BEEB -> PC COMMUNICATIONS
Simon Godfrey (K5U) Feb95
Last year I had to transfer some wordprocessor files between my BBC
model-B and an IBM PC. In order to do this I had to find out how the
serial/comms ports worked on each machine. This took hours of trial
and error before I got it sussed. I hope this article will be of help
to anyone else wishing to embark on a similar project.
I'm not sure what serial ports Archimedes or Apple Macs have but the
comms lead described in this article should(?) be compatible with ANY
computer with an RS232 port.
Incidentally, if you need to connect your Beeb to a modem, this
requires almost the same lead. On the RS232 connector just
swap-around the Request-To-Send / Clear-To-Send wires and the
Transmit-Data / Receive-Data wires. (I think this will do the job)
BACKGROUND TO THE RS232 / RS423
The standard serial port on most computers is the RS232 which
typically has 25 pins, which send or receive different signals to
enable the computer to communicate with other devices. As many of the
pins are only used for very specific tasks (and some of them serve no
purpose at all), over the past few years a cut-down version of the
RS232 has become more popular, using just 9 pins. The BBC has an
RS423 port which is designed to do the same job as the RS232 but has
only 5 pins, which can sometimes be rather limiting.
One such limitation is encountered when connecting the BBC to a modem.
Modems send signals such as "the phone is ringing" and "another
computer has connected" to pins in the RS232 port, and these are then
interpreted by the computer at the other end. If, however, there is a
Beeb at the other end then such signals cannot be made use of as they
aren't catered for in the RS423. Some modems were designed to combat
this problem by redirecting some of the RS232 pins to the Beeb's
User-Port but this I feel can just complicate matters further...
Problems such as this arose in connecting the Beeb to a PC, as the
PC's serial/comms port 'expects' signals the BBC can't send via the
RS423. Anyway... enough of the problems, here is the cure:
I'm afraid I've only made a cable for the 9-pin RS232 as this was the
type on the PC I was using and most PCs nowadays use this type. My
diagram for the 25-pin RS232 is an educated guess and may or may not
work. 8BS and myself accept no responsibility for loss of data, time,
life or anything caused by the project described in this article.
I was able to buy all of the materials from a local electronics shop
(Tandy) for under 5.00. Presuming you have access to all the
necessary tools - soldering iron, wire strippers etc, all you need is:
* 1.5m of telephone cable (6-core colour-coded)
* a male 5-pin domino-type DIN plug (for the Beeb)
* a female 9-pin or 25-pin RS232 serial connector (for the PC)
* plastic casing for the RS232 connector
Tandy actually stocked a ready-made cable but I was unsure whether
this would do the full job (as I didn't know whether or not it would
'trick' the PC-end as mine does - explained later). It was called an
RS423/RS232 null modem lead or something like that.
I suppose you could use any length of lead, but the longer it is the
more risk you run of the data you send along it being corrupted.
I shall refer to the colour of the telephone cable's wires as follows:
K=Black, Y=Yellow, W=White, B=Blue, G=Green, R=Red.
Such cables are probably not all exactly the same, so my apologies if
yours has wires of different colour.
RS423 CONNECTOR - BBC END
This view of the RS423 pins is from the back of the BBC computer
1 - Signal Ground / B W \
2 - Clear To Send | 5 4 |
3 - Transmit Data \ R |
4 - Receive Data / 1 |
5 - Request To Send | G Y |
\ 2 3 /
Solder the wires to the ---------
pins according to their colour.
Also solder the Black wire to the metal tube of the casing.
The indented notch in the plug's metal tube (here shown on the left)
can be on either the left or the right. I have shown it on the left
to correspond with a diagram in the BBC User Manual.
It is important to make a mark on the top of the plug to denote which
way up it should be inserted into the computer, as it is possible to
insert it upside down
RS232 9-PIN CONNECTOR - IBM PC END
This view of the RS232 pins is from the back of the PC computer
1 - Carrier Detect
--------------------------- 2 - Receive Data
\ % Y W % R / 3 - Transmit Data
\ 1 2 3 4 5 / 4 - Data Terminal Ready
\ / 5 - Signal Ground
\ % G B . / 6 - Data Set Ready
\ 6 7 8 9 / 7 - Request To Send
----------------- 8 - Clear To Send 9 - Ring
Solder the wires to the pins according to their colour. Also solder
the Black wire to the metal casing. The pins marked % should be
joined together by two short pieces of wire. ie. connect the ends of
one wire to pins 1 and 4 and the other wire to pins 1 and 6.
As mentioned earlier, the BBC cannot provide certain signals the PC
will need. These wires marked % will trick the PC into thinking,
"there IS something properly connected to the other end of this wire"
and, "it IS sitting ready for action".
