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RE: PC to RS485 bridge
- Subject: RE: PC to RS485 bridge
- From: "DynamoBen" <ben@xxxxxxxxxxx>
- Date: Sun, 30 Jan 2005 10:19:03 -0600
Something to consider: I have a device that transmits an account code when
a
card is scanned. It is much easier for me to just broadcast the account
code
to the master right away. However, if you plan to make a polling
environment
we need to determine the polling interval. Idea being if the node gets no
response from the bridge over a period of time it will fail.
Additionally, we need to determine the type of checksum for the bridge and
nodes. We need to determine what is included in the length byte. Finally is
the device address going to be a 9bit address?
Make sense?
-----Original Message-----
From: Neil Wrightson [mailto:neilw@xxxxxxx]
Sent: Sunday, January 30, 2005 4:43 AM
To: ukha_xpl@xxxxxxx
Subject: [ukha_xpl] PC to RS485 bridge
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]
xPL Links: http://www.xplproject.org.uk http://www.xplhal.com
http://www.xpl.myby.co.uk
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