15-08-2011, 05:58 PM
I recently had a problem of trying to remote control up to 8 devices and all I was given for communications was a fibre optic link driven by an RS232 to Optics control box. A similar fibre optic box was positioned at the other end of the line to recover the RS232 signals. My original thought was to buy off-the-shelf units but all required the use of a PC at each end! So could I use a PIC, turns out this is not so easy either as you need to be able to program the device first and I did not want to spend a fortune just for that. My limited budget meant a cheap option was required so I chose the rather ancient 6402 UART. OK, have a chuckle if you like but the beauty of this chip is that it was pre-PC era, capable of operating on its own without any software supporting it. I was successful in making both a transmitter and receiver as separate stand alone units and was able to go one cleverer by sending back the signal to the transmitter, the RS232 link is bi-directional. So when I activate circuit 1, the corresponding piece of equipment in another room is turned on. This sends a signal back and illuminates an LED in the Transmitter to show that circuit 1 was successfully activated. It works a charm, so well in fact that I built another set for another area. The units are used as a safety interlock so are very important and have to be fail safe. Anyway, I anticipate making more of these units so I thought a test board would be useful based on my original design. With this I can check that all eight circuits can be sent or received on either Transmitter or Receiver and I have attached the circuit and board layouts for this. The switching is Active LOW, done on my test board by the DIL switches. The serial data is fed to a MAX232 for conversion to RS232. By looping back the signal into the MAX232, a serial stream at TTL levels is sent into the UART and the corresponding output is made to go Low. Eight segments of the Bar graph display are used to indicate the status of the UART outputs, segment 10 is used simply as a +5V presence indicator whilst segment 9 is used to show data activity. It was implemented into the designed so that if somebody unplugged an RS232 cable then it would shut down the remote units. By using separate switches instead of the DIL, or even using something like opto couplers, you can use the board to transmit a copy of the switch set up to a remote location. Another board being used as the receiver could be fitted with Relays instead of the bar graph but this is a test board and the presence of 'always on unless active' LED's was not a problem. You may wish to fit inverters between UART and source driver to make them fail safe in operation, the way my Receivers are configured and anyway you don't want relays turned on all the time!
So there you have it, this item was knocked up on the kitchen table one weekend so I think it can be classed as home brew. I think the novel piece of design on my part was not just using a counter with built in oscillator to produce the 4800 Baud clock (all done before) but to use another counter output to provide the data clock. I fitted rubber feet on each corner of the board so it can stand safely on the workbench.
Les
So there you have it, this item was knocked up on the kitchen table one weekend so I think it can be classed as home brew. I think the novel piece of design on my part was not just using a counter with built in oscillator to produce the 4800 Baud clock (all done before) but to use another counter output to provide the data clock. I fitted rubber feet on each corner of the board so it can stand safely on the workbench.
Les
