Serial Port Protocol; Figure 20 Serial Port Flow Control Using Dtr - Ithaca 150 Programmer's Manual

Communications
Serial Port

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
that it is not ready. The figure below illustrates how the Ready/Busy protocol works.
Communications
Serial
data in
Inquire
Serial
response
data out
(Not used for
flow control)
DTR
RTS
RTS -> Request to send

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.
Page 118
PcOS Series 150
Serial Port Flow Control
Using DTR
port
High
speed
Data Data
FIFO
ENQ response
Clear Buffer filling up
DTR
Set
Buffer emptying
Select key
DTR -> Data terminal ready
Rev G
Programmer's Guide
15
bytes of data after it indicates
ENQ
Print
Data
proc.
buffer
256 - 6K
Printer
control
software
Data
12/14/99