Serial Protocols
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- USB (Universal Serial Bus)
- CAN (Controller Area Network)
EIA RS-422 similar to RS-232 each signal uses a differential voltage between two wires rather than single ended voltages relative to GND Tx is two wire twisted pair, Rx is two wire twisted pair ±6 V multi-drop with up to 10 receivers per transmitter one master, 10 slaves not all slaves can talk back to master unless they have tri-state outputs
higher speed up to 100K baud longer distance up to 4000 feet typically used to boost RS-232 over longer lines RS-485 similar to RS-422 but only one twisted pair true multi-point bus up to 32 drivers and 32 receivers on one twisted pair of wires up to 4000 feet requires tri-state Tx drivers requires device addresses requires bus arbitration – usually software master-slave
listens for end of current byte being sent on twisted pair can sense bus collision by current draw recommend bus terminating resistors Rt = 120 requires TTL/CMOS logic to differential drivers - MAX485 chip 
1 RO Receiver Output if VA > VB+200mV then RO=HI if enabled if VB > VA+200mV then RO=LO if enabled 2 /RE Receiver Output Enable if /RE=LO then RO is enable if /RE=HI then RO=hiZ 3 DE Driver Output Enable if DE=HI then driver outputs are enabled if DE=LO then driver outputs are hiZ 4 DI Driver Input if DI=LO then VB > VA if enabled if DI=HI then VA > VB if enabled 5 GND 6 A Noninverting I/O 7 B Inverting I/O 8 VCC Positive supply voltage 4.75 V < VCC < 5.25 V USB (Universal Serial Bus) two wire differential multipoint bus four pins – twisted pair data D+ and D-, +5V, GND USB 1.1 up to 12 Mbps USB 2.0 up to 480 Mbps with 5 m cable USB 3.0 up to 4.8 Gbps
reduced voltage levels in high speed (HS) mode - low = 0V to 10mV, high = 360mV to 440mV uses non-return to zero inverted (NRZI) logic logic 0 transitions the voltage state (low to high, or high to low) logic 1 does not transition state
master device arbitrates traffic flow on the bus significant protocol at packet level three types of packets - handshake, setup, data six styles of connectors for USB 1.1/2.0 – A, B, miniA, miniB, microAB, microB  
maximum unit load of 150mA per device on +5 power line maximum of 5 unit loads from bus master
provides TTL level TX, RX, RTS, CTS requires USB drivers on PC that act like COM serial port http://www.usb.org/home CAN (Controller Area Network) two wire differential multipoint bus requires CAN controller (similar to UART) and CAN transceivers (similar to MAX232) many microprocessor families now provide CAN support up to 1 Mbps at 40 m nominally uses 0 to 5V no standard connectors – often use D9 or RJ45 can provide power to slave devices requires bus terminating resistors Rt = 120  
significant protocol requirements at packet layer includes collision arbitration numerous SAE and ISO standards 
msb first identifier = 11b address of destination control = 4b number of data bytes per packet (up to 8) data = 8b per byte CRC = 15b cyclic redundancy check Download 284.67 Kb. Share with your friends:
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