New csa2 FAQs: F002t1TCOM (Part 2 of 2)

[Rubywand <rubywand@swbell.net>]

From: Tony/ JoePRO Electronic Systems <anomaly3@mc.net>

016- How do I make an Apple 5-pin serial interface-to-DB9 cable?

If it's the standard 5-pin DIN connector used for Apple //c's ...

Get a "straight through" DB9 cable - it's got DB9 female connector on
one end, and DB9 male connector on the other - mainly used for extending
length of modem cables, etc.

Cut the male end off (if your serial port is female, then cut the female
end off ...) and strip the jacketing so all wires are exposed.

Now get out a continuity tester and a 5-pin DIN male plug, and connect
the wires as follows:

DB9 Connector     5-pin DIN Connector
-------------     -------------------

    N/C                   1

     3                    2

     5                    3

     2                    4

     4                    5

That's all there is to it!


The pin-outs for the plugs are as follows:

5-pin DIN - position the plug with the pins facing you, the "notch" on
top to the pins are in a "U" shape. Going clockwise, the rightmost pin
is 1, the leftmost pin is 5.

DB9 - position the plug so it is facing you, and that the 5-pin row is
on top. Pins 1-5 are on top, from right to left. Pins 6-9 are on the
bottom, again from right to left.

       DB9                 5-pin DIN
------------------      ---------------

 5   4   3   2   1        5         1
   9   8   7   6            4     2
                               3

_____________________________


From: David Empson

017- What kind of cable should I use to connect a modem
     to my IIc?

Here is the pinout of the IIc serial port.  Looking at the computer
from the back, the pins are numbered:

5       1
 4     2
    3

The functions are:

1  Handshake Out (nominally DTR)
2  Data Out (TxD)
3  Ground
4  Data In (RxD)
5  Handshake In (nominally DSR)

To wire up a IIc to a simple modem without hardware handshaking, use the
following pinout:

IIc     Modem
DIN-5   DB-25M

1 ----- 20 DTR
2 ----- 2  TxD
3 ----- 7  Gnd
4 ----- 3  RxD
5 ----- 6  DSR   (you might want to use pin 8, DCD in some cases)

The IIc cannot do hardware handshaking very well, but this is as close
as you can get:

IIc     Modem
DIN-5   DB-25M

1 ----- 4  RTS
2 ----- 2  TxD
3 ----- 7  Gnd
4 ----- 3  RxD
5 ----- 5  CTS

The IIc's handshaking lines have annoying side effects, which cause
problems with hardware handshaking:

1. The "Handshake Out" signal is implemented to mean "I want to send
data" (the official and original meaning of RTS).  If you turn off the
output handshake line, the IIc will stop sending data.  For a hardware
handshaking modem, RTS is supposed to mean "You are allowed to send me
data" (from the computer's point of view).

If the computer tells the modem to stop transmitting, the computer will
also be unable to transmit.  This will reduce the rate at which data can
be transferred bidirectionally, but doesn't cause any other problems.

2. The "Handshake In" signal is implemented to mean "There is receive
data present" (the official meaning of DCD).  If the incoming handshake
line is disabled, the IIc will stop receiving data (ignore any data on
RxD). For a hardware handshaking modem, CTS is supposed to mean "You are
allowed to send me data" (from the modem's point of view).

If the modem tells the computer to stop transmitting, the computer will
also be unable to receive, and will discard any data sent by the modem
while CTS is not active.  This can cause screen corruption and loss of
data blocks or acknowledgements during a file transfer, which will
require retransmission. It is only likely to be a problem while a lot of
data is being sent, so is more likely to cause problems during a file
upload than a download. If the comms software is quick enough, it can
drop RTS immediately when CTS is lowered, which will prevent the modem
from sending any more data.

The original IIc motherboard has another problem: it uses a cheap method
of generating the clock frequency for the serial ports.  Most
implementations based on the 6551 chip use a 1.8432 MHz crystal, which
gives exact baud rates, but the IIc takes the master system clock
(14.31818 MHz in an American IIc) and divides it by eight to produce
1.7898 MHz.  The 3% decrease in clock frequency produces a 3% drop in
the baud rate, which is enough to prevent operation with some serial
devices, particularly intelligent modems running at 1200 bps or faster.

This not always a problem, and I have successfully used one of these
IIc's with a ZyXEL U-1496E modem and a direct connection to a IIgs at
9600 bps.  I have had problems in other cases.

You cannot identify whether you have this motherboard except by opening
up the computer and looking for a tell-tale component.  (I don't have
the details handy.)

You can make a reasonable guess at whether you have the original
motherboard by checking which firmware version is installed in the IIc.
Get into BASIC, and type PRINT PEEK(64447).

If the value displayed is 255, then you have the original ROM, and
almost certainly have the original motherboard, though it might have
been modified to use a crystal.

If the value displayed is 0, then you might have the original
motherboard or the revised one.  (This ROM version supports the UniDisk
3.5 drive.)

If the value displayed is 3 or 4, then you have the "memory expansion"
version of the firmware, and almost certainly have the latest
motherboard, which includes a memory expansion slot under the keyboard.

In theory, the IIc's maximum baud rate is 19200.  Whether it can
actually keep up with that rate is another question.  9600 should be
fine.