Asynchronous Serial Communications Programmer's Reference Manual (32022-90052)

Chapter 6 129

Typeahead Mode

Programming for Typeahead Mode

Bypassing the Typeahead Buffer

Figure 6-3 is an example of using FDEVICECONTROL to bypass the

typeahead buffer. You could use this capability to send a status request

and obtain a reply directly from a terminal without affecting the data

already in the typeahead buffer. This FDEVICECONTROL is only valid

if typeahead is enabled. The default is not to bypass the typeahead

buffer.

This FDEVICECONTROL call will cause the typeahead buffer to be

bypassed for a single read only; normal typeahead mode processing will

resume for any following reads, and data will be taken from the

typeahead buffer if data is present.

When you use this request, you call FDEVICECONTROL with parm1

set to 61, bypass typeahead mode, and pass a value of 1 in the buffer

parameter (value :=1). As in the previous examples, Write access is

speciﬁed in parm2, and the Pascal/iX

ccode

function is used for error

checking.

NOTE

In cases where there is no read trigger (read trigger set to null)

bypassing the typeahead buffer may have unexpected results. If, for

example, your program is swapped out between the FWRITE and

FREAD of a terminal status request, the terminal may appear to

become “hung”, since there is no read trigger to synchronize terminal

response with the posting of the read.

To avoid this problem, instead of bypassing the typeahead buffer to

send a status request when no read trigger is present, the following

should be the sequence that occurs:

1. = Disable typeahead mode

2. = Post a 0 byte read

3. = Enable typeahead mode

4. = (Lock keyboard) (FWRITE) Send status request

5. = FREAD of status

6. = FWRITE (unlock keyboard)