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From: dempson@atlantis.actrix.gen.nz (David Empson)
Subject: Re: Apple IIc 'Real World Interface'?
Message-ID: <D9wJM8.LzA@actrix.gen.nz>
Sender: news@actrix.gen.nz (News Administrator)
Organization: Actrix - Internet Services
Date: Fri, 9 Jun 1995 10:54:08 GMT
References: <3r49j4$phf@styx.uwa.edu.au>
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In article <3r49j4$phf@styx.uwa.edu.au>,
Antony Lord <macgyver@tartarus.uwa.edu.au> wrote:
>
> Recently I have been toying with real world interfaces for computers :
>
> I have built several for a C64 (with its 8 channel I/O user port)
>
> I have designs for RS232C devices that work with PC's so I have set about
> asking questions on how to go between RS232C and RS422 (the printer /
> modem socket on a Mac.)
The Mac and IIgs serial ports are actually RS-422 or RS-423.
RS-422 is a balanced system - each data signal has positive and
negative signal wires, and the difference between them determines the
logic level of the signal.
RS-423 is an unbalanced version. When transmitting, you simply avoid
the use of the TxD+ signal, and use TxD- as your data output. On the
receive side, you must connect the RxD+ signal to ground (to correctly
bias the receiver) and use the RxD- signal as your data input.
RS-423 is reasonably compatible with RS-232. The only problem is that
the voltages are lower. The Mac and IIgs have a maximum output
voltage of almost +/-5V, while RS-232 specifies an input voltage range
of +/-3V to +/-15V (or thereabouts), and an output voltage of at least
+/-5V. The Mac and IIgs do not generate enough output voltage to
comply with the standard, but they still work with the vast majority
of RS-232 devices (modems, serial printers, etc.)
I have had one case where a device didn't recognise transmit data from
the IIgs because the voltage was too low.
The flow control signals have no positive pin - only the negative
input/output is provided.
The Mac/IIgs serial port pinout is as follows, looking at the back of
the computer:
8 7 6
5 4 3
2 1
The pin functions are:
1 Handshake Out
2 Handshake In
3 TxD-
4 Ground
5 RxD-
6 TxD+ (only used with RS-422)
7 General Purpose Input (not available on many low-end Macs)
8 RxD+ (ground if used with RS-232/RS-423)
The GPI pin goes to the carrier detect input of the serial chip. Its
usual function is carrier detect when used with with a modem.
With special software, this input can also be used to provide an
external clock for the serial port, which allows unusual baud rates to
be generated. For example, Apple's MIDI interface feeds a clock into
this pin to produce a speed of 31250 bps (or whatever the MIDI speed
is - something around there), which cannot be generated from the
internal crystal.
The GPI input isn't available on the Mac Plus, Mac Classic and most
models in the Mac LC family (and possibly others).
On the Mac, the handshake input signal can be used as a second clock
input, so in the case of a Mac with GPI, separate transmit and receive
clock frequencies may be provided. The IIgs doesn't support this.
> Inbetween all of this I have my trusty Apple IIc sitting on the desk. I
> have little idea how I could interface anything in terms of output to it.
>
> Does anyone have schematics or pin wiring diagrams? I have looked for
> various RS232 / Apple / Mac faq's without success.
The IIc serial port pinout is as follows, looking at the back of the IIc:
5 1
4 2
3
The pin functions are:
1 DTR (flow control out - actually the RTS pin on the serial chip)
2 TxD (data out)
3 Ground
4 RxD (data in)
5 DSR (flow control in - actually the DCD pin on the serial chip)
The same pinout applies for both ports.
--
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand