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Data Communications
Receive Buffer
The terminal's receive buffer is a first in/first out (FIFO) storage area for accepting data from
the remote device. When you are using any type ofreceive pacing, the buffer is partitioned into
a working buffer and a 63 byte overrun area. In particular, the buffer size is 255 bytes, thus if
receive pacing is being used: the working buffer is 192 bytes long and the overrun area is 63
bytes long. If the received data overflows the working buffer and intrudes on the overrun area,
the terminal will exercise whatever receive pacing mechanism is currently enabled (send an
XOFF, for example, ifXON/XOFF receive pacing is enabled) at that time to temporarily halt
the flow of data from the remote device. When enough data has been processed so that the
receive buffer is only one quarter full (64 bytes), the terminal then signals the remote device to
resume transmission (by sending an XON, for example, if XON/XOFF receive pacing is
enabled).
Receive Errors
When receiving data from the remote device, the terminal can detect the following three types
of error conditions (in addition to parity errors):
1. Character overruns - a character is received before the preceding character was processed
by the terminal's datacomm firmware.
2. Framing errors - no stop bit was detected at the end of a character.
3. Buffer overflows - the entire allocated buffer space is filled (both the working buffer and the
overrun area). The last character in the buffer will be overwritten by a ((DEL" character.
Note that if the remote device is using the selected form of pacing, this condition should
never occur.
Receiver errors, when detected, cause a <DEL> character to be displayed on the screen at the
point of the error. The host may determine ifa datacomm error has occurred by inspecting byte
5 of the primary terminal status bytes (refer to Section 8, Status, for information on terminal
status). The host computer will not be able to determine which type of error occurred.
Local/Remote Modes
The data communications portion of the terminal operates independently of Remote and Local
modes. If the terminal is switched from Remote to Local while data is being received from the
remote device, the datacomm portion of the terminal continues receiving data, and storing it in
the buffer. In such a case, any data received in Local mode which overflows the buffer is
discarded by the terminal's firmware. Then, when the terminal reenters Remote mode, the data
stored in the buffer in Local mode will be processed and sent to the screen. (To prevent buffer
overflow errors when you switch from Remote to Local mode to stop data from being trans-
ferred to the screen, XON/XOFF receive pacing should be used.)
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