SJ Bridge

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What it does
Acorn bridge
SJ bridge

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Misc info

The SJ bridge

The SJ bridge is powered by a 6502 processor using the ADLC (unlike the SJ file servers which are Z80 designs utilising the Z80-chipset SIO for network comms).

The SJ bridgeAn LCD display and three buttons on the front of the box allow instant status checks and also a friendly configuration. As this is SJ kit, it is not unusual to see a keyswitch for Run/Admin selection.

The box itself is a solid grey ABS plastic box with a white front panel. In side, it is about the same size as a decent book on learning to program in C. For those who have never attempted, it is not unlike PRM 1 in size. If you're still stuck, forget it, it isn't that important!

There are three network sockets on top. From left to right:

  • Net B - Clock Out
  • Net B - Clock In/Out
  • Net A
You may use the bridge as a simple link between two existing networks (Net A and Net B Clock In/Out) or you may use the bridge to commission a new network to operate fairly autonymously to the existing one, whilst allowing data transfer between all stations if necessary. In this case, the SJ bridge can also operate as a clock... and what a clock! The setup allows you to set the clock intervals down to the quarter microsecond!

With this level of configurability, a suggested use for the SJ bridge is to run your old 6502 equipment (FileStore, BBC micro, etc) on network A with a fairly slow clock. Then you can use the bridge to provide clock facilities on network B with faster RISC OS machines. Given the extensive flexibility of the SJ bridge's clock, you can eke out the very best clock settings for your particular installation. Rather like tuning a car - you can tune your network!


Setting up the bridge

If you are using the bridge in a safe environment, such as your bedroom, then set the keyswitch to "Admin" and leave it there.

When you power on, the bridge will display a scrolling text saying "SJ Research Econet Bridge ver 3.04". Afterwards, it may report an error or say:

x Clks x

To set up the bridge, assuming the keyswitch is in the 'Admin' position, press and hold the Menu button for a second.

The first thing the display will ask is:
NetA: 1
You can use the No/↓ and Yes/↑ buttons to select a network, between 1 and 127.

When you have selected the number for network A, press Menu and the display will change to:
NetB: 1
Same thing. Choose a different number for network B. If in doubt, just go for NetA = 1, NetB = 2. If this is a home-bedroom installation then the numbering doesn't really matter. It only starts to get complicated if the network contains multiple bridges.

Press Menu once more and the display will change to:
Clk: 5us
You should choose the overall period of the clock pulse. Your options are:

13µs, 12µs, 11µs, 10µs, 9µs, 8µs, 7µs, 6µs, 5½µs, 5¼µs, 5µs, 4¾µs, 4½µs, 4¼µs, 4µs, 3¾µs, 3½µs, 3¼µs, 3µs, 2¾µs, 2½µs, 2¼µs, 2µs, 1¾µs, 1½µs, 1¼µs, 1µs, Arc, MDFS, BBC, or Off.
Once you have selected the clock period, press Menu to choose the duration of the mark pulse. The display will show:
Your options are:
2µs, 1µs, 0¾µs, 0½µs.
Now press Menu again and you will see:
You should now press the Yes/↑ button to instruct the bridge to begin operating as a bridge.

At this point, the bridge will probably display:

/ Clks x
(the '/' is a tick, but this isn't an HTML glyph)

Connect network A, if it isn't already connected, and/or switch on the clock. The display should then change to:

2      1

This is correct. When there is traffic between the networks, you will see this reflected in the LCD, showing the direction of data transfer too.


The SJ bridge in use

Apart from the method of detecting a bridge, the SJ bridge is completely transparent in use. The display will keep you informed, additionally reporting if there are any problems.


Error messages

I do not know where my SJ bridge instructions are (last seen 1999!), so I cannot currently tell you anything about the error messages or the self-test.
If you have a copy of this document, please contact me!


Help! I have no admin key!

