vxb@mhuxh.UUCP (Vern Bradner) (01/04/88)
I posted the following request for information on writing C routines to
communicate through the PC serial port:
Can anyone share their knowledge/experiences in writing C routines to
communicate with remote systems via the PC serial port?
I tried inp and outp without much success. Are there any C libraries
that might make this job easier?
I have seen requests for this information recently, but I have not seen
the answers posted (I will post a summary if anyone is interested).
A summary of the replies is listed below. Several people suggested using
the Greenleaf communications library and Joe Cambell's book: " C
Programmers Guide to Serial Communications". Also a couple people sent
good descriptions on PC serial communications for beginners like myself.
Thanks to all who helped me with this!
Vern Bradner ihnp4!mhuxh!vxb
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Several people suggested this library:
The easiest thing to do is purchase the Greenleaf communications library for
your compiler. It includes all sorts of buffers and most (all?) of the routines
you would need for most any communication (RS-232) program.
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Several people mentioned this book:
There is an excellent book by Joe Campbell -- "C Programmer's Guide to
Serial Communications." It covers IBM PC serial port (as well s the
Kaypro CP/M machine serial port). It is published by Howard W. Sams
& Company, Indianapolis.
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From: osm@metavax.UUCP (Owen Scott Medd)
I've just glanced at the documentation on writing interrupt handlers in
Microsoft C 5.0. This looks like a very nice facility to avoid coding
serial drivers in assembler. Has anyone made an attempt to rewrite any
of the public domain serial drivers in MSC5.0?
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From: rutgers!eddie.mit.edu!genrad!jpn (John P. Nelson)
You don't say what you've already tried, so I don't really know what level
to approach this at.
Using DOS level functions to access a serial port is useless, except
perhaps for writing to a serial printer. You have no control over baud
rate etc, and DOS does not provide any buffering whatsoever. If IBM
had done this right, the DOS serial driver would be interrupt driven,
buffered, and you would use ioctl calls to change baud rate etc.
The next level, the standard serial ROM BIOS routines, give you a better
level of control. However, there are still problems. For instance, the
standard IBM bios refuses to send characters (and the machine hangs)
if DTR is not asserted. This is not a hardware problem, but a software
problem.
Another (more serious) problem is that polling the serial port can cause
lost characters. Any lengthy operation (including scrolling the screen)
may take so long that an overrun occurs. I found that even at 300 baud,
a trivial communications program using the ROM bios routines would lose
characters when scrolling the screen (XT class machine: your milage
may vary).
Any serious communications program MUST use interrupts. The serial port
can be set up so that when a character is recieved, an interrupt occurs.
Your interrupt handling routine can queue data so that long operations
do not cause overruns.
You must enable the interrupts in several places. First, in the
interrupt enable register, you select which events will cause an
interrupt to occur. Second, bit 3 of the MODEM control register (this
bit is output to an interrupt ENABLE circuit) must be set to a 1.
Third, you must enable the appropriate interrupt on the 8259A interrupt
controller on the system board.
When an interrupt DOES occur, you must service the 8250 (communications
chip) so that the interrupt is no longer asserted, then you must send
an EOI instruction to the 8259 interrupt controller. If you skip this
last step, the system will hang, because no further interrupts will be
processed.
Since I don't know what level of expertise you are at, I don't know
if I am answering your questions. I could send you some working Turbo
C code fragments, if you want.
>I tried inp and outp without much success. Are there any C libraries
>that might make this job easier?
Several libraries are advertized in all the magazines. I have no direct
experience with any of them, though.
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From: manes@dasys1.UUCP (Steve Manes)
Writing code to control the PC's serial port can range from fairly simple
to gruesomely elaborate depending on what you want to do. However, in most
applications you want to avoid the IBM's built-in ROM BIOS serial support
because it doesn't support interrupt-driven character receipt, i.e. it
doesn't buffer. If you're not all that experienced writing code for
hardware and particularly the PC's 8250/16450 UART and 8259 interrupt
controller the first thing you'll need to do is study how they work. <PC
Tech> did an excellent series on the IBM hardware interrupt structure
several years ago. Another good source of reference is <The IBM Personal
Computer for the Inside Out> by Sargent & Shoemaker (Addison-Wesley). The
8250 can be tricky to work with if you're starting out with a blank page.
I probably spent 6 months tweaking and refining (and fixing many subtle
bugs) in my comm library. You can save yourself a lot of aggravation by
purchasing the Greenleaf comm library, or equivalent. The downside is that
you'll lose a valuable education in hardware programming. The upside:
you'll sleep better knowing that you've got a predebugged library to work
with.
You can't just pop bits in and out of the comm port if you want an
effective serial device controller. The UART has to be prepared and
initialized beforehand. The Sargent/Shoemaker book explains this well.drew@ncspm.ncsu.edu (Drew Wilhite) (01/07/88)
I was following this discussion with great interest and was pleased to learn of the existence of the Greenleaf communications library. Unfortunately when I called around trying to find out more about it, nobody had ever heard of it. Is it public domain? Is it distributed by a company with a different name? Is it a library for a specific C compiler and distributed with that compiler? Am I missing something obvious? Does anybody know? Thanks in advance for any information. ------------------------------------------------------------------- Drew Wilhite uucp: ...!mcnc!ncsuvx!ncspm!drew Domain: drew@ncspm.ncsu.edu internet: drew%ncspm@ncsuvx.ncsu.edu
dunn@ihlpf.ATT.COM (01/07/88)
(Drew Wilhite writes: ) >I was following this discussion with great interest and was pleased to learn >of the existence of the Greenleaf communications library. Unfortunately when >I called around trying to find out more about it, nobody had ever heard of it. >Is it public domain? Is it distributed by a company with a different name? Is >it a library for a specific C compiler and distributed with that compiler? Am >I missing something obvious? Does anybody know? 1) Pick up a issue of BYTE or PC Magazine 2) Look for a full page or double page ad by any software vendor who sells multiple packages (i.e. - a mail order software house ) 3) Look under : "Tools", "C Tools", "Communication Tools", etc... 4) I am sure you will see the Greenleaf Communications package listed price varies depending on vendor, so shop through the magazine. Also, there is a Greenleaf sampler available which contains a subset from each of the Greenleaf packages, i.e. - a little communications, a little graphics, a little windowing system..... Good luck, Bill Dunn Bill Dunn ihnp4!ihlpf!dunn
pjh@mccc.UUCP (Peter J. Holsberg) (01/08/88)
Greenleaf Comm Library $185.00
Greanleaf Software, Inc.
800-523-9830
--
Peter Holsberg UUCP: {rutgers!}princeton!mccc!pjh
Technology Division CompuServe: 70240,334
Mercer College GEnie: PJHOLSBERG
Trenton, NJ 08690 Voice: 1-609-586-4800