from the Galaxy. Now it does not matter that 5, 6 and 8 are in each case an
input connected to an input, but it may matter that 4 and 20 are outputs
connected to outputs, and these outputs may be attempting to assert different
values. Fortunately RS232 devices seem very resilient, and I have not heard of
any damaged printers because of the two devices arguing over lines 4 or 20.
Similarly if you want to connect the Galaxy to a modem or acoustic
coupler, then not only is the socket the wrong type, but lines 2 and 3 need to
be reversed. But pins 4, 5, 6, 8 and 20 are now correct.
How to wire up the connectors correctly
The ideal solution is to wire two sockets from the present one. One would
be as a host, and the other as a terminal. The socket acting as a terminal is
identical to the present socket with two changes. Firstly it is a male socket,
and secondly the lines to pins 2 and 3 are reversed. The socket acting as a
host is identical to the present socket, except that the RTS line from the
Gemini is wired to pins 5, 6 and 8 of the socket, providing the signals
expected by a terminal. Also RTS should be wired back to the CTS, DSR and CD
lines leading to the Gemini UART, since this provides the signals that it
expects. Pins 4 and 20 on the socket contain signals from the terminal which
should not be connected to the Gemini board. The RTS line from the Gemini
should not be connected.
To make this clearer, here are the exact connections from the Gemini
serial connector to a host socket, which should be female.
|Gemini Serial Connector||Cannon D (Female)|
|Data in||3||<-----||2||Data from terminal kbd|
|Data out||6||----->||3||Data to printer|
|RTS out||10||--X X--||4||RTS out|
|CTS in||8||<---->*||5||CTS in|
|DSR in||9||<---->*||6||DSR in|
|CD in||1||<---->*||8||CD in|
|DTR out||12||--* X--||20||DTR out|
X means no connection. In addition to the connections shown, the DTR
signal from the Gemini is connected to 5, 6 and 8 in the Cannon connector.
A particular advantage of this wiring is that the DTR signal from the
Gemini provides a useful indication of the state of the Gemini RS232 interface.
When the Gemini is Reset, the Data out line gives an invalid data signal, since
it is not yet conditioned to even operate at the correct speed. But the DTR
signal informs the terminal that the host is not ready, and the terminal will
ignore the input. For example when connected to a Diablo terminal, pressing
Reset causes the audible alarm on the terminal to sound. Then a moment later
the DTR signal is restored and the terminal will function correctly. This is
most important if there is no IVC in the system, and a serial terminal is being
used as the console, either of RP/M or of CP/M. If you simply fix the input
Signals at the terminal so that it is always ready, then the initial messages
may be corrupted for about ten characters until bit alignment is achieved.
Serial Connections for the Nascom 2
These connections do not work for the Nascom 2 serial connector, since
this is not fully compatible with the Gemini. [Ed. – The Nascom 2 does not have
modem control signals, and although a handshake signal can be provided, this is
not at RS232 levels.] However you may find it useful to modify an N2 to conform