...my colleague Brian wants all of his house rewired. Also, we have been toying around with placing temperature sensors around our office (which we need to prove that our airconditioning doesn't work).
After some thought about power-line RF drivers and such, we decided on RS485-type network at a low data rate (16kbps approx). When used with appropriate slew-rate limited drivers (MAX487) you can have relatively long stubs on your network. The network is terminated on both ends with 100R, with two 1k resistors biasing the network into a '0' state.
The temperature sensors I've built use just four active components: a DS1621 I2C temperature sensor, a MAX487 line driver, a PIC 12C509 and a 78L05. We're planning to use four-wire cable at home (+12V, GND and the two differential data lines) and cat-5 network cabling in the office.
Below is the code for the RS485 protocol that we use. This code is heavily macro'd, but should be readable. If enough people show interest then I'll put the include files needed and the rest of the (not quite working) temperature sensor application up on my or Brian's home page.
Frank
Note: This source requires PC graphics to show the diagrams.
Note 2: JBS - BTFSC+GOTO
JBC - BTFSS+GOTO
LOOP - DECFSZ+GOTO
LOOPI - INCFSZ+GOTO
STR - MOVWF
LD - MOVF,W
JZ - SKPNZ+GOTO
FRAME..FEND sets up a set of local registers
etc.
; IMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM;
; : :
; : 485.I :
; : :
; : RS-485 home automation bus interface. :
; : :
; : Copyright 1997, Frank A. Vorstenbosch. :
; : :
; HMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM<
;
; Uses: FLAG_IDSTRING - a flag, somewhere in a register
; FLAG_BROADCAST - a flag, somewhere in a register (can be same as
FLAG_IDSTRING)
; RxTxD - a port pin for received and transmitted data
; TxEN - another port pin set high to enable transmitter
frame 485
byte Shifter485
byte Counter485
byte TempA485
byte CRCLow485
byte CRCHigh485
byte Timer485
static OurTC
fend 485
#define Timer485 TempC485
; ZDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD?
; 3 3
; 3 Description. 3
; 3 3
; @DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDY
; Two twisted pairs in cable, one for data and one for power/ground. Power is
; supplied as 9..12V as measured on the peripheral -- a transmitter may take
; approx. 120mA when using 100j terminators, or 104mA using 120j terminators
; in addition to any local power needs.
;
; The network protocol is designed to run on 4MHz 12-bit PICs at ~16kb/s in
; software. The longest packet takes less than 23ms to send, the shortest
; packet less than 2.5ms. CSMA is used to prevent two devices transmitting
; at the same time.
; Synchronous message format:
;
; - Start bit (100-250us duration)
; Peripherals try sending a start bit of 100-200us, then float the
; line, see if the line goes back to 0, and if it does, then drive
; it low for the remainder of the 20us, then start sending data.
; If the line is still driven to 1, then the peripheral will back
; off until the bus is idle for >100us, then try sending another
; start bit. The peripheral will try progressively longer start
; bits up to the limit of 200us. The host uses a 250us start bit
; to give it priority over all peripherals (this also means that
; most peripherals can ignore start bits <250us long).
; - Flags byte
; 7..6 - transfer direction:
; 00 - peripheral to host
; 01 - peer to peer (UNIM)
; 10 - host to peripheral
; 11 - reserved
; 5 - reserved
; 4..0 - data length
; - Peripheral or destination address byte or 255 for broadcast
(Flags<7..6>=10)
; - Optional source address byte (Flags<7..6>=01) (UNIM)
; - Command byte (Flags<7..6>=10) or status byte (Flags<7..6>=00)
; Command
; 0 - Are you there? (Returns up to 31 byte ID string)
; 1 - Data poll
; 2..255 - User defined commands
; Status
; 0 - OK
; 1 - CRC error
; 2 - Command error
; 3 - Parameter error
; 4 - No (more) data
; 5..254 - User defined errors
; 255 - Data broadcast
; - 0..31 data bytes
; - CRC-16 over Flags, Address and Data bytes
; ZDDDDDDDDDDDD//DDDDDDDDDDD? Z
; Start line idle 3 100-250us 325us3
; DDDDDDDDDDDDDDDDDY @DDDDY
; ZDDDD? Z
; 0-bit 325us325us3
; Y @DDDDY
; ZDDDDDDDDDD? Z
; 1-bit 3 50us 325us3
; Y @DDDDY
;
; The low period at the end of each byte is 50us rather than 25us, giving
; devices time to process the byte. Thus, the byte 0x54 would be transmitted
as:
;
; ZDDDD? ZDDDDDDDDDD? ZDDDD? ZDDDDDDDDDD? ZDDDD? ZDDDDDDDDDD?
