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Communications PcOS Series 150 Programmer's Guide
Serial Port
Page 118 Rev G 12/14/99
Serial Port Protocol
The serial port supports two flow control standards, XON/XOFF and Ready/Busy (sometimes
called DTR or hardware handshake.)
When Ready/Busy flow control is selected, the printer can be configured to use DTR, RTS, or
both for flow control. If only DTR is selected for flow control, RTS will indicate that the printer
has faulted. The following discussions will assume the DTR is being used for flow control.
The Ready/Busy protocol generally uses the DTR signal to indicate to the host computer that the
printer is not ready to accept data. The host should stop sending data to the printer as soon as
possible. Because the host may not notice the DTR signal until it has transmitted several bytes of
data to the printer, the printer will continue to except up to 255
15
bytes of data after it indicates
that it is not ready. The figure below illustrates how the Ready/Busy protocol works.
Serial
Communications
port
Data
Buffer emptying
Select key
Printer
control
software
Print
buffer
Serial Port Flow Control
Using DTR
RTS -> Request to send
DTR -> Data terminal ready
data in
Serial
data out
(Not used for
flow control)
DTR
RTS
DTR
Clear
Set
Inquire
response
256 - 6K
Data
Buffer filling up
Data
High
speed
FIFO
ENQ response
Data
ENQ
proc.
Figure 20 Serial port flow control using DTR
15
The buffer always signals it is full before it overflows. The size of the reserve depends on the buffer size
selected. It is always at least 255 bytes.