From: tbc@netcom.com (Mike Garvey)
Subject: Re: Question about //e serial ports
Organization: Netcom - Online Communication Services (408 241-9760 guest)
Date: Thu, 12 Nov 1992 01:25:03 GMT

In a prior missive, sbennett@gnh-starport.cts.com (Stephen Bennett) eloquently
scribed this stuff:
 
>> 2) Does the port support 9600 baud (I have
>>   a Concord Data Systems modem)?
> 
>Yup, it will:  Here's how to set it.  There's a black jumper block on the
>right side of the SSC as you look at the components.  This is how you switch
>it between use for a modem or printer.  Pull out the jumper, and put it in
>so that the point of the triangle is pointing UP (there should be the word
>MODEM over the block) instead of down (where the SSC should say TERMINAL).
>You also need to move switches 1-5, 1-6 and 1-7 to ON, and 2-7 to OFF.
>(There are two types of switches that have been used on the SSC.  If you
>have rocker switches, the switch is ON when the top of the switch is pushed
>in.  If you have sliding switches, the switch is ON when the switch has been
>slid to the top.)


The Super Serial Card can support a maximum speed of 19.2Kbps. The actual
functions of SW1-7 and SW2-7 are only documented in Apple IIe Technical
Note #7. When SW1-7 is on, pin-8 of the DB-25 is the Data Carrier Detect
Signal (DCD). When SW2-7 is on, pin-19 is a Secondary Clear-to-Send (SCTS)
signal.

It is ironic that these two signals aren't fully documented except in a
technote. It is unfortunate that DCD and CTS aren't handled properly by the
SSC either. Let me explain. For most applications, a cable that ties the
DSR line (pin-6) at the port to the DCD line (pin-8) of the modem, is
required. Without this connection, software cannot know when the modem has
disconnected with the remote modem. Most Apple II communications software
thus reads the DSR line for carrier status. Also, the SSC cannot
communicate with the modem unless it believes that a carrier is present.

High-speed modems require hardware flow-control for maximum speed.
Unfortunately, the SSC doesn't like a 'straight-through' Clear-to-Send
(CTS) signal on pin-5. Paul "The Oggman" Parkhurst has been recently
working on some high-speed modem drivers for GBBS and could only get
flow-control working correctly by using the following cable:

Apple Super Serial Card to Modem

SSC (DB-25)	Modem (DB-25)
-----------------------------
  2 (TxD)  ------>  2 (TxD)
  3 (RxD)  <------  3 (RxD)
  4 (RTS)  ------>  4 (RTS)
  6 (DSR)  <------  8 (DCD)
  7 (GND)  <----->  7 (GND)
  8 (DCD)  <------  5 (CTS)
 20 (DTR)  ------> 20 (DTR)
              |
  5 (CTS)  ---

The problem is that the first character sent after the modem resumes
transmitting (CTS falls, modem pauses, modem empties its buffer, CTS rises,
modem resumes) is corrupted. This cable basically asserts CTS constantly
during a connection and uses the DCD line for transmit flow-control. So now
the working interface has DSR acting as DCD, and DCD acting as CTS --
confused yet?

>> 3) How do you select the speed?
> 
>Switches 1-1 through 1-4 are for the baud rate settings.  For 9600 baud,
>you should set them as:  1-1 through 1-3 OFF, 1-4 ON.

The baud rate of the SSC is only read when power is first applied; most
communication software ignores the settings of these switches. In fact,
I've gotten best results with SW1-1 through 1-4 on (i.e. the null,
undefined setting).

>Switch 2-6 controls forwarding of interrupts to the Apple II.  I believe
>that this should be set to OFF for the //e, so that interrupts won't be
>forwarded to the computer.
 
You definitely WANT interrupts forwarded to the computer when using a
high-speed modem, otherwise you are sure to lose data; SW2-6 should be set
to on.
 
>> 4) What are the pinouts?
> 
>It's a standard DB-25 connector, and if you've switched the jumper, that has
>the effect of having switched the Transmit Data and Receive Data pins (pins
>2 and 3, as I recall) that's normally done in the modem cable, so you can
>use a standard straight through DB-25 cable, probably the same one as you
>were using to the printer, if I remember all of this correctly (It's been
>awhile since I've done much with SSCs, but this should be correct).
>If I'm incorrect about the cable, I'm sure someone will correct me, I'm
>pretty sure everything else is correct...  :)

I recommend using the cable I described above.
 
(Wanders off mumbling something about Paul has a similar cable hack for the
GS (again, CTS on the DCD line) that also neatly sidesteps the problem with
the 26LS32 chip that Brendan Hoar reported here. Funny thing, Apple IIs and
the CTS signal)

Be seeing you...
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