Date: Thursday, 26 February 1987  02:29-MST
From: larry at Jpl-VLSI.ARPA
To:   Info-IBMPC at MIT-MC
Re:   CTTY Answer

An answer to someone else's question follows, on how to set up a cable
to use another PC as the system console.  First you need a serial
port, of course.  The two kinds I've used for this purpose were both
from IBM: their Asynchronous Communications Adapter and
Serial/Parallel Adapter.  The following cable works on both when
connected to another ACA or SPA or to a VT100.

         pin 2 --------------\  /----------- 2     Transmit Data
                              \
             3 <-------------/ \-----------> 3     Receive Data

             4 --------------\  /----------- 4     Request To Send
                              \
             5 <-------------/ \-----------> 5     Clear To Send

             6 <-+-----------\  /--------+-> 6     Data Set Ready
                 |            \          |
             8 <-+           / \         +-> 8     Carrier Detected
                            /   \
            20 ------------/     \---------- 20    Data Terminal Ready

             7 ----------------------------- 7     Signal Ground

In case the diagram isn't clear, pins 2 & 3 criss-cross, as do 4 & 5
and 6 & 20.  Pins 6 & 8 are jumpered together.  The usual protocol is
that DTR comes on when power does and goes off only when a terminal
goes into setup mode or issues a break.  In effect, each device says
"I'm awake!"  The other device recognizes this via its DSR input and
prepares to cooperate.  When a device wants to transmit it activates
RTS ("Can I give you stuff?") and waits until its partner answers
("Socket to me!") into its CTS pin.

After the physical hookup is made, enter the appropriate mode command.
For a VT100 it's typically

                       MODE COMn:9600,N,8,1

for COM1 or COM2 at 9600 baud, No parity, 8 bits per byte and 1 stop bit.

Then type              CTTY COMn
to give control to the terminal (or terminal emulator) on the serial line.

To return type         CTTY CON
on the terminal.

You can only run programs that use DOS call for input and output.
BASIC and many word processing programs don't.

                                               Larry @ jpl-vlsi.arpa