RS232 25-PIN CONNECTOR - IBM PC END (alternative)
This view of the RS232 pins is from the back of the PC computer
\ . . . . . % R % B G Y W K /
\ 13 12 11 10 9 8 7 6 5 4 3 2 1 /
\ . . . . . % . . . . . . /
\ 25 24 23 22 21 20 19 18 17 16 15 14 /
1 - Protective Ground 12 - Data Signal Rate Detect DCE
2 - Transmit Data 15 - Transmitter Signal Element
3 - Receive Data 17 - Receiver Signal Timing
4 - Request To Send 20 - Data Terminal Ready
5 - Clear To Send 22 - Ring Indicator
6 - Data Set Ready 23 - Data Signal Rate Detect DTE
7 - Signal Ground 24 - Transmitter Signal Element
8 - Carrier Detect
I have included details of all the assigned pins but for the purpose
of this project you need only regard the nine that are marked.
Solder the wires to the pins according to their colour. The pins
marked % should be joined together by two short pieces of wire. ie.
connect the ends of one wire to pins 6 and 8 and the other wire to
pins 8 and 20.
The easiest way I've found of transfering files between the two
computers is by using simple comms software. Such programs were
designed primarily for use with modems for uploading or downloading
files from bulletin boards but Beeb to PC transfer is in effect the
Xmodem seems to be the most popular file transfer protocol in BBC
comms programs, which sends data as blocks and when receiving, checks
for mistakes (corrupt data) after each one - If it detects any, the
program will ask the other compuer to re-transmit the previous block
again. This way you are more or less guaranteed 100% error-free data.
I'm not sure what the easiest to use comms package is for PCs, I use
either RIPterm or Terminal (which comes as part of Microsoft Windows).
There are however many good public-domain comms packages available for
the Beeb. I personally prefer 'Quercus' but there are quite a few
others in the 8BS pool - two such programs are on the Mad Rabbit Comms
Disk (TBI-59-1). Any 'terminal' or 'scrolly' package worth its salt
should support Xmodem transfers.
There will probably be transmit/receive settings that you can adjust
when the comms software is running. These should be set to the same
values on both computers. I tend to stick to full duplex, 8 bits, no
parity, 1 stop bit (shorthand: 8N1). If this sounds like complete
gobbledy-gook don't worry, chances are the comms programs will default
to these settings anyway. The higher the baud rate, the faster data
will be transmitted/received. I use 9600 as the next speed up of
19200 tends to be very easily corrupted unless you have a very short
comms lead connecting the two computers.
If there are 'modem initialization strings', 'auto-dial' or 'AT'
commands set in your comms programs (which is usually the case with
PC-based programs), these should be set to blank as they only apply
when you have a modem connected.
Once you have the computers connected and the software up-and-running
you can test the link by typing something on one of the computers.
What you type should appear on the screen of the other computer.
To transfer files, you just start an upload on one computer and start
a download on the other. If you are using Xmodem then the upload &
download must be in this order - otherwise the transfer may not engage
CONVERSION - UNDERLINES, LINEFEEDS AND BITMAPS
Of course, what use is file transfer if you can't use the files once
they've been transferred?
Some Beeb Wordprocessors such as Wordwise use very few 'hidden' codes
within the text (apart from the green 'embedded commands' you insert
yourself). Other wordprocessors hide codes (such as 'underline on',
'justified text on' etc) within the text and if these are transfered,
for example, from a Beeb running InterWord to a PC running
WordPerfect, the PC will not understand the codes and will output an
'incompatible file format' error message.
It is therefore very rare that you can transfer a wordprocessor file
from the Beeb directly to a PC (or vice-versa) and use it straight
away. Most wordprocessors offer a 'spool to file', 'save as generic
WP' or 'output as DOS text' option. These options will strip the
document of any underlines, bolds, etc which would otherwise make the
file difficult for other wordprocessors to read. It may be necessary
for you to use one of these. 'Save as Generic WP' is highly
recommended - it's by far the best as it leaves the paragraphs intact
rather than splitting them with a 'return' after each line.
Another incompatibility is that of the Beeb's and PC's handling of the
'Return' character at the end of lines and paragraphs. Here it is
important to understand the difference between 'carriage return' and
On the BBC the return key is stored as ASCII character 13 (carriage
return), which sends the cursor to the far left hand side of the
screen or printed page. On reaching one of these characters the BBC
automatically inserts a 'linefeed' to take the cursor down one line.
On PCs, the return key is stored as both 'carriage return' AND
'linefeed' together. (linefeed is ASCII character number 10).
Without the 'linefeed' characters, you can't use the DOS 'TYPE'
command on PCs to type a text-file to the screen, nor can you load
these text files into some of the more fussy PC-based wordprocessors.
