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

Chapter 3 85

Common Device Control Functions

Altering Terminal Deﬁceﬁles

Setting Binary Editing Mode

In some cases you may need to disable all special character processing

and pass everything through as data. Binary editing mode allows you to

transfer information in this manner.

Binary editing mode treats all characters as data and passes all eight

bits of every character through without performing any special

operations. Fairly obviously, this restricts the way in which data

transfers can take place. You cannot use parity checking in binary

mode, since all eight bits of each character are treated as data. Block

mode transfers are also impossible, since no special handshake

characters are recognized.

The only way that a binary read will terminate without returning an

error is by reaching the byte count speciﬁed in the FREAD, READ, or

READX call that initiated the read. Read timeouts will terminate a

binary read in error and no data will be transferred. It is a good idea to

specify a read timeout value when posting binary reads to avoid

problems that can occur if the read does not reach the speciﬁed byte

count.

Binary mode is enabled by calling FCONTROL with a controlcode

value of 27 and disabled through FCONTROL with a controlcode

value of 26. With either of these FCONTROL calls the third parameter

has no meaning and should be invoked with a dummy value of 0.

Figure 3-9 provides an example of enabling binary editing mode in a

program. You should note several features of this code fragment, as

explained below, to understand how to use binary mode for your own

programming needs.

Note ﬁrst that a call to FCONTROL(13) precedes any call to enable

binary mode. FCONTROL(13) disables character echo at the terminal.

If character echo is not disabled prior to transferring data in binary

mode, any ASCII DC3 (XOFF) character in the data (CONTROL S

typed at the terminal) would be echoed by the DTC back to the terminal

and it would appear to the terminal that the DTC had been XOFFed.

This would cause the terminal to hang, since the terminal would

suspend transmission to await on XON signal that would never arrive.

An alternative to disabling character echo is to turn XON/XOFF ﬂow

control off at the terminal. This is a dangerous alternative, however,

since disabling ﬂow control may result in write data being lost at the

terminal.

The next feature of this program you should note is that separate calls

to FCONTROL(27) are posted against a read port and a write port on

the device. FCONTROL(27) is one of several terminal controls that

affect the data transfer characteristics of a speciﬁc ﬁle, rather than of

the device itself. This makes it necessary to explicitly specify binary

mode for both the read port and the write port opened on the device.