ZDDDD? ZDDDD? Z
; 325us325us3 50us 325us325us325us3 50us 325us325us325us3 50us
325us325us325us325us3 50us 3
; Y @DDDDY @DDDDY @DDDDY @DDDDY @DDDDY
@DDDDY @DDDDY @DDDDDDDDDDY
; ZDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD?
; 3 3
; 3 Local definitions. 3
; 3 3
; @DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDY
ifndef FLAG_IDSTRING
ifndef FLAG_BROADCAST
error "FLAG_IDSTRING and/or FLAG_BROADCAST must be defined"
endif
#define FLAG_IDSTRING FLAG_BROADCAST
endif
ifndef FLAG_BROADCAST
#define FLAG_BROADCAST FLAG_IDSTRING
endif
; ZDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD?
; 3 3
; 3 Receive a message from the RS485 bus. 3
; 3 3
; @DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDY
;
; In: W - Size of receive buffer
; fsr - pointer to first byte of receive buffer
; Act: Receive a message from the RS485 bus.
; Out: Receive buffer filled in with command and data bytes
; Counter485 - Number of data bytes
; W - error code
; 0 - no error
; 1 - message was not addressed to us
; 2 - message was too long for our buffer
; 3 - CRC error
; Regs: Shifter485, Counter485, fsr, tmp, TempA485, TempB485, TempC485
; Note: Assumes that the start bit has already been detected.
; Uses one stack level internally, can only be called from top level
ReceiveMessage str Counter485
TRACE 1,"ReceiveMessage"
clr CRCHigh485
clr CRCLow485
jbs RxTxD,$ ; wait for start bit to end
call RM_Byte ; receive flags & length
byte
retbc Shifter485,7,1 ; if peripheral-to-host then
ignore it
ld Shifter485
str TempA485
subwf Counter485,w
retc 2
movlw 31
andwf Shifter485,w
str Counter485
inc Counter485 ; count command as a data
byte
bcf FLAG_BROADCAST ; assume is's not a
broadcast
call RM_Byte ; receive device ID
call DeviceID
xorwf Shifter485,w ; is the message addressed
to us personally?
jz RM_Loop
incf Shifter485,w ; or is it a broadcast?
retnz 1
bsf FLAG_BROADCAST
RM_Loop call RM_Byte ; receive command/data byte
ld Shifter485
inc fsr
str ind
loop Counter485,RM_Loop
call RM_Byte ; receive first CRC byte
call RM_Byte ; receive second CRC byte
ld TempA485
str Counter485
ld CRCHigh485
xorwf CRCLow485,w
retnz 3 ; CRC error
movlw 31
andwf Counter485 ; clear unwanted bits in the
length byte
retlw 0
; zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz
RM_Byte ldk Timer485,8<<<4
RMB_Loop jbc RxTxD,$ ; wait for bit to start
clc
RMB_Inner incfsz Timer485 ; time the length of the bit
jbs RxTxD,RMB_Inner
; here the low nibble of Timer485 should be 6 or 13
btfsc Timer485,3 ; detect 1 bit if 9 counts
or more
stc
ld status ; carry is LSB of status reg
ister
rl Shifter485 ; shift bit into register
xorwf CRCLow485,w
clrc
rr CRCHigh485
rr CRCLow485
andlw 1
xorwf CRCLow485
movlw 0xa0
skipz
xorwf CRCHigh485
movlw 0x0f
iorwf Timer485
loopi Timer485,RMB_Loop
retlw 0
; ZDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD?
; 3 3
; 3 Entry point to subroutines for SendMessage. 3
; 3 3
; @DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDY
SM_Byte_tmp ld tmp ; [1]
SM_Byte str Shifter485 ; [1]
SM_Byte_Sh ldk Timer485,8<<4 ; [2]
jp SMB_Loop ; [2]
; ZDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD?
; 3 3
; 3 Send a message on the RS485 bus. 3
; 3 3
; @DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDY
;
; In: W - Number of data bytes in message
; tmp - Status byte
; fsr - pointer to first byte to transmit
; Act: Send a message on the RS485 bus.
