Multiple Serial Port Switcher to Various Outputs

[Brent Benade]

I have the following challenge and would like to discuss options and work on a possible solution.
Infrastructure:

6-8 CNC machines that can accept code via a parallel port interface
2 Windows 7 Pc's with multiple serial ports used to send data to the above CNC machines
3 Lathes
1 Windows 7 Machine multiple serial ports to send to the Lathes above

Each device has a Null modem with loop back handshaking cable connected to it and runs back to the main office where it is manually connected to the Pc's and changed when each machine needs to be used.

Project scope:
Design a switching panel that accepts an serial input from each of the 3 computers (possibly 4 for future growth but not critical)
Manual selector switch that connects the input signal to the correct output device without interference on any other output device.

Thoughts and designs:

Rx,
Tx,
Ground
Range of +3 to +15 volts or the range −3 to −15 volts

Seeing that the RS-232 Null modem cable with loop back handshaking only uses 3 wires to communicate first line of thought was on a 3 pole 8 selection switch per input. Thus each of the 8 devices connected to 2x 3 pole 8 selection switches and each switch connected to the 3 pins of the RS232 to the input device. The challenge here could be, if Computer A is sending data via Switch A to device 3 on selection 3 and then Computer B wants to be connected to Device 4 on switch B. Switch B could connect to Device 3 for a moment while turning the selection switch to get to 4. Might cause data interference. Or if both selection switches were left on the same selection.

Thus they would have to be segregated that no connections could be made except to the desired selection.

First looked at relays but too complex and not cost effective 3 n/o 2 n/c Relay per 4 pole selector switch.

Then 3 transistors per device line with each B pin of each group of 3 transistors connected to a 1 pole 8 selection switch.
Thus when the selection switch is on lets say selection 2 it connects a trigger voltage to the transistor group that connects E to the output of Device 2 while all other group on C are still connected to the Serial port but with no output connections.

The same solution can be duplicated for the Lathe machines on a smaller level or added for a possible 4th programming computer.

I know this is a very crude and rough thought idea, but this is why I am asking for some advice or an easy way to solve this problem.
Cables are constantly broken by manually switching the device and large Serial Switching devices are very hard to find and proves to be fairly costly. Then where is the fun in purchasing something then you can make it. ;-)

Thanks for any help in advance

 

[Ian Rogers]

A small pic with a PSP and a serial connection on each lathe... All setup to use RS485... If you read through the EUSART part of the data sheet the pics can do automatic addressing using the ninth bit.... You can then send out codes via the PC to each lathe in turn..... Each pic will effectively be a serial to parallel convertor..

I'm currently doing something similar at the moment...

 

[Brent Benade]

Ian, thank you for the advice and direction.
I do understand the direction and can work out what to read up more about as I am probably a bit rusted in electronics, but love to learn and take on projects.
Would you care to elaborate a bit more on you project, and or give me a bit more direction for my project.

Thanks

 

[4pyros]

I would web enable your machines and make a network with your computers.
All you need are some web server converters like these;
https://www.usconverters.com/rs232-ethernet-converter

 

[Ian Rogers]

RS485 is used exactly the same way as RS232... But because its omnidirectional its half duplex... Which means one conversation at a time... So the PC would place an address on the bus and then some data... Each pic receives the same address and data, but each has a different address, so the hardware in the pic's allows you to only act on the data directed to that device... This way your pic will be a serial to parallel convertor....

 

[KeepItSimpleStupid]

I wonder if any of these https://www.moxa.com/product/Serial_Device_Servers.htm could work for you?

The bigger problem is how well serial to parallel conversion actually works.

I just got a Moxa 4-port serial server off of ebay for about $100.

 

[dougy83]

The interlocking and number of connections make this problem more complex. It can be done using electronics, but will require quite a few components.

Would it be suitable to simply replace the serial connectors with connectors that are easier to insert and remove? e.g. make a crosspoint switch similar to those used in the first telephone exchanges; you could have four thin cables (connected to the computer ports) terminated in audio jacks. These four jacks can then plug into any of 12 sockets (that will then connect to the equipment). You don't have to worry about accidentally connecting multiple PCs to the same CNC (or vice versa), nor do you have cables breaking all the time and the selection is intuitive and quick.

 

[KeepItSimpleStupid]

Matrix switching systems are quite expensive. I spec'ed two of them. One had a nice light-pen interface.

The other had to deal with switching picoamps of current.

And their were manual/ home brew ones two. Back in the day we had to "SHARE" two high speed 1200 baud modem lines. Most of the time they were connected to CRT terminals, but we had to "dump" data to a mainframe.

We also made a 4-wire remote switch for in house use, relay based and rotary switch based. Probably something like 4p8T

I did find two companies that make RS232 switches.