***********************************************************
* HOW TO MAKE A SERIAL CABLE FOR BBC <-> PC FILE TRANSFER *
***********************************************************


Ingredients
===========

- shielded cable with at least four wires; length depending on needs, up to
  many meters
- 5-pin Domino DIN-plug (you might have to shop around for this item; they're
  not that common anymore)
- 9-pin D-type plug or 25-Pin D-type plug, depending on your PC's serial
  port(s)


Recipe
======

Make the following connections:

Pin   Pin
(9 P) (25 P)    PC                              BBC           Pin
----- ------    --                              ---           ---
 5     7        0V (Ground)                     Gnd (Ground)   1
 2     3        RxD (Data in)   --------------  TD (Data out)  2
 3     2        TxD (Data out)  --------------  RD (Data in)   3
 1     8        DCD             ---+----------  RTS            5
 6     6        DSR             ---+
 4     20       DTR             --------------  CTS            4
 7     4        RTS             ---+
 8     5        CTS             ---+

(The +'s mean that locally on the PC DSR has to be connected to DCD and that
RTS has to be connected to CTS.)


Pin numbering
-------------

PC:                                           BBC:
9-pin D-type       or: 25-pin D-type          5-pin Domino DIN-plug
                                              (male)
_____________      _______________________      _____ 
\ 1 2 3 4 5 /      \ 1  2 .  .  .  .  13 /     /     \
 \ 6 7 8 9 /        \ 14 15 .  .  .  25 /     / 5   3 \
  ---------          -------------------      |   1    |
                                              \ 4   2 /
                                               \     /
                                                -----

The views of these figures are all from the outside (the backs of the
computers) into the sockets, i.e. they show the wiring sides of the plugs.


Some tips
=========

If you plan to wire up your BBC to a PC, you might learn from what I
have done wrong:

- You can insert the BBC-plug into the RS423 port in two ways. Do it
  the right way! (A handy tip I received is to put some Tippex on the
  top of the plug.)
- Find out which PC's RS232 port is COM1 and which one is COM2. I thought
  COM1 is always the 9-pin socket and COM2 always the 25-pin socket, but
  on the PC I used it was exactly the other way round! (Btw. Does anyone
  know of an easy way to find this out?)


# The Theory
# ==========
#
# It will be clear that the Beeb's Data-In has to be connected to the
# PC's Data-Out and vice versa.
#
# It is more complex with the other connections. These are meant for the
# hand-shaking between the Beeb and the PC. This basically means that the
# Beeb and the PC can tell each other when they are ready to send/receive
# data. A handshaking mechanism prevents one of the computers to send,
# when the other one isn't ready to receive, in which case data would get lost.
#
# The reason for the unclarity of the handshaking pins, and also the reason
# why there appeared to be two "schools" of people on how to connect a Beeb
# and a PC during a recent discussion on the BBC-mailing list, is two-fold:
# 
# - the RS423 and the RS232 are only *interfaces*. An
#   interface definition only defines the *local* aspects of an
#   interconnection between the computers and not the *remote* aspects.
#   (Both the local and remote aspects together form the communication 
#   *protocol*, which is the important thing in our case. For a good
#   treatise on interfaces and protocols read "The design of Telematics
#   Systems" by C. Vissers who is my PhD supervisor :)
# - the RS232 interface definition (CCITT V.40 or V.42, I believe) is just
#   plain bad (its definitions of the functions of the pins are ambiguous).
# 
# If the above sounds a bit too theoretical to you: the problem is that
# you have a lot of freedom in connecting the pins, depending on the
# communication protocol you use. 
# 
# On the Beeb the above is not much a problem, because the RS423 is very
# simple: if the Beeb wants to send it raises its RTS signal and waits
# until its CTS signal is raised by the other computer; for receiving it's
# the other way round. 
# 
# However, the RS232 of a PC is more complex. Its definition suggests (but
# not necessarily prescribes) that the DTR pin has a function similar to
# the Beeb's RTS pin and the DSR pin a function similar to the Beeb's CTS
# pin. Most protocols implemented on the PC (and all I tried) adhere to
# this. This is why you have to connect the Beeb's RTS to the PC's DSR and
# the Beeb's CTS to the PC's DSR. 
# 
# But things are more complex. The RS232 also has a DCD (data carrier
# detect) pin, which can be used to detect if there is a device connected
# to it. Most (but not all) protocols implemented on a PC use this. As
# the Beeb has no complementary signal, a "circumvention" is needed. The
# easiest way to circumvent this is to connect this pin to the PC's DSR
# pin, so that every time it receives a DSR-signal, the PC automatically
# knows there is also a device connected. 
# 
# To make things even more complex, the RS232 also has an RTS and a CTS
# pin, albeit with different functions than on a Beeb. Some protocols
# implemented on the PC use these for a second handshake after the
# handshake with DTR/DSR/DCD. (Don't ask me what it is good for.) The
# easiest way to circumvent this, is to connect the PC's RTS pin simply
# to its own CTS pin.
# 
# If you want to experiment with the different types of handshaking
# described above: it is easy to do so with QBASIC (included in MS-DOS
# versions 5 to 6.20, I think). Look at the help item for "OPEN COM".
# 
# 
# To conclude
# ===========
# 
# - The wiring required for the handshake depends on the protocol you use.
# - Nearly all protocols implemented on the PC treat the DTR pin as the
#   Beeb's RTS pin and the PC's DSR pin as the Beeb's CTS pin; so you need
#   to cross-wire these.
#   (To the people who think RTS/CTS should be cross-wired on both computers:
#   I'm not saying you were wrong or that your suggestion will never work;
#   however, if it works for you you're using a pretty esoteric protocol
#   and it will probably not work with most other protocols.)
# - Many protocols implemented on the PC also wait for a DCD signal during
#   handshake; therefore it's useful to connect DCD to DSR on the PC.
# - Some protocols use a second hand-shake. In order to "circumvent" this, 
#   connect the PC's RTS pin to its CTS pin.

$Id: SerCable.txt,v 1.1 1999/11/04 09:21:16 angus Exp $
