10</A> lists the signal used in HDX B. The terminal uses only the connector +signals listed.</P> +<P>The terminal uses ETX and EOT for turnaround characters. You can also +select DC3, FF, and CR; however, these characters do not comply with +ANSI X3.4-1977. If you set the line turnaround SET-UP feature for auto, +the line automatically turns around when the terminal transmits:</P> +<UL> +<LI>The answerback message</LI> +<LI>A cursor position report</LI> +<LI>A device status report</LI> +<LI>A device attributes</LI> +</UL> +<P>or</P> +<UL> +<LI>You press <SPAN CLASS="keyname">RETURN</SPAN>. If the turnaround character is CR, only one CR +character is transmitted.</LI> +</UL> +<P>If you set the line turnaround SET-UP feature for manual, you must select +the turnaround character manually.</P> +<H4 ID="S6.2.6.6">HDX B Connection</H4> +<P>Before communicating, the terminal verifies the connection +to the computer by using DSR. The terminal prepares to connect +to the computer when DSR turns on. The terminal prepares to communicate +by:</P> +<UL> +<LI>Erasing the keyboard buffer and turning off KBD LOCKED (if a keyboard +locked condition occurred)</LI> +<LI>Stopping print operations and ignoring new print operation requests</LI> +<LI>Using the character set selected by the US/UK character set SET-UP +feature. (Computer-selected character sets are not used).</LI> +<LI>Selecting numeric keypad mode. (The keypad generates numeric +characters. The cursor keys transmit cursor movement commands).</LI> +<LI>Selecting replace mode. (Turning off insert mode. INSERT is off and +all received characters appear at the cursor position, replacing old +display character).</LI> +<LI>Selecting the initial direction of transmission by using the initial direction +SET-UP feature.</LI> +</UL> +<H4 ID="S6.2.6.7">HDX B Character Transmission and Reception</H4> +<P>During a communication +line connection, the initial direction SET-UP feature selects which +device transmits. The transmitting device determines when the communication +line turns around. Line turnarounds begin with the transmitting +device sending a turnaround character. Both devices then change the +condition of RTS. This causes a line turnaround.</P> +<P>The terminal receives data as follows.</P> +<TABLE> +<COL SPAN="2" WIDTH="50%"> +<THEAD> +<TR> +<TH>Computer</TH> +<TH>Terminal</TH> +</TR> +</THEAD> +<TBODY> +<TR> +<TD>Computer turns RTS on.</TD> +<TD></TD> +</TR> +<TR> +<TD></TD> +<TD><P>This turns RLSD on at terminal.</P> +<P>Terminal's RTS is off; therefore, CTS to terminal is +off. Terminal receives characters from computer.</P></TD> +</TR> +</TBODY> +</TABLE> +<P>The terminal transmits characters as follows.</P> +<TABLE> +<COL SPAN="2" WIDTH="50%"> +<THEAD> +<TR> +<TH>Computer</TH> +<TH>Terminal</TH> +</TR> +</THEAD> +<TBODY> +<TR> +<TD></TD> +<TD>Terminal turns RTS on.</TD> +</TR> +<TR> +<TD>This turns RLSD on at computer.</TD> +<TD></TD> +</TR> +<TR> +<TD></TD> +<TD>This turns CTS on at terminal. Terminal can transmit characters.</TD> +</TR> +</TBODY> +</TABLE> +<P><A HREF="table6-11.html">Table 6-11</A> summarizes the modem control signal conditions when transmitting +and receiving characters.</P> +<H4 ID="S6.2.6.8">HDX B Disconnection</H4> +<P>The terminal hangs up the phone line by turning +DTR off. A disconnect occurs when the terminal:</P> +<UL> +<LI>Switches off-line</LI> +<LI>Performs a recall, reset, or general default</LI> +<LI>Receives a disconnect character with the disconnect character enable +SET-UP feature on</LI> +<LI>Transmits the long break disconnect using <SPAN CLASS="keyname">SHIFT</SPAN> and <SPAN CLASS="keyname">BREAK</SPAN>. +(Transmits a disconnect character if the disconnect character enable +SET-UP feature is on).</LI> +<LI>Loses DSR during communication</LI> +<LI>Loses RLSD for more than 5 seconds without receiving a turnaround +character</LI> +<LI>Line turnaround does not complete within 5 seconds.</LI> +</UL> +<H3 ID="S6.2.7">Input Buffer Overflow Prevention</H3> +<P>The terminal places all received characters (other than NUL), in a 128-character +input buffer. The input buffer holds received characters until +processed. After processing, the terminal removes characters from the input +buffer and transmits or displays them.