If you open the box up and look on the lower right of the main board, you will see the key connection:
SJ bridge key connection

The easy way...
The only difference between Run and Admin mode is that you can access the settings when in admin mode. The bridge will run as a bridge in either mode.

Therefore, the simplest way to lock the bridge into admin capabilities is to unplug the key connection and wrap some wire or (as I have) a short length of solder around the terminals of the switch connector:

Forcing admin mode, the easy way

The hard way...
You may prefer to have the switch operational, at least to fool others into thinking that a special key is required.
Here's a step-by-step guide to making that happen.

The first thing you must do is take the keyswitch out. That big brass nut simply unscrews half a donen times, then the whole arrangement can be pulled through the front panel and away.

The keyswitch, alone.

Next you need to remove the electric switch. This is the white part and it clips on to the key mechanism. The plastic is quite brittle so be very gentle or you risk breaking bits off.
You will be left with the key mechanism.

The key mechanism.

Now here's the really hard bit. If you are a child this is probably the time to call for an adult to help. And if you are an adult, you might find a child can do this better. In any case, don't use knives!

Okay... Pick up the key mechanism and look at the business end. If you are staring at the key hole, you're looking at the wrong end!
You will see that it is a circular sort of thing inside the outer tube, and that the only thing stopping the inner sliding out from the outer is what appears to be a small metal ring.
This metal ring has a miniscule split in it. If you require glasses to see the split, you may find this to be somewhat tricky.

The idea, and I leave exact implementation up to you, is to force the split wider, a millimetre or two, and then to knock the ring out of its groove, then slowly force it away from the groove until it can be completely removed. Note that you must be careful for final removal as it could 'ping'.

The key mechanism, split ring removed.

The way I did this was a fairly large straight-head screwdriver. Wedge the tip between the two sides of the split, then twist to pop the ring out of the groove. Then just faff around until it comes free.
That worked for me, however I am going to let you think up your own method here due to the potential for solid metal tools impacting into soft squishy fingers. And, as you would imagine, I disclaim all liability - especially if you attempt this with a samurai sword and the all the tact of Alan Sugar, then wonder why the keyswitch is in one piece but various limbs are lying on the floor...

Eventually you will be able to slide out the inner part of the key mechanism. It will look like this:

The keyswitch, in bits

If you are at all interested, you ought to be able to download an 'anarchy file' to do with lockpicking that should give a pretty good résumé of how the lock works, though note that these days this mundane information is probably considered a "terrorist weapon" in countries that really should be more enlightened.

Whatever, the next stage is to tip all the tumblers out of the barrel. There will be five flat metal things, and five amazingly tiny springs.
You might like to put these someplace safe in case you ever want to play with the lock in the future?

The next step is, essentially, a reversal of the above. Slide the barrel into the outer tube. Have loads of fun trying to get that stupid split-ring back on. Pop the switch onto the back, fit the whole thing into the bridge, do up the nut, plug it in...

I make it sound simple. Actually it was a doddle really, the only complication was the split-ring. Using the screwdriver I wedged the split about 3-4mm apart, along the edge of the screwdriver's blade. This allowed me to hoop a large part of the ring over the key mechanism without it pinging. I carefully removed the screwdriver, then used my thumbs to ease the ring down and into the groove.

Here's the final result:

Using the key
As you can see, I am using my flat-blade screwdriver to activate the switch. Anything that fits will work, however you can easily fool people by using a key that fits. Unless they've read this document, the majority of people don't tend to think to stick screwdrivers into locks to see if they really work!
Thanks to the definite click-click action of the electric part of the switch, the lock is held in place quite firmly. It is always a giveaway if you can turn the lock slightly with your finger. Not so in this case!


The main board

Here is an annotated diagram of the SJ bridge main board:
The SJ bridge main board, annotated
(click for large non-annotated image, 1855×1234 800K)

A dump of the v3.04 firmware, should you require it, is available in the main index.

Copyright © 2008 Rick Murray