80-Bus News


May–June 1983, Volume 2, Issue 3

Page 39 of 59

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 ConnectorCannon D (Female)
Data in3<-----2Data from terminal kbd
Data out6----->3Data to printer
RTS out10--X X--4RTS out
CTS in8<---->*5CTS in
DSR in9<---->*6DSR in
CD in1<---->*8CD in
DTR out12--* X--20DTR 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

This is an OCR’d version of the scanned page and likely contains recognition errors.

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