Abstract: Block mode is a method of formatting data when sending it to and from the host computer. Some host computer operating systems make it difficult for the user's program to send or receive the full ASCII character set. The block mode protocol lets you send and receive messages which use the full ASCII character set (including lowercase characters and control characters), even though your host computer's operating system makes it difficult to send and receive certain ASCII characters. (Indeed, even full eight-bit binary data bytes may be sent to or from the terminal in block mode.) This is accomplished by means of a packing scheme, in which messages using the full character set are packed into character strings using a subset of that character set.Also, the block mode protocol provides error detection and automatic retransmission of bad data blocks. This lets you transfer data to and from the terminal, without errors, despite occasional noise on the communications line.