</P> +<P>The input buffer fills when it receives characters faster than the terminal +can process them. When the buffer is full, the terminal loses received +characters and displays the substitute character (<IMG SRC="char141.png" ALT="[]" HEIGHT="20" WIDTH="12">). There are three +methods of input buffer overflow prevention:</P> +<UL> +<LI>XON/XOFF characters. (Recommended with full-duplex communication).</LI> +<LI>Fill characters</LI> +<LI>Low-speed operation.</LI> +</UL> +<P CLASS="note">NOTE: In applications using continuous 19,200 baud communication, +occasional data errors may occur. When the terminal detects errors, it replaces +the characters in error with the substitute character (<IMG SRC="char141.png" ALT="[]" HEIGHT="20" WIDTH="12">). In these +cases, use a lower baud rate.</P> +<H4 ID="S6.2.7.1">XON and XOFF Characters</H4> +<P>When you select the auto XON/XOFF, the +XON and XOFF control characters prevent input buffer overflows. These +characters indicate when this 128-character buffer is almost empty or full. +When the input buffer holds 32 characters, the terminal automatically +transmits XOFF (DC3, octal 023). The computer should stop transmitting +characters to prevent an input buffer overflow.</P> +<P>If the computer fails to respond to XOFF, the input buffer continues to fill. +The terminal transmits a second XOFF when the input buffer holds 112 +characters. This second XOFF is a last request to the computer to stop +transmitting characters.</P> +<P>The terminal continues to remove characters from the input buffer. When +the input buffer holds 16 characters, the terminal automatically transmits +XON (DC1, octal 021), requesting the computer to continue transmission.</P> +<P>Use the following formulas to determine how fast the computer must respond +to the first XOFF character to avoid input buffer overflow.</P> +<BLOCKQUOTE CLASS="equation"> +<P>C = 96 - [3 × (receive speed / transmit speed)]</P> +<P>R = C × (bits per char. + parity bit + no. of stop bits + 1) / receive speed</P> +</BLOCKQUOTE> +<P>where</P> +<BLOCKQUOTE CLASS="equation"> +<P>C = number of characters to overflow</P> +<P>R = response time to XOFF (seconds)</P> +</BLOCKQUOTE> +<P><STRONG>Example 1</STRONG> - The terminal is transmitting 8-bit characters with no parity at +1200 baud and receiving at 1200 baud. When the terminal transmits the +first XOFF, the computer must stop transmitting within 0.775 seconds or +the input buffer overflows.</P> +<BLOCKQUOTE CLASS="equation"> +<P>C = 96 - [3 × (1200 / 1200)] = 93 characters</P> +<P>R = 93 × (8 + 0 + 1 + 1) / 1200 = 0.775 seconds</P> +</BLOCKQUOTE> +<P><STRONG>Example 2</STRONG> - The terminal is transmitting 7-bit characters with parity at +300 baud and receiving at 300 baud. When the terminal transmits the first +XOFF, the computer must stop transmitting within 3.1 seconds or the input +buffer overflows.</P> +<BLOCKQUOTE CLASS="equation"> +<P>C = 96 - [3 × (300 / 300)] = 93 characters</P> +<P>R = 93 × (7 + 1 + 1 + 1) / 300 = 3.1 seconds</P> +</BLOCKQUOTE> +<P CLASS="note">NOTE: Immediately after sending the reset or invoke confidence test sequences, +the computer may assume an XOFF from the terminal. The computer +stops sending characters until it receives XON. The terminal transmits +XON only after completing the specified function.</P> +<H4 ID="S6.2.7.2">Fill Characters</H4> +<P>The computer can use fill characters (NUL, octal 000 +recommended) to prevent input buffer overflows. The terminal ignores received +null characters. Since the NUL character is not processed, the terminal +can process characters already received while receiving fill characters.</P> +<P>The computer transmits fill characters to the terminal after each control +function or display character. The number of fill characters needed depends +on the control function transmitted and the terminal receive speed. +<A HREF="table6-12.html">Table 6-12</A> lists the number of fill characters used when transmitting to the +terminal.</P> +<H4 ID="S6.2.7.3">Low-Speed Operation</H4> +<P>A slow terminal receive speed prevents input +buffer overflows. Low-speed operation lets the terminal process a character +before receiving the next character. Therefore, the input buffer is always +ready to receive characters. Use the following rules during low-speed +operation to prevent input buffer overflows.</P> +<UL> +<LI>Do not send the ESC code to the terminal.</LI> +<LI>Use terminal receive speed of 4800 baud or less.