AMT Datasouth 41AY89AR777 - Manual (Page 55)

As already mentioned, the printer must signal to the host when it is ready to
receive another character. This is an extremely important task, and most
compatibility problems center aroimd this function. Garbling or loss of data
will occur if the host does not honor the ready status or if the printer does not
transmit it.
Two signals on the interface are associated with the printer-ready status. They
are referred to as BUSY and ACKNOWLEDGE and are described below:
ACKNOWLEDGE; This signal is a pulse (normally negative going) which is
sent to the host when the printer is ready to accept a new character. It is often
referred to as the handshake signal since it is the signal that the host is expected
to await before sending more data. The electrical characteristics and timing of
this pulse are described later.
BUSY: The BUSY line is provided to indicate to the host when the printer is
performing an operation such as printing or moving paper. BUSY is asserted
upon receipt of a character. After processing the character, BUSY is removed
unless the PTFO is full.
The printer parallel interface was designed around the Centronics standard since
most systems in use today are compatible with the Centronics specifications.
Unfortunately, there exist many interpretations as to what the Centronics
specifications actually are. The following variations have been observed:
IGNORING HANDSHAKE; Many designers take advantage of the hard-wired
input circuitry of some printers by sending data without regard to the
ACKNOWLEDGE or BUSY signals until a complete line has been sent.
Althou^ tins is strictly against the specification, it works fine as long as the
printer can accept data as fast as the host can send it.
Printers which utilize microprocessors (with software driven data acquisition)
cannot accept data at an arbitrary rate and will lose data unless the host honors
the handshake signal.
HANDSHAKE ON BUSY: A look at the Centronics specification will show
that BUSY is not raised on a character-by-character basis. Thus, a system
designed to look at BUSY (for a ready condition) after transmission of each
charactw, may cause loss of data unless the printer has a fast front-end. Being
microprocessor controlled, the printer carmot accept data at a rate equal to
printers with a hard-wired front end. This makes it imperative that the host honor
the ready status sent by the printer. Since many systems want to handshake using
BUSY, the printer manipulates the BUSY signal on a character-by-character
basis. The ACKNOWLEDGE signal is also issued just as before. This enables
the printer to operate with more systems without sacrificing compatibility with
others complying with the strict Centronics specifications.
POLLING ACKNOWLEDGE; A few designers have attempted to handshake
by polling the ACKNOWLEDGE pulse with a software routine at the host end.
This will not work because the ACK pulse is too fast. The ACK pulse should be
used to set a latch whose output can be “READ” by the host, then reset.
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