TPDU is an acronym for Transport Protocol Data Unit. TPDU is commonly used in the banking/financial transaction processing industries. Many vendors have used the basic concept and added their own variations. In general, the TPDU is a message encapsulation format where there are several bytes of routing header added to the front of the payload message.

A TPDU header is 5 bytes in length and consists of 1 byte message type, 2 bytes destination address, 2 bytes source address.
Each byte represents 2 BCD

Binary-coded decimal

In computing and electronic systems, binary-coded decimal is a digital encoding method for numbers using decimal notation, with each decimal digit represented by its own binary sequence. In BCD, a numeral is usually represented by four bits which, in general, represent the decimal range 0 through 9...

digits. A TPDU header is sometimes erroneously referred to as a TPDU.

OSI-model:
A TPDU is the name for the messages that the protocols in the transport layer of the OSI-model use for communication. It adds a transport header to the APDU to make a TPDU which is then sent to Network layer. Most protocols will implement a CONNECT, SEND and DISCONNECT TPDU.