Application Brief

Design Goals

Design Description

In spacecraft applications RS-422 might be used as a bus and payload telecommand and control interfaces. RS-422 was designed as serial communication methods to convey information between equipment by using balanced and differential twisted-pair cable. RS-422 is specified for multi-drop applications where only one driver connects up to 10 receivers through a single differential twisted bus. The following figure shows a typical half-duplex isolated RS-422 interface circuit.

The following circuit uses the ISOS141-SEP digital isolator and SN65C1168E-SEP transceiver devices to achieve a space-grade isolated RS-422 driver.

Design Notes

The previous circuit uses the SN65C1168E-SEP driver to create an isolated RS-422 driver to isolate the primary device from the secondary devices. SN65C1168E-SEP consists of two receivers and can be used as secondary devices.

Design Steps

• Determine signaling rate versus cable length

RS-422 system can transmit up to 10Mbps, or over a 1500-m cable at a lower signaling rate. The signal rate is reciprocal to the cable length. Hence, it is important to figure out the maximum signal rate for a given cable length or vice versa. The following figure shows the RS-422 recommended signal rate over the cable length in its annex.

This system design uses a 1500-m cable. From the previous figure, the maximum recommended signal rate is 90kbps.

• Calculate the path delay
  Path delay = Propagation delay of digital isolator + Propagation of RS-422 TX to bus + Propagation delay of wire
  + Propagation delay of bus to RS-422 RX Calculate bit time

  Parameter	Conditions	Typ (ns)	MAX (ns)
  Propagation delay of ISOS-141-SEP	With RL = 50Ω, CL = 15pF	10.7	16
  Propagation delay of TX to bus	With RL = 50Ω, CL = 40pF	8	16
  Propagation delay of 1500-m wire	Approximate delay is 5ns/m (Typ), 5.3ns/m (Max)	7500	7950
  Propagation delay of bus to RX	With RL = 50Ω, CL = 50pF	15	27

  Path delay (Typ) = 10.7+8+7500+15 = 7534ns
  Path delay (Max) = 16+16+7950+127 = 8109ns
• Calculate bit time
  $Bit time=\frac{1}{Bit rate}= \frac{1}{90kbps}=11100ns$
• Check path delay < bit time
• This design meets the timing requirement as the approximate worst-case round-trip delay is 8109ns which is less than 11100ns.

  Note: If the path delay is greater than the bit time, the following options apply:

  1. Pick components with shorter propagation delays
  2. Shorten the cable length
  3. Lower the maximum signaling rate
• It is recommended to place a 120-Ω termination resistor at the other end from the driver.

Reference