From: Johannes Wolf (mp2100_at_mail-gw.estec.esa.int)
Date: Tue Mar 22 2005 - 06:06:38 PST
Again, the Mac provides a symmetric RS-422 interface (RX+ and Rx-, Tx+ and
Tx- & several handshake signals like HSKo and HSKi).
On the other side the MAX 232 - as the name already says, is a TTL to RS-232
level shifter!
Therefore you do NOT have RX- and NO Tx-.
In order to make an adaption RS-232 to RS-422 you need to connect Tx- to Rx
(MAX232) and the Rx- to Tx (MAX232)while the Rx+ (Mac side needs to be
connected to GND in order to interprete the UNSYMMETRIC signals coming from
the MAX 232 correctly)
Sorry, you are talking about serial channel2 on the NIC port???
I do only see channel0 and channel3 there.
channel 3 are TTL signals that could be used for yor project.
But the main problem I see here is the software bit - you need to address
this channel in order to use it, of course it will not work automatically.
As far as I know the serial channel2 (internal) is implemented into the OS
such that you can use PCMCIA card modems (or other serial devices like the
GPS cards).
Cheers
Johannes
> -----Original Message-----
> From: newtontalk-bounce_at_newtontalk.net
> [mailto:newtontalk-bounce_at_newtontalk.net]On Behalf Of Adriano
> Sent: Monday, March 21, 2005 7:31 PM
> To: newtontalk_at_newtontalk.net
> Subject: Re: [NTLK] Interconnect second serial port (was: FDTI usb)
> Thanks to your help something is clearer to me now, but i still do not
> know how to use this infos for communicating with the second serial
> port on the interconnect port.
> Please let me explain better the tests and results i found working
> around this:
> the TX and RX output signals on the second serial port are not
> specified if plus or minus, and i do not have the instrument for
> accurate measurements. Although the lack for the right instruments
> (i.e. oscillometer), i tried to have the second serial port
> communicating using a TTL (Max232) level shifter adapter.
> So i connected the Messagepad second serial channels to a rs-232 level
> shifter using the 5V+, TX, RX and GND signals from the interconnect.
> Then on the level shifter output i have connected a mac serial 8din
> female connector which shows the level shifter output signals on TX-
> (pin 3) , RX- (pin 5), with RX+ (pin 8) grounded on pin 4.
> I have leds connected on both sides of the level shifter, so the led on
> the Newton side light-up when i start any connection from the
> MessagePad (thanks to the 5V output), instead the led on the level
> shifter output light-up only if connect a rs-232 to usb adapter, NOT if
> the adapter is a keyspan which translates a mac serial 8din connector
> to usb.
> Although i could assume that the board i have is pretty functional (as
> the leds light-up on both sides), instead i still haven't had the
> chance to see the MP2100 communicating anyway.
> I tried to connect:
> - Newton Keyboard (no success)
> - Newton Fax (no success)
> - Docking on a Mac (again no success)
>
> I am getting mad, so any help will be a breathe....
> Thanks,
> Adriano
> P.S.: pardon me if am offending any good technician present in this list
> Il giorno 21/mar/05, alle 11:00, Johannes Wolf ha scritto:
> Indeed it looks like that,
> but Tx+ and Tx- are SYMMETRIC differential output signals of the RS 422
> driver:
> Tx+ ranges from 0 to + 12V and Tx- from 0 to - 12V.
> When you apply an unipolar logic high signal (i.e. from the Cirrus
> chip),
> you get +12V at the Tx+ and -12V at the Tx- output at the same time,
> but the
> Tx- is NEGATIVE.
> converting the symmetric signals back to unipolar logic using a
> differential
> input, you recover the original signal in the first place.
> But dependent on the levels, a differential input connected to 0V and
> Tx-
> might interprete the signal correctly and indeed then you have the
> inverted
> signal of the original input.
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