Files sent using the TRANSMIT command contain: control records, data records, and a trailer record (a special type of control record). Several control records begin the file. They are followed by data records, and finally a trailer record. An acknowledgment file is composed of only control records.

Data is actually transmitted in card image format (80-byte records). However, the record descriptions that follow are those of ‘logical records’; card boundaries are ignored. Each control record begins in the byte immediately following the end of the previous record. The first data record begins in the byte immediately following the end of the last control record. The trailer control record begins in the byte immediately following the last data record.

Control records and data records have the same format. The records of the file to be transmitted are broken into segments whose format is shown in the next figure. A segment has a maximum length of 255 bytes, including the 2-byte header. If the length of a record in the file is greater than 253 bytes, the record is sent as multiple segments.

Figure 1. Format of control records and data records