RS485 Bus Requirements - (Joe Novello)
1st Draft RS485 Bus Design Standard
The following is my cut at a standard to be used in designing a
common hardware interface to LENA's RS485 bus. Please provide your
inputs so we all will be working to common rules. Note area's I
have a question mark (?) where I would like conformation that I
got it right.
The LENA system RS485 bus peripherals hardware interface shall comply with or
be equivalent to following design rules:
- Baud rate ----------------------------9600
- Data field ----------------------------8 bits/ASCII format
- parity bit -----------------------------none
- start bit ------------------------------1
- stop bit------------------------------none ?
- unit load -----------------------------1U.L. /RS485
specification (?)
- Power/Comm. device connector -------------Molex 03-06-1044 male shell and
02-06- 6103 male pins
- Connector pin assignments
- Pin 1 ------------------------------ power return
(black wire)
- Pin 2 ------------------------------Comm. A termin.
(+ on binary 0 signal, space or on state)?
- Pin 3------------------------------+12 volt dc
input power
- Pin 4------------------------------Comm. B terminal
(- on Binary 0)
- Pin 5------------------------------Shield 1
- Dc power voltage -----------------------+12 +/- 10% (?)
- Max. DC current ------------------------20 MA ?
- Line driver/receiver ----------------------Maxim MAX487
IC (or equivalent slew rate/voltage tol.)
- Line termination (between pin 2 & 4) -----------120 Ohm carbon
resistor1
- Stub Length (new designs) -----------------Max. length from IC
driver to connector pin = 4 “and/or use of 120
ohm PC run/twisted pair cable from driver/receiver
to connector pins.
- 1-a jumper shall be provided to allow field connect/disconnect to be achieved.
Original e-mail: July 21, 1998, Joe Novello
Last modified: July 29, 1998