VT102 User Guide| Chapter 5 | Contents | Chapter 7 |
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.
NOTE: See Chapter 8 for a description of the 20 mA current loop interface.
The VT102 communication (modem) interface is a DB-25 male connector mounted on the back of the terminal (Figure 6-1). 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.
The terminal connects to a computer through a common carrier facility (telephone line) or directly (Figure 6-2). 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.
The terminal modem must be compatible with the computer modem. Table 6-1 lists the possible modems for public switched telephone lines. Table 6-2 lists the possible modem configurations for dedicated telephone lines.
NOTE: These tables list examples of possible modem configurations. You can use equivalent modems supplied by other manufacturers.
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.
Compare the modem's interface information to the interface information provided under Modem Control in this chapter. Determine which lines are not used and disconnect these signals by using the internal communication switches.
You must select these features to match the type of communication used by the computer. For a description of communication features, see Modem Serial Characters, Break, and Modem Control in this chapter. See Chapter 3 for more information about SET-UP communication features.
The terminal has a set of ten internal communication switches. These switches allow some communication signals to disconnect from the modem connector.
These switch settings usually stay at their factory settings. However, you must configure these switches for some modems. Figure 6-3 shows the communication signals which have switches.
NOTE: The internal communication switches are ignored when using 20 mA current loop communication.
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. Figure 6-4 shows the serial character format.
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).
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.
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.
The stop bits SET-UP feature selects the number of stop bits (one or two) in a character.
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.
If you hold down SHIFT and press BREAK, 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.
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. Table 6-3 describes these selections and their usual applications.
The modem control feature must be compatible with the computer and modem. Table 6-4 matches feature selections and possible modems. You can use equivalent modems supplied by other manufacturers.
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.
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.
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. Table 6-5 lists the signals used in FDX A. The terminal uses only the connector signals listed.
NOTE: FDX A is used with the 20 mA current loop option (VT1XX-CA).
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.
The terminal prepares to connect to the computer immediately after DTR turns off. The terminal prepares to communicate by:
The terminal disconnects (hangs up) the communication (phone) line by turning DTR off. A disconnect occurs when the terminal:
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. Table 6-6 lists the signals used in FDX B. The terminal uses only the connector signals listed.
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.
The terminal prepares to connect to the computer when DSR turns on. The terminal prepares to communicate by:
The terminal hangs up the communication phone line by turning DTR off. A disconnect occurs when the terminal:
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. Table 6-7 lists the signals used in FDX C. The terminal uses only the connector signals listed.
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.
The terminal prepares to connect to the computer when DSR turns on. The terminal prepares to communicate by:
The terminal hangs up phone line by turning DTR off. A disconnect occurs when the terminal:
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.
NOTE: In half-duplex, characters typed on the keyboard do not appear immediately on the screen unless you selected local echo.
This selection lets the computer control line turnarounds by using the secondary channel control signals. The secondary channel does not transfer data. Table 6-8 lists the signals used in HDX A. The terminal uses only the connector signals listed.
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:
After the terminal connects to the computer, the secondary channel modem control signals control line turnaround. The terminal receives data as follows.
| Computer | Terminal |
|---|---|
| Computer turns secondary request to send (SRTS) off. | |
| This turns secondary receive line signal detector (SRLSD) off at terminal. Terminal turns request to send (RTS) off. (CTS turns off). |
|
| This turns RLSD off at computer. Computer turns RTS on. |
|
| This turns RLSD on at terminal. Terminal turns SRTS on. |
|
| This turns SRLSD on at computer. | |
| Terminal receives characters transmitted by computer. |
The terminal transmits data as follows.
| Computer | Terminal |
|---|---|
| Computer turns RTS off and SRTS on. | |
| This turns RLSD off and turns SRLSD on at terminal. Terminal then turns SRTS off and RTS on. |
|
| This turns SRLSD off and RLSD on at computer. | |
| Terminal waits for CTS to turn on, allowing transmission of characters to computer. |
Table 6-9 summarizes the modem control signal conditions when transmitting and receiving characters.
The terminal hangs up the phone line by turning DTR off. A disconnect occurs when the terminal:
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. Table 6-10 lists the signal used in HDX B. The terminal uses only the connector signals listed.
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:
or
If you set the line turnaround SET-UP feature for manual, you must select the turnaround character manually.
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:
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.
The terminal receives data as follows.
| Computer | Terminal |
|---|---|
| Computer turns RTS on. | |
This turns RLSD on at terminal. Terminal's RTS is off; therefore, CTS to terminal is off. Terminal receives characters from computer. |
The terminal transmits characters as follows.
| Computer | Terminal |
|---|---|
| Terminal turns RTS on. | |
| This turns RLSD on at computer. | |
| This turns CTS on at terminal. Terminal can transmit characters. |
Table 6-11 summarizes the modem control signal conditions when transmitting and receiving characters.
The terminal hangs up the phone line by turning DTR off. A disconnect occurs when the terminal:
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.
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 (![[]](char141.png){kind=small}
). There are three
methods of input buffer overflow prevention:
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 (). In these
cases, use a lower baud rate.
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.
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.
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.
Use the following formulas to determine how fast the computer must respond to the first XOFF character to avoid input buffer overflow.
C = 96 - [3 × (receive speed / transmit speed)]
R = C × (bits per char. + parity bit + no. of stop bits + 1) / receive speed
where
C = number of characters to overflow
R = response time to XOFF (seconds)
Example 1 - 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.
C = 96 - [3 × (1200 / 1200)] = 93 characters
R = 93 × (8 + 0 + 1 + 1) / 1200 = 0.775 seconds
Example 2 - 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.
C = 96 - [3 × (300 / 300)] = 93 characters
R = 93 × (7 + 1 + 1 + 1) / 300 = 3.1 seconds
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.
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.
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. Table 6-12 lists the number of fill characters used when transmitting to the terminal.
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.
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.
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.
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.
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. Table 6-13 lists recommended DIGITAL printers, printer options, and cables.
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.
The VT102's printer interface is a DB-25 male connector mounted on the back of the terminal (Figure 6-5). 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. Table 6-14 lists pin assignments for printer interface signals. The terminal uses only the connector signals listed.
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.
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. Figure 6-4 shows the serial character format.
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).