olson@modular.UUCP (Jon Olson) (04/08/88)
We have recently developed a multiplexor board which has eight RS232
ports. This board which allows a single RS232 port on a SUN
workstation to support seven ports (the eighth port on the multiplexor
board connects to the RS232 port on the SUN).
A MUX daemon on the SUN provides the interface software between the
single RS232 port and seven pseudo-terminal (pty) ports. Although the
interface was simple to develop and debug there are a few shortcomings
with the `tty' and `pty' drivers which would be useful in this
application (and many other applications).
1) I have found that SUNOS 3.4 does not support hardware (CTS/RTS)
handshaking on RS232 lines. Although the dedicated
microprocessor on the MUX board can handle 19,200 baud input
without flow-control, the standard Unix TTY driver (either the
old or new discipline) cannot. What I need is for Unix to
generate a ~RTS whenever the input buffers are in danger of
overflowing. Since the MUX must pass any binary information
through the TTY port, ^S/^Q handshaking would require messy
encoding of the data stream.
2) The standard TTY driver does not have sufficient buffering to
accept large data packets. I have limited the size of packets
sent to the SUN to 64 bytes to overcome this limitation. It
would be useful to have an `ioctl' which configures a large
buffer for a TTY port.
3) I tried the `bk' driver but it doesn't work with the `select'
system call. When given a file descriptor of a tty using the
`bk' line discipline, `select' always returns immediately even if
there is no data to be input.
4) The `pty' driver has a TIOCPKT mode which gives one of the
following control flags.
TIOCPKT_FLUSHREAD
TIOCPKT_FLUSHWRITE
TIOCPKT_STOP
TIOCPKT_START
TIOCPKT_DOSTOP
Ultrix (DEC's Unix) has an extra (very useful) control flag which
SUN leaves out, TIOCPKT_IOCTL, which indicates that the slave
process has executed an ioctl operation. This allows the master
to manipulate the device to emulate the ioctl operation that the
slave just performed.
Any comments on hardware handshaking or high-speed input of data over
RS232 lines would be appreciated.
--
Jon Olson, Modular Mining Systems
USENET: {ihnp4,allegra,cmcl2,hao!noao}!arizona!modular!olson
INTERNET: modular!olson@arizona.edu