››        'The McHugh Patch'››     Split Baud Rates With Any›       Atari RS232 Interface››          By Geoff McHugh›››       Did  you  know  that   it   is›possible,   despite   what     the›'experts'  say,  to obtain split baud›rates  on   the   Atari  850   (or›similar- including P:R connection)›RS232 interface? No, neither did I,›until very recently.›››       It all started when I received›a  copy  of the source code  for  the›RS232  handler  which I use with   my›Datatari  interface.  While   I   was›looking  through  the   listing,   It›dawned on  me   that   it  should  be›possible  to  get  the   handler   to›operate at split  rates   (which   it›was not originally intended to  do)›by using  a  few  direct  POKEs  from›within a terminal program. Since  at›the  time the  expert  opinion  was›that   the   850  interface    was›totally   incapable   of  split  baud›rates, I naturally  assumed that this›would  only  work  with  the Datatari›interface, so   I   constructed  some›program PATCHes for two  versions of› the   Amodem    terminal     program›(Amodem4  and  Amodem7),   labelled›them as being for  use   with   the›Datatari  interface  and  the   RHAND›RS232   handler  (which  I  use)  and›uploaded them to  my favourite  BBS,› The    ARK.     I     was  naturally›curious as to   whether   this  WOULD›work with the 850  interface,   so  I›asked the sysop to try it  out,   and›thought no  more  about  it,  until›I noticed a message from a   fellow›user to the sysop claiming  that  the›patched program  had  worked  with›his   P:R Connection   (supposedly›more-or-less  identical  to  the  850›interface).››       I   was,   of   course,   very›interested by this, so I again  asked›the sysop if he  would  try   again›with  his   850 interface. Imagine my›surprise when  I  next  logged-on  to›find that   my   patch  was   being›hailed  as  the   greatest discovery›in   Atari   8-bit    communications!›Apparently, the  patch had worked  on›the 850, and  the  sysop (James›Bastable)  was  quite  understandably›excited. I hardly  need to  tell  you›that I was just as excited as  him!›The     repercussions     of     this›discovery should strike  far   into›the world of  Atari  8-bit   comms.›Anyone  James  contacted  about  this›still   said  it  was   impossible,›until  he  proved them wrong. As  far›as I can tell, even Atari themselves›do  not  know  about this,  otherwise›they surely would have capitalized on›it! Maybe now the Atari will  benefit›from a   widely   available  ViewData›compatible terminal program.››     Now, I think I  have   waffled›on far too long, so I will  get  down› to the nitty-gritty of explaining to› you how to obtain split  baud  rates›on your humble  Atari  850  Interface›Module!››         How To Do It Yourself››      Ok,  where  should  I  start?›I suppose a very  basic  explanation›of what  happens  in  your  interface›handler is as good as any.››     Basically, the  handler   sets›up the  baud  rates   using    the›POKEY hardware  timers,  which  are›stored   in   the   Audio   Frequency›registers AUDF1  -  AUDF4  (locations›$D200, $D202,  $D204, $D206)  What  I› discovered  was  that AUDF1 &  AUDF2›store   the   timer     value   which›represents the transmitting baud rate›(ie the speed  at  which  your  Atari›sends data  out),  and  that  AUDF3›& AUDF4  store  the   timer   value›which represents the  receiving  baud›rate (ie the speed at  which   your›Atari  reads data in).››     The timer values  are   stored›as two-byte integers (ie have a value› of between 0  &  65535)  and   the›values corresponding to each   baud›rate  are shown in the table below:››     ----------------------------›      Timer Value      Baud Rate›          2976       300     Baud›         19661        45.5   Baud›         17891        50     Baud›         15727        56.875 Baud›         11925        75     Baud›          8128       110     Baud›          6646       134.5   Baud›          5958       150     Baud›          2976       300     Baud›          1484       600     Baud›           739      1200     Baud›           490      1800     Baud›           366      2400     Baud›           179      4800     Baud›            86      9600     Baud›     ----------------------------›       Table of timer values for›           specific baud rates›     ----------------------------›››     Let's take an   example.   Say›you  wanted  1200/75   split   baud›rates   to  access  your  favourite›Bulletin  Board (obviously     The›ARK).     So,      your  transmission›speed   is   75   baud,    and   your›receiving  speed  is  1200  baud.›Look at  the   table   above.   The›timer value  corresponding  to   75›baud  is 11925, and the  timer  value›corresponding to 1200 baud is 739.››    First, set your other parameters,›parity,  translation  mode,   word›ength, etc. When you use the XIO  36›command (or if you are using m/c then›when you call CIO with the cmdno  36)›then treat the baud value as  0  (300›baud   -     this   is   simply   for›convenience). After  you have entered›CONCURRENT I/O mode  (XIO 40)   then› POKE   (or    equivalent, depending› on    your     programming language)›the timer  values   into   the  POKEY›registers AUDF1-4.››     In this  example, the  addresses›and values to be stored in  them  are›as follows:››    Memory Location    Byte Stored›      53760 ($D200)      149 ($95)›      53762 ($D202)       46 ($2E)››      53764 ($D204)      227 ($E3)›      53766 ($D206)        2 ($02)›››     The values  can  be  explained›like this:›› Tx rate:149+256*49 = 11925 (75 baud)› Rx rate:227+256*2  = 739 (1200 baud)››››     Well,  that's  about   it!   I›would just like to make it clear that›I have made this important discovery,›and  it would be a sad reflection  on›any Atari owner if he tried to claim›otherwise.›What the Atari  8-bit   needs   now›are friends. It has enough    enemies›already, with Atari seeming  to  be›one of  them  (I  won't  complain  if›anyone at Atari  wants  to  prove  me›wrong,  but  I doubt if anyone will)››      I  would  like  to  thank  some›people   for   their   help   and›inspiration.  This may seem  trivial,›but it means  a  lot to  me  to  know›they are there  to  help  if  I  need›them.››     First of all,  I  would   like›to  thank  James  Bastable,  and  his›Bulletin Board, The ARK. If  it  were› not  for both James  and  The  ARK›BBS,  I  would never have  made  this›discovery,  for  it  was  from   this›source that  I  obtained the  Source›listing  for  the   RS232 handler.››     I would also like  to  thank  an›ARK user who I know  only  as  NITRAM› (or Martin), who convinced   James›to  try the  patch  out  on  his  850›interface.››     Last, but certainly not least,›I must thank Page 6 magazine,   who›have REALLY  supported  the   Atari›8-bits,   when   others   have   only›pretended. If it were not for Page 6,›I would not  even have  found   The›ARK  BBS,  which  has featured  so›prominently   in   this breakthrough.›››     So remember,  even   you   are›told something is impossible,   the›chances are that it's not!››Geoff McHugh,›  Co. Armagh››9th October, 1988››