National Instruments 320682C Using RS-485, Reporting RS-232 Errors, XModem File Transfer Functions, Troubleshooting

Chapter 5 RS-232 Library

Using RS-485

You can use all of the functions in the RS-232 Library with the National Instruments RS-485

AT-Serial board. The ComSetEscape function allows you to control the transceiver mode of

the board.

Reporting RS-232 Errors

The functions in the RS-232 Library return negative values when an error occurs. In addition, the

global variable rs232err is updated after each function call to the RS-232 Library. If the

function executes properly, it sets rs232err to zero. Otherwise, it sets rs232err to the same

error code that it returns. A list of the possible error conditions that can occur while using the

RS-232 Library functions are at the end of this chapter.

XModem File Transfer Functions

With the XModem functions, you can transfer files using a data transfer protocol. The protocol

uses a generally accepted technique for serial file transfers with error-checking. Files transfer

packets that contain data from the files plus error-checking and synchronization information.

You do not need to understand the protocol to use the functions. To transfer a file, open the com

port, use the XModemSend function on the sender side of the transfer and the

XModemReceive function on the receiver side of the transfer, and then close the com port.

The XModem functions handle all aspects of the transfer protocol.

You can treat the XModem functions as higher-level functions that perform a more precisely

defined task than the functions ComToFile and ComFromFile. Use ComToFile and

ComFromFile if you need finer control over the file operations. Remember that the Xmodem

functions calculate the check sum and retransmit when an error is detected, whereas

ComToFile and ComFromFile do not do so.

Troubleshooting

Establishing communication between two RS-232 devices can be difficult because of the many

different possible configurations. When using this library, you must know the device

requirements, such as baud rate, parity, number of data bits, and number of stop bits. Basically,

these configurations must match between the two parties of communication.

If you encounter difficulty in establishing initial communication with the device, refer to an

elementary RS-232 communications handbook for information about cable requirements and

general RS-232 communication. Refer also to the section RS-232 Cable Information later in this

chapter.