PC to RS485 bridge

["Neil Wrightson" <neilw@xxxxxxxxxx>]

Hi All,

It seems I have put up my hand to do a PC to RS485 bridge, which is fine
with me, but as yet I haven't done any thing with xPL yet, so please excuse
a few stupid questions along the way.

I have been looking over the doc's with regard to xPL protocols and I need
to bounce a few ideas off people.

Basically from a RS485 point of view, the smaller we can make the messages
the faster the whole system will be.
So to send the following command out to a node would consume lots of
valuable time.

xpl-cmnd
{
hop=1
source=XPL-XPLHAL.MYHOUSE
target=ACME-CM12.SERVER
}
x10.basic
{
command=dim
device=a1
level=75
}

Basically I think that we need to come up with some software in the PC that
maps a device to a specific I/O bit on a certain port of a certain slave
device.
It then passes a certain message to the bridge saying set this bit on this
port on this slave.
Hmm, I think we should start by deciding on interfacing to a basic I/O
module, perhaps we should layout some devices and design a protocol that
will interface to these generic devices
Possible RS485 slaves
- Simple I/O module I.e. 8 Inputs and 8 Outputs
- Keyboard
- Display
- On Screen Display module.

My suggested protocol is as follows

1 Stx             Byte,#02
2 Addr            Byte 0..255      Note Generally RS485 limits 32 devices
to
be connected to the bus
3 Message Length  Byte              Allows messages up to 256 - Message
size
4 xpl-cmnd        Byte              I.e.Clear LCD display, Obtain I/O
status
etc
5 Data if any Variable size based on message length. could be zero
6 Checksum        Byte             Checksum of bytes from Stx to Checksum
location minus 1
7 Etx             Byte, #03


Values in quotes are deemed too be ASCII characters, xx = a calculated byte
value.

So if the PC wanted to know the status of the bridge it would send the
following command
02   Stx
00   Bridge is address 0
06   Length of message
'S'   xpl-status                 S=Status, 'T' = Trigger, 'H' = Heart beat
xx   Checksum
03    ETX

The bridge would then respond with
02   Stx
00   Bridge is address 0
08   Length of message
'S'   xpl-status                 S=Status, 'T' = Trigger, 'H' = Heart beat
xx   This byte could have bits associated with various status flags I.e.
Power failure
05   5 slaves are connected
xx   Checksum
03    ETX


If the PC wanted to check for a heart beat on the bridge it could send
02   Stx
00   Bridge is address 0
06   Length of message
'H'   xpl-status                 S=Status, 'T' = Trigger, 'H' = Heart beat
xx   Checksum
03    ETX

The bridge would then respond with
02   Stx
00   Bridge is address 0
06   Length of message
'H'   xpl-heartbeat
xx   Checksum
03    ETX


If the PC wanted to set the output port of a 8 input / 8 output I/O model
were the slave was address 9
02   Stx
09   Slaves Address
07   Length of message
'O'   xpl-cmnd
xx   Data (Byte) to be output
xx   Checksum
03    ETX

The bridge would then respond with
02   Stx
09   Slaves Address
06   Length of message
'A'   Acknowledge
xx   Checksum
03    ETX


OR perhaps send the port address, bit reference and output value were the
slave was address 9
02   Stx
09   Slaves Address
09   Length of message
'O'   xpl-cmnd
'2'   Port 2
'3'   Bit 3
'1'   Set to Logic 1
xx   Checksum
03    ETX

The bridge would then respond with
02   Stx
09   Slaves Address
06   Length of message
'A'   Acknowledge
xx   Checksum
03    ETX


Any comments? Any better ideas?

Do we have any volunteers to for the PC software side of the bridge?

Regards,

Neil Wrightson.




[Non-text portions of this message have been removed]



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