IBM 1 Series Manual page 120

Commands That Initiate Receive and Transmit Operations
Read
Write
The teletypewriter adapter has two commands that must be described
before presenting timing diagrams of transmit and receive operations: Read
and Write.
A character of data is received and deserialized in the receive
serializer/deserializer (SERDES) register. After the last bit of the
character has been clocked into the receive SERDESregister, the character
is moved to the receive buffer register.
A Read command transfers the contents of the receive buffer register to
main storage.
In normal operation, an attention interrupt indicates to the processor that
a data character has been received andis in the receive buffer register. A
Read commandis then executed as a part of the interrupt service program.
The receive buffer register is then free to be used to hold another
character.
If the receive control logic attempts to transfer a second received character
into the receive buffer register before the first received character has been
read from the receive buffer register by the program, an overrun occurs.
The first characteris still in the receive buffer register, and the second
characteris lost.
The program has one full character time to read the receive buffer
register.
A byte of data is transferred from main storage to the transmit SERDES
register; the internal clocking necessary to serially transmit the characteris
initiated.
At stop-bit time of the transmission of this character, a device-end
interrupt signals the processor that character transmission is complete and
that the adapter is available to accept another Write command.
Teletypewriter Adapter Feature 4-15