Optional RS-422/485 Operation
This section covers RS-422 and RS-485 serial communication standards, wiring configurations, and operation modes for Brainboxes Serial Solutions RS-422/485 compatible cards.
The RS-422 Standard
The RS-422 standard defines a serial communications standard. RS-422 is designed for high speed and/or long distance data transmission. Each signal is carried by a pair of wires and is thus a differential data transmission system.
RS-422 Specifications
| Line Length | Max Baud Rate |
|---|---|
| 40 feet (12m) | 10 Mbits/sec |
| 400 feet (122m) | 1 Mbits/sec |
| 4000 feet (1219m) | 100 Kbits/sec |
- Maximum receivers: 1 Driver up to 10 receivers
- A 120-Ohm resistor should be used to terminate the receiving end of the line
- Generally used between one transmitter receiver pair to ONLY one other transmitter receiver pair, but each output can drive up to 10 receivers
The RS-485 Standard
The RS-485 standard is similar to the RS-422 standard upon which it is based. The main difference is that up to 32 transmitter receiver pairs may be present on the line at one time.
RS-485 Specifications
| Line Length | Max Baud Rate |
|---|---|
| 40 feet (12m) | 10 Mbits/sec |
| 400 feet (122m) | 1 Mbits/sec |
| 4000 feet (1219m) | 100 Kbits/sec |
- Maximum pairs: Up to 32 Driver/Receiver Pairs
- A 120-Ohm resistor should be used to terminate either end of the main line
- If more than one device may transmit data, the RTS line is used as transmit enable signal, preventing contention between talkers
Terminating Impedances
RS-422 and RS-485 lines should be terminated at the end of the main branch of the RECEIVER, in the cable's characteristic impedance. These terminating impedances stop echoes caused by the serial data being reflected back at the cable ends.
It is not necessary to terminate the transmitter end of the twisted pair.
The AT Dual Port RS-422/485, Opto Isolated AT Dual Port RS-422/485 and AT Velocity RS-422/485 cards have the correct 120 Ohm (nominal) terminating resistors for the RXD twisted pair line and the CTS twisted pair line fitted on the RS-422/485 card for both the serial ports on the card.
There is no need to add any more terminating resistors at the PC end. The terminating impedances shown in the wiring diagrams are automatically provided by the on-board resistors and do not have to be added by the user.
Fail Safe Open Circuit Detection
Open circuit occurs when there are no drivers on the circuit. This happens by design in party line multi driver/receiver systems and unintentionally when the twisted pair line is accidentally cut or disconnected or the transmitting device fails.
In RS-485 party line systems, there are extended periods of time when none of the many possible talkers are gated onto the bus. This is known as the line idle state and occurs when all the driver outputs are in the high impedance state. The lines float, perhaps being pulled to the high or low state by noise or other voltages on the line.
Without fail safe open circuit detection, false start bits are detected by the receivers, either corrupting good communications or causing noise to masquerade as good data.
The on-board fail safe open circuit detection causes the receiver to go to a known, predetermined state and prevents false start bits and bad data being detected during open circuits.
Fail Safe Short Circuit Protection
Short circuits occur when the two lines of a twisted pair are connected together, either due to accidental damage to the cable or failure of one or more transmitter/receivers on the line. The short circuit condition is dangerous since damage to the receiver may occur and communication may be corrupted or prevented.
The on-board fail safe short circuit detection prevents the line impedance from going to zero and thus protects the inputs of receivers and the outputs of drivers.
RS-422 Operation
Generally, in RS-422 systems all 8 signal lines from the 9-pin D connector participate in the data transfer sequence, using 4 twisted pair cables:
| Twisted Pair | Function |
|---|---|
| Pair 1 | Carries TXD data outwards |
| Pair 2 | Brings RXD data inward |
| Pair 3 | Carries RTS handshake outwards |
| Pair 4 | Brings CTS handshake inwards |
There is no need to carry the ground from one device to another.
This RS-422 arrangement allows data to be transmitted and received simultaneously since each signal has its own data cable pair. Additionally, the receiver can set RTS true, telling the transmitter on its CTS input that the receiver is ready to accept data. In this way, no data will ever be transmitted when the receiver is unable to accept it, preventing data loss.
