Need help connecting Raspberry Pi to old Com-trol MCS-4000 boards.

Question

Hi All, I have a Raspberry Pi V2 that I'm trying to see if I can connect them to an old Com-trol MCS-4000 control boards. The Com-trol system had digital & analog input boards and relay output boards. The input and output board use the same IC chip. The IC chip is a p87c51sbpn. It uses UART to communicate. The Com-trol system is powered with a 24vac transformer. They connect to the boards as (12vac, ct, ground, 12vac). Then boards are then connected together and to the main console (MCS-4000) via the local bus. The local bus is a serial type communication. How do I check to see if I can connect it to my Pi? How would I get them to talk to each other? If I can't us the boards can I just use the p87c51sbpn IC chip. I do have PDF's for the Com-trol system if anyone needs them. They are very hard to find. Teeterbuilt Message was edited by: Teeterbuilt Added PDF's 12/27/13. 
 Attachments: F_27262_4.pdf F_28077_1.pdf P87C51SBPN datasheet.pdf

Former Member · Answer

Hi Teeterbuilt,

I was little bit short on time.

How do I check to see if I can connect it to my Pi?

It should but there is some work to do:

- The Phi has an RS232 serial bus, your Com-trol system uses an RS485 bus (see your specs)

On the RS232 the transmit and receive line are separated, on the RS485 they are combined. But this problem can be overcome

- On the marked are convertor modules available or convertor chips (just what you want, see RS232 to RS485 Converter and Adapter Wiki)

- Just use an convertor or build jour own convertor (its not that complex)

How would I get them to talk to each other?

- First you have to deal with the hardware (see How do I check to see if I can connect it to my Pi?)

- Then you have to figure out the protocol,( I haven't seen this specs yet, a quick Google search reviled no specs on the protocol)

- For the protocol, try an email comtrol for the protocol specs...

If I can't us the boards can I just use the p87c51sbpn IC chip.

- I think its not useful, the program ROM is a OTP (One time programmable).

I hope this info is of help.

greetings Robert

michaelkellett · Answer

Somehow this discussion seems to be in two places - never mind !

The problem with the RPi RS232 port is that it doesn't actually have one - it has a logic level serial port which can be converted to RS232 (btw RS232 specifies the hardware and signalling voltages and timing of a serial port) by means of an additional chip or board. You could then use one of the readily available RS232-RS485 convertors.

The big problem is the protocol - if you can't get the supplier (TAC is part of Schneider) to give you documentation you are faced with a big task in reverse engineering it and you would need a complete working system.

Are you planning on using these boards for a new job or do you have existing installation that you must support ? Is there any reason why you can't just use the original software on a PC ?

MK

robotop · Answer

Hello, looking at the PDF you supplied, the communications acts via RS485 interface, with baud rate of 19200 or 38400 BPS. You can simply use an USB to RS485 interface that can be recognized by the Raspberry PI board, then use it in your program as TTYUSB0 (or similar), setting the appropriate baud, bits and parity parameters. Note that if you use an FTDI based interface, the handling of the bus direction (RS485 is always half-duplex) is done directly by the interface, so you have just to write to the serial channel or read from it, without the need for the direction control (usually involving one output line as TX enable). The FTDI chips have an automatic "1 bit before start" activation of transmission and "1 bit after stop bit" release of the bus. So, when you don't transmit any data, the receiver is enabled, and when you transmit data, the transmitter is enabled just for the requested time, freeing the bus immediatly after the data has been transmitted. Obviously, the hardware part is very simple, but you must know the protocol for sending and retrieving data. Note that the RS485 bus is multi-point, so every device connected has its own "address". I noted on the schematic that there are two rotary switches to set the address for the peripheral. However, if you don't have the protocol specs (a list of commands and the packet structure) it's almost impossible to handle it. It may be "easily" decoded if you have some controller that actually works with the device. In such case, you can use the USB to RS485 interface as a "sniffer" on the bus, capturing the packets that go from the controller to the device and vice-versa. Hope this helps...