Xon/Xoff Protocol - NCR 7158 Owner's Manual

6-6 Chapter 6: Communication Interface

XON/XOFF Protocol

[DTR SPACE] - When ready to accept data.
- Remaining space in the 4K buffer rises above 512 bytes. **
[XON sent] - When ready to accept data after power on.
- Remaining space in the 4K buffer rises above 512 bytes. **
[XOFF sent] - In an error state.
- Remaining space in the 4K buffer falls below 256 bytes*
* - 10 bytes in line mode.
** - 20 bytes in line mode.
Sending data (status):
In DTR/DSR mode, data is sent only when DSR is SPACE.
In XON/XOFF mode, DSR state is ignored.
The XON/XOFF characters coordinate the information transfer
between the printer and the host computer. The printer sends an XON
character when it is ready to receive data and it sends an XOFF
character when it cannot accept any more data. The software on the
host computer must monitor the communication link as shown in the
following flowchart in order to send data at the appropriate times.
If XON/XOFF has been selected, the printer also toggles the DTR
signal, as described in the next section, but it does not look at the DSR
signal to transmit data.
Host Flow Control Diagram