Kidderminster Repeater Group - KIDDER Packet Node Components

The KIDDER / GB7PZT packet system consists of 3 computers, one for the KIDDER:G8PZT radio router, one for the GB7PZT BBS, and one as an internet router / firewall. A fourth computer is normally used to control and monitor everything using telnet and HTTP sessions over Ethernet links.

The radio router uses G8PZT's "XRouter" software above DOS 6.22 on a 33 MHz '486 PC.

One serial port drives 12 polled KISS TNC's (located remotely) via a long RS232 cable and a diode matrix. The Ethernet port links to the BBS and several other machines, including the Internet router. A hardware watchdog (not shown) monitors the TNC polling activity and would reboot KIDDER if polling stopped for more than 60 seconds.

The TNC's are assorted Tiny2, TNC200, Thor and Maxpak types, some with G3RUH 9600 bps modem boards. All have been modified to reduce hash emissions (an unmodified TNC may seem fairly quiet, but put several of them together and you've got a big problem!)

The radios are mostly modified ex-PMR types, with a couple of amateur rigs. All RF is filtered by band pass filters (VHF) or cavities (UHF) and duplexers are also employed on the full duplex ports.

Power for radios and TNC's is provided by a 12 volt 50 amp switch-mode unit, which also had to be modified to reduce hash.

The XROUTER software is a combined AX25 / IP network access point, Net/Rom node, IP router, web server, chat server, and APRS digipeater.You can find out more,and download a copy at G8PZT's software site.

The Internet router / firewall also uses XRouter to share the broadband cable Internet connection between all the LAN machines. A software router running on DOS was preferred for the following reasons:-

Windows 98 ICS (Internet Connection Sharing) was not up to the job. It was unstable, insecure, and needed frequent reboots.
Neither Windows ICS nor a hardware router would allow traffic to be viewed and logged.
The DOS platform is extremely stable, and boots quickly. It will run on any old junkyard machine
XRouter is flexible and secure, and allows traffic to be viewed and logged to disk.
It gives XRouter software a really good workout under hostile conditions.
The software can be rapidly adapted to cope with new requirements.

Diode Matrix

For clarity, this diagram shows only a 3 port matrix, but it is easily expanded by adding additional diode pairs for each extra port. All diodes are silicon small signal types such as 1N4148.

The polling signal from the TXD output of the computer's RS232 port (pin 2 of 25-way or pin 3 of 9-way connector) is connected straight through to the RXD inputs of all TNCs in parallel. All the grounds (pin 7 on 25 way connector) are linked together.

The TXD outputs of each TNC (which become the RXD inputs of the matrix) are diode-ORed together onto the RXD input of the computer, such that the RXD input is driven positive by a positive voltage on any of the TNC outputs, whilst current is prevented from feeding back to any TNC whose output is in the negative state. A pull-down resistor is connected to a negative supply derived from the combined negative outputs of all the TNCs (in theory, with no TNC's connected there would be no negative voltage, so a slight refinement would be to add a diode with its cathode connected to the computer's TXD, and its anode to the junction of the resistor and capacitor).

KIDDER / GB7PZT Components & Interconnections

Diode Matrix