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Communications PcOS Series 150 Programmer's Guide
Parallel Port
Page 114 Rev G 12/14/99
Parallel Port Inquire (Non-IEEE 1284)
All inquire commands require a response from the printer. The serial port is bidirectional; the
non-IEEE 1284 parallel port is not. For this reason, the parallel, non-IEEE 1284 operation of
inquire commands is different from serial and IEEE 1284 modes. In serial and IEEE 1284
operation, inquire commands are responded to by an acknowledged (ACK) or not acknowleged
(NAK) sign and then the command ID on the return link.
In non-IEEE 1284 parallel operation, there is no easy way to return complex information to the
host. The paper error (PE) signal is driven in response to inquiries. If a true paper error is being
signaled, the ERR line will also be driven. It is, therefore, possible to distinguish between a paper
error and an inquire response.
In parallel, non-IEEE 1284 mode, the printer will respond to the inquire before the parallel port
sequence is complete. When the parallel port acknowledges the ID portion (the second byte) of
the inquire sequence, the PE signal will be valid. The host must, however, remember what status
request was made, as the printer has no way of signaling that information. In addition, the host
must not interpret the PE signal as an error but as a response to an inquiry.
The PE signal can be configured to respond in two ways. The first is static. The PE will remain
valid until the next inquire is issued. It may also be configured to operate dynamically which
means the PE signal will follow the status of the feature being tracked. In all cases, a true paper
error is dynamic. True paper error status will not override any other status request. For example,
if you want true paper error operation, it must be selected after all other dynamic status requests.
To synchronize inquire commands with the operation of the paper error signal, the printer will
remain busy after the second byte of an inquire command until the inquire command is
processed. This typically takes about 100 microseconds.
PE
DATA
STROBE
BUSY
ACK
Valid
ID Next
Figure 19 PE to ENQ request timing
It should be noted that inquire commands are preprocessed (looked at and processed before they
are buffered). They are still buffered. This means that the buffer could be filled with inquire
commands if the printer is waiting for some other operation.