FUNKYWARE.ORG - MODEM-2.TXT - 28.07.1997


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FUNKYWARE.ORG - MODEM-2.TXT - 28-JUL-1997
                     Updated: 27-OCT-1997

NOTE: The following text discusses two modifications to allow
      speeds of 57600, 115200 and 230400 on Modem-2 through a
      change of SCC clock speed.

      First, a good translation of Harun Scheutzow's simplest
      modification, which involves cutting a trace and taking
      the clock from Serial-2 with a small wire.

      Second, my rough translation of an earlier modification
      by Franz Sirl, which is a bit more elaborate. It uses a
      new timing crystal to provide a clock speed of 14 MHz.

      Both modifications are recognized by HS-Modem and could
      require setting a few additional parameters in SCC.PRG,
      ESCC.PRG or, if you prefer it to HS-Modem, Fast_Serial.

      While every precaution has been made to ensure accuracy
      of the information, neither their authors nor Funkyware
      offer any guarantee about the results.

                 ATTEMPT AT YOUR OWN RISKS !!!

[ NOTE: Those interrested in finding out more about the various
        serial ports on an Atari and learn how 76800 and 153600
        bauds became plausible choices, should also read Ronald
        Anderson's text elsewhere on this site. ]

***
By: Harun Scheutzow
In: HS-Modem 7 docs (translated by M-E Racine)

SCC.PRG, ESCC.PRG, ST_ESCC.PRG
------------------------------
These are drivers for the serial interfaces realized by a SCC
or ESCC (eg: Z8530, Am85C30, Z85230). They work together with
DRVIN.PRG or an equivalent replacement.

General
-------
Only an Z85230 and Am85C230A can qualify as an "ESCC". These IC
have an 8 byte receiver FIFO and a transmitter FIFO not smaller
than 4 byte. An ESCC contains all functions of the SCC.

The configuration options of SCC.PRG and ESCC.PRG are slightly
different from each another.

Clock rate and baud rate
------------------------
A SCC can use different clock sources for the generation of baud
rates. The most typical source is the system PCLK of 8 MHz, in a
normal (as delivered by Atari) M-STE, TT and Falcon.

This is a nice figure, but is rather unsuitable for generation of
standard high baudrates. These quick baudrates in M-STE/TT/Falcon
are generated from other sources.

It is possible to modify the Mega-STE/TT/Falcon with a new quartz
oscillator to a PCLK of 14.7456 MHz (as suggested by Franz Sirl).
However, if one only needs baudrates of 57600, 115200 and 230400,
a simpler modification is described below in the section called:
"Using 57600, 115200 and 230400 Bauds on the TT's Modem-2"

The drivers detect automatically wheter there is a PCLK frequency
of 8MHz or 14.7456 MHz and display the rate found upon bootup, in
their installation message.

With a PCLK of 8 MHz the following baudrates are possible:

         MODEM-2:               SERIAL-2:
      new       old           new       old
      ---       ---         115200      150
      ---       ---          57600      134
     38400      110  ------  38400      110
    153600       75           ---       ---
     76800       50           ---       ---

With a PCLK of 14.7456 MHz the following baudrates become
possible on both MODEM-2 and SERIAL-2:

    MODEM-2/SERIAL-2:
      new       old
    115200      150
     57600      134
     38400      110
    153600       75
     76800       50

If the GEMDOS function TIOC?BAUD is used to access the ports,
there is no problem, because it returns the available rates as
"bit per second" values.

The M-STE/TT/Falcon contain an ESCC only if somebody changed
the original IC. The driver for SCC runs on an ESCC too, but
the ESCC driver will not correctly run on a SCC.

SCC and ESCC
------------
Remember:  only the Z85230 and Am85C230A qualify as an "ESCC".

The MegaSTE/TT/Falcon contain an SCC in their original state.

To decrease the CPU load and to improve the data safety, it
is possible to replace it with an ESCC made as PLCC-package.
The SCC and ESCC are sufficiently pin-to-pin compatible.

SCC.PRG
-------
This is the driver for MODEM-2 and SERIAL-2 on M-STE and TT.

ESCC.PRG
--------
Similar to SCC.PRG, this driver only works on computers that
were retrofitted with a Z85230 or Am85C230.

SCC-drivers can work on an ESCC too, but don't take advantage
of the ESCC's advanced features. However, be aware that these
ESCC-drivers are very unsuitable for an SCC!

Using 57600, 115200 and 230400 baud on the TT's Modem-2
-------------------------------------------------------
(Only for expert hardware hackers)
The circuit trace between pin 17 of the TT-MFP (MC68901) and pin
32 of the SCC (Z85C30) should be cut (or pin17 could be isolated
using the plastic strip from the socket). Pin 32 of the SCC will
then be connected with pin 13 of the SCC.

To the corresponding question (M2TT) in the configuration of the
SCC.PRG or ESCC.PRG, you then have to reply that you are using a
M-STE/Falcon (ie: enter a value of 0).

***
By: Franz Sirl
In: Fast_Serial 0.96b docs (translated by M-E Racine)

I have developped a modification using a Zilog ESCC 85230 with a
14.7456 MHZ quartz oscillator.

Parts needed:
 1 ESCC Zilog Z85230-16VSC
 1 Quartz oscillator of 14.7456 MHz
 1 Fast_Serial version for a PCLK of 14.7456 MHz

Instructions:
 1. carefully remove the old 85C30 from its socket.
 2. Locate pin 23 on the socket (count clockwise, starting from
    the mark near pin 1)
 3. On the new ESCC, _carefully_ lift pin 23 (PCLK) away from
    the body of the chip.
 4. Insert the ESCC in the socket.
 5. Connect the quartz oscillator to the nearest 14-pin TTL chip
    (ie: 74xx) pin 2 (7) and pin 4 (14). Ensure you first locate
    pin 1 correctly!  Beware:  space is limited under the PSU!
 6. Connect pin 3 of the oscillator with pin 23 of the ESCC.
 7. That's it!

With this modification, both channels are now capable of producing
baudrates up to 230400. Should one also provide pin 13 and 32 with
(/RTXCA and /RTXCB) 14.7456 MHz, baudrate up to 921600 should also
be theoretically possible.

Custom-version:
Should anyone require Fast_Serial to offer different baudrates or
higher baudrates than described above, contact me. It is possible!

***
By: Martin-Eric Racine
Re: HSM modification addendum 27-OCT-1997

There are some late-production TT where all the chips are soldered
to the motherboard, making it difficult to upgrade to an ESCC, but
one can still perform the above single-wire mod by Harun Scheutzow
to achieve higher baudrates on Modem-2.

The trick is to carefully cut Pin-32 of the SCC close to the board.
Then, carefully bend up the pin, just enough to clear the remaining
stub still attached to the motherboard.

Afterwards, it's just a matter of soldering a short insulated wire
between Pin-32 and Pin-13 of the SCC.  Keep wire length as short as
possible, but leave enough so that it arches nicely above the SCC.

Configure SCC.PRG as above, entering a value of 0 for M2TT, then
reboot to confirm the new speeds are correctly recognized.

***
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      8530 SCC (Modem-2, Serial-2)



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