Programmer's Guide
PcOS Series 90PLUS
Communications
Serial Port
12/16/99
Rev M
Page 149
When XON/XOFF protocol is implemented, it is possible for the host to miss an XON or an XOFF. To
prevent this from causing a communication lockup, the printer will send an XOFF for every character
received after the high watermark is reached.
Whenever the printer detects that the serial data link is inactive, it sends out an XON, which results in an
XON being sent out about every two seconds.
When the printer is selected or deselected (with the SELECT key), an XON or XOFF is also sent. This
happens even if the internal data buffer is past the high watermark
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and allows the host to know that the
SELECT key has been pressed. If the buffer is full and an XON is sent the next character by the host, an
XOFF will also be sent. The data that was sent by the host will not be lost unless the 255-character pad
buffer is completely used.
8.3.5
Printer Buffer Size
The Series 90PLUS Printer has a configurable buffer size, which allows an application to control how far
ahead of the printer it can get before being asked to wait. The buffer size can be set from 256 bytes to
6144 bytes. (This does not include the 64-character high-speed buffer). The smaller the buffer, the tighter
the control will be. It is up to the application developer to select the optimal buffer size.
8.3.5.1
One-line Mode
The Series 90PLUS Printer supports a one-line mode of operation. In one-line mode, the input buffer
is set to 1024 bytes; however, the printer will go busy after every [CR] is received. The printer will
stay busy after the [CR] is received and remain busy until the previous information is printed. This
mode is not recommended for serial operation unless the application looks for the line terminator
handshake. There are several features that must be considered when using one-line mode.
1.
The printer only looks at [CR]’s. [LF]’s will cause the printer to print but will not set busy.
2.
If a [CR] character is received as part of another command (i.e. as part of graphic data), the
printer will go busy after the [CR] is received, but will go ready after the [CR] is processed.
3.
By definition, one-line mode is slow. The printer will not allow the application to get ahead of
the printer.
4.
In serial mode, one-line mode is difficult to control. Because of the 64-character high-speed FIFO
buffer, the printer will not go busy immediately after the [CR] is sent. It may take up to 25
milliseconds for the printer to process the [CR]. In this time, it is possible for the application to
send up to 48 bytes of data to the printer. When the [CR] is found in the input stream, the busy
status will be set. If the application continues to send information, the input buffer will be
overrun as it only has 16 bytes left. In addition, the application will not truly be in one-line
operation. The application must wait for at least 100 milliseconds after a [CR] before looking for
busy or trying to send more data.
5.
In XON/XOFF mode, the printer is heavily burdened by the task of transmitting XOFF
characters while printing and receiving data. The application should be written to look for the
XOFF after every [CR]. The Series 90PLUS Printer will lose data if too many characters follow
the XOFF.
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If the host sends more information, it will be responded to by an XOFF. If the printer cover is opened and
closed in this mode too many times, it is possible for the buffer to overflow.
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