serial port data aquisition via Tcl

The current on-line documentation for Tcl (i.e. the Tcl reference
manual) still reads "Opening a serial port is not currently
implemented under Macintosh".  Hence, last week, when I inquired as to
whether or not it was possible to acquire data from a usb serial
device from within R, I was pointed to Rpy, which is somewhat indirect
in that communication is unidirectional from Python to R. While it
allows R functions to be called from within Python, it is not a very
familiar environment from an R-user perspective.

As it turns out, the Tcl documentation is not up to date, since the
open/ puts/ gets/ read commands all allow access to the Macintosh
serial (USB) port from within the Tcl interpreter. Consequently, it is
in fact very easy to access the serial port using the tcltk package
from within R. On the off chance that someone else may need to acquire
data directly from an external device, I thought I should document how
this can be done using a few Tcl calls, so that data can be read into
R in real time, where it can be parsed, processed, and displayed using
a familiar R environment.


require("tcltk")

# Under Unix, hardware ports are mapped to file descriptors. So we
open the serial port by specifying its file descriptor (typically /dev/
tty.* or /dev/cu.*. Under Windows, this would be COM1, COM2 etc).
Here, the resulting channelID (e.g.file10) is assigned to the Tcl
string 'serial' and the port (file) is opened for read/write access.
Other options are covered in the Tcl reference manual - see [open].
.Tcl("set serial [open /dev/cu.usbserial r+]" )

# display the channelID for later references
.Tcl("puts $serial")

#configure the port setting, here for a non-blocking stream. If
blocked, make sure the -timeout option is set or it will hang
.Tcl("fconfigure $serial -mode \"19200,n,8,1\"  -blocking 0")

# read and display individual lines (to the newline character if
that's how your device is configured)
.Tcl("puts [gets $serial]")

# or read all the data in the buffer
.Tcl("puts [read $serial]")

# remember to close the port or you will have to reboot
.Tcl("close $serial" )

To write to the port, just send "puts $serial <output>" to the Tcl
interpreter.  In the absence of an R serial library, I think this is
the simplest approach in that it is platform independent and minimizes
reliance on external programs other than Tcl, which is supported by R-
Core.

Atul

-----
Atul Sharma MD, FRCP(C)
Department of Pediatrics,
McGill University



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