TransAct Ithaca 8000, Programmer'S Manual

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Communications Ithaca
®
8000 Programmer’s Guide
Page 274 Rev D 100-06908
Parallel Port
Parallel Port Protocol
The Ithaca
®
8000 parallel port behaves just as any printer connected to a personal
computer. The parallel interface accepts 8-bits of data from the host. The strobe signal
from the host is used to indicate that data is available. When the printer sees the strobe
signal and accepts the data, it asserts a busy signal. The busy signal indicates to the
host that the printer has accepted the data and is working on it. After the printer absorbs
the data and is ready to accept another byte, the printer asserts acknowledged (ACK),
negates busy, and then finally negates ACK.
The host computer should meet the following parallel-port specifications and timing. In a
standard personal computer, the strobe signal is generated by software writes to the
parallel-port control port, which is typically done in the bios or some parallel-port driver.
As personal computers become faster, it is up to the software to assure that the strobe
signal does not get too narrow. One microsecond is the minimum pulse width that
should be sent down a cable. Shorter pulse widths (500 nanoseconds) will be accepted
by the printer. The cable can introduce significant signal degeneration and skew.
The data must be valid before the strobe signal is asserted and remain so until the
strobe is removed. A 500 nanosecond setup and hold time is required by the printer.
The following chart illustrates parallel-port timing.
INIT
Pin 16
DATA
Pins 2-9
STROBE
Pin 1
BUSY
Pin 11
ACK
Pin 10
<-- Dhld
<--Tstb
Dstu---> < >
>
Bhld---> <
Ahld ----> <
Sdly -----> <
Dstu 500 nS (Min)
Dhld 500 nS (Min)
Tstb 1 uS (Min)
Bhld Approx. 5 uS
Ahld Approx. 5 uS
Sdly 3 uS (Min)
Data Data
< ---- Rdly
Rdly Approx. 2 seconds
<---------------- ---------------> Tcycle
Tcycle 200 - 280 uS Typ.
Figure 16 Parallel-port Data Timing
To implement flow control, the busy signal is asserted by the printer outside the normal
data-transfer sequence. The busy signal has several uses, but it always indicates that
the printer cannot accept information. The busy signal may happen at any time and may
not adhere to the above timing chart in all cases. It is up to the host's parallel-port driver
to handle all possible busy states. It is important that the host driver does not hang up if
it takes some time for an acknowledged (ACK) response to a strobe signal. Standard
personal computer parallel-port hardware implements an interrupt on the ACK signal to
make flow control easier.