</LI> +<LI>Do not use the smooth scroll feature.</LI> +<LI>Immediately after sending the reset or invoke confidence test sequences, +use a delay of no less than 10 seconds to allow the terminal +to complete the function. This does not guarantee against loss of +characters when a test error is detected.</LI> +</UL> +<H3 ID="S6.2.8">Keyboard Transmit Buffer</H3> +<P>The keyboard transmit buffer holds characters generated by the terminal +before they are transmitted to the computer. When using auto XON/XOFF +in full-duplex, the computer can use XON (DC1, octal 021) and XOFF +(DC3, octal 023) characters to control character transmission from the +terminal.</P> +<P>After receiving XOFF, the terminal stops transmitting any characters except +XOFF and XON. A keyboard buffer stores keystrokes. If the keyboard +buffer overflows, KBD LOCKED turns on; keyclicks also stop if the keyclick +SET-UP feature is on.</P> +<P>When the terminal receives XON, the character transmission continues. +Also, entering and exiting SET-UP turns off KBD LOCKED and allows the +terminal to transmit characters. However, characters transmitted after entering +and exiting SET-UP may be lost if the computer is not ready to receive +characters. When the keyboard buffer is empty, KBD LOCKED turns +off; keyclicks return if the keyclick feature is on.</P> +<H2 ID="S6.3">Connecting to the Printer</H2> +<P>The terminal connects to a local serial printer via a printer interface. The +printer interface is full-duplex and uses the XON and XOFF control characters +to prevent input buffer overflows. <A HREF="table6-13.html">Table 6-13</A> lists recommended +DIGITAL printers, printer options, and cables.</P> +<P>When you use a serial printer, select full-duplex communication between +the terminal and computer. Also, select auto XON/XOFF to prevent input +buffer overflows. Without full-duplex communication and auto XON/XOFF, +it is impossible to guarantee correct system operation. However, a large +printer input buffer and fast printer interface speed (baud rate) decrease +the chance of problems.</P> +<H3 ID="S6.3.1">Serial Printer Interface</H3> +<P>The VT102's printer interface is a DB-25 male connector mounted on the +back of the terminal (<A HREF="figure6-5.html">Figure 6-5</A>). This interface meets Electronic Industry +Association (EIA) standards RS-423 and RS-232-C, and International +Telegraph and Telephone Consultative Committee (CCITT) recommendation +V.28. <A HREF="table6-14.html">Table 6-14</A> lists pin assignments for printer interface signals. The +terminal uses only the connector signals listed.</P> +<P>The printer interface uses full-duplex communication with auto +XON/XOFF to prevent buffer overflows. If possible, the printer should turn +off DTR when not ready to print. DTR has a higher priority than +XON/XOFF. If DTR is off, the terminal will not transmit to the printer after +receiving XON. If DTR is on, the terminal assumes XON.</P> +<H3 ID="S6.3.2">Printer Serial Characters</H3> +<P>The terminal communicates with the printer by using serial characters. +These characters have a start bit, seven or eight data bits, an optional +parity bit, and one or two stop bits. Data bits are transmitted with the least +significant bit first. See ANSI X3.15-1976 for further details on the character +format. <A HREF="figure6-4.html">Figure 6-4</A> shows the serial character format.</P> +<P>SET-UP features select the number of data bits and stop bits per character, +the parity, and the transmission speed. The printer data/parity bits +feature selects the number of data bits and type of parity. With 8-bit characters, +the terminal forces the last data bit to the space (0) condition and +ignores the last data bit when received. If you do not select parity, the +terminal cannot transmit the parity bit. The transmit/receive speed SET-UP +feature selects the number of stop bits (one or two) and speed (baud +rate).</P> +<DIV CLASS="navbar"><HR> +<TABLE WIDTH="100%"> +<COL SPAN="3" WIDTH="33%"> +<TBODY> +<TR> +<TD ALIGN="LEFT"><A HREF="chapter5.html">Chapter 5</A></TD> +<TD ALIGN="CENTER"><A HREF="contents.html">Contents</A></TD> +<TD ALIGN="RIGHT"><A HREF="chapter7.html">Chapter 7</A></TD> +</TR> +</TBODY> +</TABLE> +<DIV CLASS="navbot">http://vt100.net/docs/vt102-ug/chapter6.html</DIV></DIV> +</BODY> +</HTML> |