NCR 2900/260 LF User Manual

NCR 2900/260 LF User Manual Manual pdf 18 page image

9CommunicationsThis terminal is equipped with three communications ports. The two serial ports are intended for connection toeither a host (or modem), an alternate input device such as a scanner, or a serial printer. (The EIA port has bothan RJ45 and a DB25 interface; either, but not both, can be used.) The last port is a Centronics-compatibleprinter port which could be used with a parallel printer or other output device (such as a cash drawer) connectedto a parallel interface. The port you use as the printer port depends on whether you have a serial or a parallelinterface to the printer. Another factor affecting your port selection is whether the terminal is being used in asingle or dual host environment.Either “EIA” serial port (DB25 or RJ45) can communicate with your computer or printer at a baud rate of 110 to115,200 baud (bits per second). The second host/printer serial “Aux” port can communicate at speeds from 110baud to 38,400 baud. Any of these ports can be used to connect to a host computer/modem or serial printer.Receive Flow ControlBecause devices can receive data faster than they can process it, data flow control (selectable in the PORTSmenu of Setup) should be used to prevent data loss. Software flow control relies on the Xon and Xoff characters(“g” and “e” characters in PC-Term mode) to indicate when the terminal is able or unable to store further data.The Xon signal transmits the DC1 character (11h) and the Xoff signal transmits DC3 (13h).This terminal has a receive buffer capable of holding 256 bytes (or characters). When the buffer becomes half full(128 bytes), and “EIA (or Aux) Rcv” in the Setup menu for PORTS is “Xon-Xoff,” the terminal issues an Xoffcharacter, indicating to the host that it should stop transmitting data. The terminal will then continue to processdata until its receive buffer holds only 32 bytes. It will then issue an Xon character to the host, indicating that itcan resume sending data to the terminal.If the “EIA (or Aux) Rcv” is “No Protocol” in Setup, the terminal will continue to accept characters into itsreceive buffer until it is full. Additional characters will be lost. Xon-Xoff protocol must also be set on the hostcomputer or printer for proper handshaking.In addition to software “receive” flow control (Xon-Xoff), the serial host/printer ports support “receive” hardwareflow control. The EIA ports have an outgoing DTR (Data Terminal Ready) signal. If the DTR signal on theterminal is low and “EIA Recv” is “DTR” in Setup, then the serial device will stop sending to the terminal. On theAUX port, the incoming DSR (Data Set Ready) signal serves to signal the device that the terminal is not ready toreceive more data, if “Aux Recv” is “DSR” in Setup.Transmit Flow ControlLikewise, the terminal will understand the Xon and Xoff requests from the host when it is transmitting data(provided the “EIA (or Aux) Xmt” is “Xon-Xoff” in Setup). This is referred to as “transmit” flow control.This terminal will stop transmitting data to the host or printer when it receives an Xoff (DC3) code. If, however,the terminal needs to send a receive protocol character, it will transmit that character even if it has received anXoff code. When the terminal stops transmitting, the data will be buffered in the transmit buffer (64 charactersfor the first host/printer port and 64 characters for the second host/printer port).Once the buffer is full, additional keyboard data will be lost. When an Xon (DC1) character is received, theterminal can again send data to the attached serial device.In addition to software “transmit” flow control (Xon-Xoff), the serial host/printer ports support “transmit”hardware flow control, whereby DSR on the EIA port and DTR on the AUX port monitor serial devices to controlthe flow of data to them (provided EIA and AUX Xmit is “DSR” and “DTR” in Setup).For parallel printers, this terminal monitors the BUSY and ERROR signals which are sent by the printer todetermine when data transmission should be stopped or resumed.Host/Printer Port 1 (DB25)This port, labeled “EIA,” located to the left of the video port, is designed for connection to the host (computer ormodem) or a serial printer via a 25-pin D-shell (DB25P) female connector. This port uses an RS-232-Ccommunication interface, is configured as a DTE (Data Terminal Equipment) device, and can operate from 110 to115,200 baud. The pins supported are shown below.