If a document which includes the 'linefeed' characters is loaded into
a Beeb wordprocessor, the linefeed characters are often converted into
peculiar characters such as % signs.
The other main incompatibility I've come across is between the BBC's
UK-standard ASCII character set and the PC's MSDOS extended-ASCII
character sets. On the BBC the (pound) sign is stored as character
number 96 while on the PC it is stored as 156 and character 96 is a
left ' quotation mark. All other characters from number 32-126 appear
to be the same.
THE CONVERSION PROGRAM - TEXT
To combat these problems in particular, I have written a small
conversion program which will convert either way from a BBC-format
text file to a PC-format text file. It basically strips the file of
any characters which are not a-z, 0-9 or punctuation marks; converts
pound-signs, *bullet-points and 'quotation-marks'; and either adds or
deletes linefeeds to carriage-return codes whenever necessary.
The BBC to PC-text option will additionally convert various characters
Interword uses into standard ASCII characters, although this option
should work with any Beeb wordprocessor.
A similar conversion program to convert View files to ASCII format
appeared in 8BS issue 19. If you use the View wordprocessor on your
Beeb, in some cases this program may be more suitable than mine, but
as I don't have View I have been unable to test it. This program
re-appeared with several other text-utilities in issue 20. Another
Interword-ASCII converter was in 8BS issue 23. You may or may not
find these programs useful.
If you are transfering a long WP file from a PC to the Beeb you may
later need to use an ASCII file splitter to cut the file down to
chunks small enough for BBC wordprocessors to handle. There's one in
8BS issue 31. Other Split and Join programs also appear in 8BS issue
Trial-and-error is the only way you'll find the best solution for you
and the particular systems you use. Once you've figured how to do it
once, you'll be able to do it a hundred times.
I tried converting from WordPerfect 5.1 on a PC to Interword on my
Beeb. Using 'Save as Generic Text' on the PC and the conversion
program on the Beeb was the most succesful method. Without using the
Generic Text option I ended up with four pages of rubbish at
beginning, weird codes at the start of each paragraph and the
occasional page or two that had become underlined.
Converting the other way (IW to WP51) I only needed to run my program
on the Beeb and the file was ready to use on the PC.
THE CONVERSION PROGRAM - GRAPHICS
I have also provided options for converting between BBC mode-0 screens
and Windows .BMP bitmap-images. The program relies on BBC screens
having been saved by *SAVE filenam 3000 8000 and Windows bitmap
screens being Black & White and 640x256 in size. Other BBC screen
modes and different sizes of bitmap screens are not supported.
If you wish to convert BBC screens from anything other than mode 0,
you'll have to first use a program to convert your screen into mode 0
and then run the BBC-to-WindowsBitmap conversion. Such a conversion
program can be found in 8BS issue number 28.
If you wish to convert different sized Windows bitmaps to BBC mode-0,
you can use a windows-based picture editor such as 'Paint'. CUT an
image from a bitmap picture using the PICK tool, then edit the IMAGE
ATTRIBUTES to be 640x256 black&white, start a NEW picture and then
PASTE-in what you had cut from the previous picture. More help with
'Paint' can be found in any Windows manual.
When converting graphics between a Beeb and a PC you must note that
the BBC's pixels in mode-0 are twice as high as they are wide. This
will cause BBC mode-0 pictures to look somewhat squashed when
transfered to a PC, and likewise a picture from a PC will look
stretched when transfered to a Beeb. The best way I have found of
getting round this problem is to either stretch or squash the image on
the PC before or after the conversion.
THE CONVERSION PROGRAM - GENERAL NOTES
Standard BBC filenaming syntax applies. I've found it very helpful to
keep the BBC versions of files on one side of a disk and the PC
versions on the other. For example, to convert a file called
'BBCORIG' on side-0 of a disk and write the converted version on
side-2 under the name 'PCVERS' you would enter the Source Filename as
:0.BBCORIG and the Destination Filename as :2.PCVERS
It is worthwhile compacting your conversion disks every so often using
the *COMPACT command. This will reduce the risk of 'Cannot extend'
errors during file conversion.
The conversion program is rather slow when working with text files as
it reads-and-writes every character in turn. I wrote the program
merely as a means to an end. If any 8BS members could write a faster
version, in assembly perhaps, your efforts would be very much
SJG, 8BS member K5U
8BS Issue 33 - Joining 2 Computers Part 1 (S.Flintham)
8BS Issue 33 - What Next? (C.Richardson)
8BS Issue 34 - Joining 2 Computers Part 2
BBC User Guide - Using Printers - Serial Port (p406)
BBC User Guide - FX Calls (p421)
BBC User Guide - ASCII character set (p490)