; Out: W - error code
; 0 - no error
; 1 - failed to send message after 8 attempts
; Regs: Shifter485, Counter485, CRCHigh485, CRCLow485, Timer485, fsr, tmp,
(OurTC)
; Note: Uses one stack level internally, can only be called from top level
SendMessage str Shifter485
TRACE 1,"SendMessage"
ldk Counter485,8
clr CRCHigh485
clr CRCLow485
clr gpio ; set TxEN low
movlw NORMAL_TRIS ; float RxTxD (with pull-up)
tris gpio
SM_WaitIdle movlw 30
subwf OurTC,w ; shorter delay if longer
start bit used
str Timer485
SMWI_Loop jbs RxTxD,SM_WaitIdle ; [2] wait for bus to be
idle for 100..150us
loopi Timer485,SMWI_Loop ; [3]
movlw TxEN_MASK|RxTxD_MASK
btfss RxTxD ; last chance: if activity
on bus then don't set TxEN high
str gpio ; drive TxEN high
jbc TxEN,SM_WaitIdle ; wait some more if the STR
wasn't executed
movlw NORMAL_TRIS&~RxTxD_MASK ; quickly drive RxTxD to
high level
tris gpio
movlw NORMAL_TRIS ; and then float it again
tris gpio
ifdef HOST
movlw 36 ; host uses 250us timeout
else
movlw 13
addwf OurTC,w
addwf OurTC,w
endif
str Timer485 ; wait for 100..200us
SM_SendStartH call Delay4Ticks ; [4]
loop Timer485,SM_SendStartH ; [3]
clr gpio ; disable transmitter
brake ; it takes ~2.5us for the
MAX487 to go into read mode
movlw TxEN_MASK ; moved here to prevent
back-to-back I/O accesses
jbs RxTxD,SM_BackOff ; if RxTxD still high, then
back off
str gpio ; enable transmitter again
movlw NORMAL_TRIS&~RxTxD_MASK ; start driving RxTxD pin
(to low state)
tris gpio
call Delay9Ticks
ld Shifter485
andlw 31
str Counter485
call SM_Byte_Sh ; send flags & length byte
call Delay13Ticks
call DeviceID
call SM_Byte ; send our ID
call Delay16Ticks
call SM_Byte_tmp ; send status byte
jbs FLAG_IDSTRING,SM_LoopCode
test Counter485
jz SM_CRCNoData ; [2]
call Delay8Ticks ; [8]
jp SMLD_Enter ; [2]
SM_LoopData call Delay12Ticks ; [12]
SMLD_Enter ld ind
inc fsr
call SM_Byte ; send data byte
loop Counter485,SM_LoopData
jp SM_SendCRC
SM_LoopCode brake
movlw 31
andwf fsr,w
str tmp
movlw IDString
addwf tmp,w
call PCW
inc fsr
call SM_Byte ; send ID string byte
loop Counter485,SM_LoopCode
brake
SM_SendCRC brake
nop
SM_CRCNoData call Delay8Ticks
ld CRCHigh485
str tmp
ld CRCLow485
call SM_Byte ; send first CRC byte
call Delay16Ticks
call SM_Byte_tmp ; send second CRC byte
movlw NORMAL_TRIS
clr gpio ; disable transmitter
tris gpio ; float RxTxD
retlw 0 ; return OK
; zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz
SM_BackOff incf OurTC,w ; increment our timer
andlw 7 ; only 3 LSBs used
str OurTC
decfsz Counter485,f ; after 8 attempts, return
error to caller
retlw 1
jp SM_WaitIdle
; DDDDD Subroutine for SendMessage DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD
SMB_Loop bsf RxTxD
ld CRCLow485
xorwf Shifter485,w
clc
rr CRCHigh485
rr CRCLow485
andlw 1
xorwf CRCLow485 ; if W=0, then do nothing,
skipz ; otherwise CRC^=0xa001
movlw 0xa0
xorwf CRCHigh485
SMB_Wait1 inc Timer485 ; [1] 25us high bit for a
'0'
movlw 0xf0 ; [1]
jbc Timer485,1,SMB_Wait1 ; [3/2]
rl Shifter485
jnc SMB_ShortBit
SMB_Wait2 inc Timer485 ; [1] 50us high bit for a
'1'
jbc Timer485,3,SMB_Wait2 ; [3/2]
brake
nop
SMB_ShortBit andwf Timer485 ; W=0xf0 here
bcf RxTxD
SMB_Wait3 inc Timer485 ; [1] 25us low bit
nop ; [1]
jbc Timer485,2,SMB_Wait3 ; [3/2]
movlw 0x0f
iorwf Timer485
loopi Timer485,SMB_Loop
retlw 0
; DDDDD EOF 485.I DDDDD
GEORGE R. FORSE Says:
Hello,James Newton replies: See: http://web.archive.org/web/20030212211144/www.falstaff.demon.co.uk/picres.html Franks site is no more, but thanks to the wayback...+
I would like to use the program, but don't have the macros. When I try
to download them from sites that indicate where to download them (Google),
I am not able to access them I have spent many hours trying to find them.
Please advise. TIA George
Comments:
Code:
| file: /Techref/microchip/rs485-fav.htm, 19KB, , updated: 2009/1/3 17:03, local time: 2024/12/25 19:50,
owner: JMN-EFP-786,
3.15.17.137:LOG IN
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