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

64 Chapter3

Common Device Control Functions

Reading From Asynchronous Devices

informed that the data will be transmitted as a block, sends another

read trigger when it is ready to accept the block of data. The second

read trigger is referred to as a block mode read trigger and is the same

character as the read trigger.

It is possible to specify an alternate character for use as the read trigger

(and therefore also the block mode read trigger) through the

FDEVICECONTROL intrinsic. You must take great care if you do so,

however, to assure that no data is lost.

Terminating Reads

Reads can be terminated in a number of ways, depending on the

programmatic controls issued by your program prior to posting the

read. Both the input mode (character or block mode) and the data

editing mode (binary, transparent, or standard editing mode) affect the

termination of reads.

Binary mode reads cannot be terminated by the recognition of any

character. Binary mode reads are terminated only by the byte count

being reached or by the expiration of the read limit timer. Only byte

count termination results in a successful binary mode read.

The following summarizes the events that will cause reads other than

binary mode reads to terminate

• An EOR character is sent from the terminal. This includes the block

mode read termination character (usually RS) if you are in block

mode and any EOR character speciﬁed in an FCONTROL(41) call or

through FDEVICECONTROL(192) with a controlcode value of 15

or 39. Reads terminated in this way terminate normally.

• An AEOR character deﬁned in a preceding call to FCONTROL(25) is

sent from the terminal. The AEOR character is included in the data

and the byte count and the read terminates with an error condition

(FSERR 31). An AEOR character will be recognized in either

standard or transparent editing mode.

• The input byte count is reached. This count is set by a parameter in

the read intrinsic used for the read (FREAD, READ, or READX).

Reaching the input byte count causes all reads to terminate,

regardless of any other condition.

• The read limit timer set by a preceding call to FCONTROL(4)

expires. Setting this timeout value prevents a device from hanging

because of an incomplete read. The read data is transferred to the

user buffer and the read length is returned. The length is the

number of characters entered at the time of the timeout, and the

read terminates with SOFTWARE TIMEOUT (FSERR 22). Reaching

the timeout limit causes all reads to terminate, regardless of any

other condition. (During block mode reads, the timer halts when the

DC2 character is received.)