Lecture 15 - Interfaces

Question 1

What is a parallel transmission standard?

Answer 1

• 8 bit concurrently → parallel
• on µC: use digital I/O lines or add special interface chip
• Bi-directional send and receive over the same lines
• Half-duplex either send or receive,
  but not both at the same time
• Send 8 data bits concurrently
  → usually faster than serial transmission
• Requires additional control lines
  → use flat ribbon cable
• Can only be used over a limited distance
  ~ 1-2 m

Question 2

What is a print routine and handshakes?

Answer 2

Refer to slides

Question 3

What is parity for parallel transmission?

Answer 3

• Is a method of detecting errors in data transmission
• Typically a single byte parity checksum is transmitted after a block of bytes (e.g. several KB of data)
• Checksum calculation is: ( transmitted bytes) MOD 256
• Receiving side calculates checksum for transmitted data bytes and compares with transmitted checksum
• Parity generators and checkers can be implemented in software or hardware

Question 4

What is a serial transmission standard?

Answer 4

• 8 bits consecutively → serial
• on µC: use 2 digital I/O lines or built-in interface chip
• Bi-directional send and receive over different lines
• Full duplex send and receive at the same time
• Send single bit at a time
  – usually slower than parallel transmission, but technology gets faster
• Minimal configuration has only 3 wires (Transmit, Receive, Ground)
  – Maximum cable length about 20 m (longer than parallel transmission)
  – Optional: additional control lines
• Handshake
  – Software (special control characters XON, XOFF)
  – Hardware (requires 2 additional lines)
  – None (may not detect transmission errors)
• Can be used over telephone (2 wires)
  – then only in half duplex (send - listen - send - listen - …)

Question 5

What are some examples of serial connections (serial transmission)?

Answer 5

Serial Connections
* RS232C standard (slow) serial connection
e.g. used on all PCs
→ will be discussed here
* RS422 fast serial bus using differential data lines:
Tx+,Tx-, Rx+,Rx- (e.g. original Macintosh)
* USB Universal Serial Bus (high speed serial connec.) today’s standard for PCs and Macs

REFER TO SLIDES FOR SPECIFICS

Question 6

What are signals in reference to RS232C serial connections?

Answer 6

• Voltages: +12V (high), -12V (low)
• Not TTL compatible (transistor-transistor-logic: 5V, 0V)
  – requires additional chip for voltage conversion
  e.g. Maxim Max232
  – or system needs power supply with 3 different voltages:
  +12,-12,+5
• Relatively simple for system with mains power supply
• Requires more conversion hardware for embedded system
• Idle state of line: “high

Question 7

What is the order of transmission for RS232C?

Answer 7

a) 1 start bit (low)
b) 7 or 8 Bit, LSB first (least significant bit) basic ASCII code is only 7 bits, so this saves transmission time
c) 1 Parity bit (odd/even) or none
for transmission error detection
d) 1 or 2 stop bits (high)
e) Line remains idle until next byte is sent

Question 8

What is the data transmission rate for RS232C?

Answer 8

Data Transmission Rate
* Measure in bits per second (bps)
* For binary signaling, bits-per-second (bps) equals the Baud rate (bd), measured in symbols per second
* Typical Baud rates:
1’200, 2’400, 4’800, 9’600, 19’200, 38’400, 57’600, 115’200, …

Question 9

What is parity for serial transmission?

Answer 9

• A method of detecting errors in data transmission
• Each data byte (8 bits) gets one additional bit appended
• Algorithm for Odd Parity [Even Parity is analog.]
  – Count the number of 1s in a byte
  – If this number is odd, then append ‘0’, else append ‘1’
• Examples:
  1001 0110 has 4‘1’ (even number) → 1001 0110 1
  1001 1110 has 5‘1’ (odd number) → 1001 1110 0
• Parity generators and checkers can be implemented in software or hardware

Question 10

What is the general understanding of parity?

Answer 10

• The effect of odd parity is that the sequence of 9 bits (8 data bits + 1 parity bit) has always an odd number of 1s
• This property can be used to check whether a byte has been transmitted correctly
• Examples:
  1001 0110 1 →transm. 1001 0110 1 has odd number of 1s ➔ OK
  1001 0110 1 →transm. 1001 0100 1 has even number of 1s ➔ Error
• Only single bit errors can be detected
• The parity bit will be stripped after transmission

Question 11

EXAMPLES OF RS232C

Answer 11

REFER TO SLIDES