🚀 go-pugleaf

I'm building a TDD (Telecommunications Device for the Deaf) interface
for my IBM PC.

Several years ago, I built a phone line coupler, and analysed TDD signals
by means of timing loops on my Apple ][.  From this, I determined a bunch
of stuff, but due to the nature of my testing mechanism, I'd like
confirmation of these details:

a)  TDD baud rate is 45.5 baud (sheesh).
b)  Basic carrier (space tone) is 1400Hz.
c)  Mark frequency is 1800Hz.
d)  There is always 1 mark start bit.
e)  There are 5 data bits.
f)  There is always 1 space stop bit.  (Would 1.5 or 2 work too?)

IBM PC specific questions:

a)  What UART chips do the default OS/2 drivers talk to directly?
b)  What UART chips do the default MS-DOS drivers talk to directly?

Electronics misc:

a)  I have a 4-port serial card made by "STB Peripherals" containing
    a "StarTech ST16C554CJ" chip for all UART functions.  Does this chip
    allow access to the "External clock rate" pins, or did they just
    dump that functionality when they crammed 4 UARTs onto the one die?
    If so, what pins?

b)  I also have a combo card (IDE/Parallel/2-Serial).  It has a single
    (see above about lost part #) "Acer M5103A3E" (or maybe
    "TS6 A20448", it's an unfamiliar number) chip.  Same questions as
    above -- does it allow access to the "External Clock rate" pins,
    and if so, what pins?

c)  Alternative to a) and b) would be can you tell me how to get info
    on these chips, eg a manufacturer's address/phone, or a parts
    reseller that could get me the info?

d)  What's your favorite means of connecting your analog circuitry to
    the phone line?  Is there one particular chip/transformer/whatever
    that would make The Phone Company happy and provide me signal
    passthrough with isolation?  For testing, I just used an old
    audio transformer I had in a box under the workbench.