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

Chapter 3 63

Common Device Control Functions

Reading From Asynchronous Devices

Standard Mode

Standard editing mode is enabled by default for asynchronous

terminals. All special characters recognized by MPE/iX are available in

standard editing mode.See Conﬁguring Systems for Terminals, Printers

and Other Serial Devices for a complete description of all the standard

special characters.

Transparent Mode

In transparent editing mode, only a limited number of special

characters retain their meanings and are acted on by the system. Also

referred to as unedited mode, this facility allows many of the characters

that would otherwise be treated as special characters to be passed

through as input, without causing any control action to be performed or

being stripped from the input data stream. Table 8-6 lists the special

characters that retain their meaning in transparent mode.

FCONTROL(41) can be used to enable transparent mode. The

FDEVICECONTROL intrinsic can also be used to enable transparent

editing mode. You will ﬁnd examples of invoking transparent editing

mode later in this chapter and in Chapter 4, “Using

FDEVICECONTROL.”

Binary Mode

In binary editing mode no special characters are recognized. All

characters are treated as data and passed through without any

terminal control actions being taken. Only character mode processing is

possible in binary mode, since there are no special characters available

to control block mode processing. Any carriage control information

contained in output data transmitted in binary mode will be stripped.

Use FCONTROL(26,27) to enable and disable binary mode transfers.

Triggering Reads

With some exceptions, every time a read is issued on an asynchronous

device a read trigger character is sent to the device, indicating that the

system is ready to receive input. The read trigger is not transmitted

over PAD connections and may not be issued if typeahead mode is

enabled at a terminal and the typeahead feature is being used.

The normal read trigger character is the ASCII DC1 character (the

same as XON). All terminal devices supported for use on HP 3000

Series 900 computers are capable of recognizing and responding to the

DC1 read trigger character.

The read trigger is generated at the beginning of each read and signals

that the system is ready to receive data. If the device is operating in

character mode, the device can begin transmitting without any further

exchange of protocol characters. If, however, the data is to be sent

through block mode, the device must inform the DTC by sending an

ASCII DC2 character in response to the read trigger. The DTC, now