The lead probably reads B/Cast standing for Broadcast. These till were able to pass info to each other via the broadcast link, starting from the U500 an open daisy chaining to each till finishing at the last till in the chain. The cable is a four core cable ( white green black and red) connected on a 6 pin molex connector. Each till on the chain will have all four strands connected (brack/red transmit to next till, green white receive from previous).
Printers could be connected direct from the u500 or could be on print through ( the printer data could be transmitted to the till and the till would transfer the data to the printer and the till data cable would be marked up to show this config)
The barmux and the unilink are a simple multiplexers. They have the ability to connect various systems together (such as the flow meters in the cellar or the pads in the bar) and transmit that data to the u500 which can then process that data and identify which till such data is associated to.
The max trans distance on the old rs232 cable is 100m. if the distance was more the till data etc could be plugged into a unilink unit which would re-transmit the data for another 100m. In large hotels whole areas could be linked, transmitting to the u500 on a single cable.

[quote up to show this config)
The transformer in the cellar is probably a solar banner which is suppused to clean the in-cumming mains. It is a UPS (uninturrupted power supply) and has some batteries inside which supply power for some 15 mins to enable a safe shut down in the event of a power cut. If the batteries haven't been replaced in the last say four years it will not work as designed. Chloride make much better ups's with a much better back up time and filter capabilities. The molex connectors (and the d type connectors) are available from rs components and the tool cost just a few pounds. The usual reason for a fault is damp/rotting of the connections.
If you are intending to carry out your own repairs take note of the termination numbers (printed on the face of the molex and the back of the d types. New cable can be purchased from the likes of city electrical and is calles two pair rs232 screened.
Hope this is of help!

Thinking about it, having the classic system, the broadcast link will be on a 9 pin d type male using pins 2 3 and 7 ( 2 and 3 having the green and red and 7 having the two earth strands, black and white, twisted around the transmit and receive data strands within the screening shield of the Belden cable).
When some pubs were sold or tennented a lot of were left with some of their old systems remaining but never the beer flow system that was in the cellar. These flow meters fed data to the back office system )and HQ) warning the managers when to order new stock. There was even a lazer system available that shone a light through the optic rail that monitored the dispensing of spirits ( useful when the optics registered a sale but the tills did not!) In some establishments even the gas end electric meters were tied in so night time activity could be monitored and if the tills were as active! The system was developed to monitor all sorts of activity within the premises ranging from the bars to resturants to what went on in the kitchens and cellars in just about every type of business. the start of big brother I think!

OMG, seeing this topic and the photo has brought back many memories of working in a Bass managed pub (well three actually) - have you got any more pics of the tills screens (home, login, mgt menu etc), would love to familarise myself with the system again, it was simple to use yet as you mentioned so very effective for the role is served...

Yeah used to find a lot of xn700's running ICR Touch. Those XN's are tough little tills.
ICR Touch is a good bit of kit, fairly stable, skinable, report designer, inbuilt loyalty system. And the ICR TMS for back office is good bit of kit too.

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Here are 2 pictures. One of server keys and the other of the main screen. I will get some more as soon as I can.

I have spent time scanning the old sales information pack which I found at work. So here it is, enjoy!

It appears I can only upload 3 pictures per post, so here's the next 3.