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

66 Chapter3

Common Device Control Functions

Reading From Asynchronous Devices

However, for asynchronous devices, the actual data transfer always

occurs on a byte-by-byte basis. If the number of bytes input reaches the

value speciﬁed, the read will terminate on byte count.

For asynchronous terminals, the maximum supported read length is

four kilobytes (4 KB) in standard or transparent editing mode and

128 bytes in binary editing mode.

transfercount

is used, the actual number of bytes transferred as a

result of the read will be returned as its value. If you speciﬁed NOWAIT

I/O in the FOPEN or HPFOPEN call, however, this value will be 0 and

the actual byte count will be returned in the IOWAIT or IODONTWAIT

call. See Chapter 8, “Intrinsics Reference,” of this manual for more

information on IOWAIT and IODONTWAIT.

The program fragment shown in Figure 3-2 illustrates the use of the

FREAD intrinsic. NOWAIT I/O is not used in this example. A ﬁle is

opened as $STDIN with 80 byte ASCII records speciﬁed and the ﬁle

number is returned in the

fileid_in

variable. A read is then posted

against this ﬁle using the FREAD intrinsic to read up to 80 bytes and

store the data as the value of read_buffer. Note that read_buffer is

declared as a string large enough to hold up to the maximum amount of

data that could be transferred in a single FREAD call.