RS-422 Serial Port Cables
Use screened twisted pair cables to make the RS-422 connection:
- Belden cable 9729 and 9829
- UL type 2493 and 2919
- IBM Part No 4716748
Unscreened Belden type 8795 may also be used in less noisy environments.
The on-board resistor networks terminate the receiving end of the twisted pair cable in its characteristic impedance.
Serial Port 1 to Other PC Cable
RS-485 Operation
The RS-485 standard is intended for up to 32 driver receiver pairs on the bus. The line drivers used in the Serial Solutions RS-422/485 card are designed to work correctly in both RS-422 and RS-485 systems.
Though the card uses a 9-pin D connector, in general, not all the lines are used for RS-485 systems:
- The RTS+/- and CTS+/- lines, though driven by the card, are usually not connected
- In two-wire, Half-Duplex configurations the TXD+ line is connected to the RXD+ whilst the TXD- line is connected to the RXD-
- Only one pair of twisted wire cable is used in RS-485 Half Duplex communications
The hardware handshaking performed by the CTS+/- and RTS+/- lines in RS-422 systems are handled by a software protocol in RS-485 systems. In situations where more than one device may transmit data on the shared data line, each card's RTS line is used as a gating signal to enable the TXD driver only when that card needs to transmit data.
Revision 3 and higher versions of this card have a facility which automatically "gates" the RTS line, thus enabling the transmitter independently of any software.
RS-485 Cable
For best noise immunity, use twisted pair cables to make the RS-485 connection:
| Configuration | Cable Pairs Needed |
|---|---|
| Half Duplex | 1 twisted cable pair |
| Full Duplex | 2 twisted pair cables |
Recommended cables:
- Screened twisted pair Belden cable 9729 and 9829
- UL type 2493 and 2919
- IBM Part No 4716748 cable
Terminate the twisted pair cable with the correct impedance.
RS-485 One Talker - Many Listeners, Half Duplex
This scheme is used when there is only one device which can transmit data and all the others simply listen to it. This is commonly used for theatrical lighting intensity control in the DMX512 standard.
For the talker, the RS-485 TXD GATE jumper should remain in the factory set position (transmitter is always enabled). There is NO multiplexing of the TXD and RXD lines. Data is only flowing one way, from PC outwards, and is thus a Half-Duplex configuration so only one twisted pair cable is needed.
The Receiver end of the MAIN line is terminated in characteristic impedance by ON-BOARD resistor networks. Stubs off the main line are not terminated.
In this scheme, one RS-485 device is talk only (transmits data, does not receive any). The other RS-485 devices are receive only (do not transmit any data at all).
Half Duplex Settings of RS-485 Multiplex
Both jumpers shorted, causing the RXD- & TXD- and the RXD+ & TXD+ signals to short together at the 9-pin connector.
RS-485 Many Talkers - Many Listeners, Half Duplex
This configuration is also known as "party line" transmission. It is imperative to have some method of preventing two devices trying to drive the data lines at the same time.
The normal method is to use the RTS line as a talk enable. The RTS line should go true immediately prior to the data transmission and go false immediately after the last byte in the stream is sent.
BOTH ends of the MAIN line must be terminated in characteristic impedance, since both ends receive. The twisted pair ends are wired to both RXD+ & TXD+ and RXD- & TXD- at each RS-485 device. Stubs off the main line are not terminated.
RS-485 Many Talkers - Many Listeners, Full Duplex
The RS-485 many talkers, many listeners, Full Duplex system can be used when all the RS-485 devices have separate Transmit and Receive channels. There is NO multiplexing of the TXD and RXD signals on the same device.
This system is especially useful when there is no flow control available on the PC, usually due to the use of a third party communications program that prevents the use of the RTS signal as a "transmit enable" control.
Use Cases
This configuration can be used in the following situations:
a) Point-to-Point Connection The PC is connected to only ONE RS-485 device.
b) Addressable Multi-Device The PC is communicating with several RS-485 devices that are each able to recognize and respond to their own unique address. The RS-485 devices only drive their TXD lines when they are responding to requests from the PC to send data.
Advantages
- No new communications software is needed
- The PC can talk and listen at the same time
- The handshaking is performed by the intelligence of the RS-485 devices attached to the PC
When wired correctly:
- The PC can transmit data at any time and all the RS-485 devices (#1 to #n) simultaneously receive it
- Only one of the RS-485 devices may talk (transmit data) at any one time
- Each RS-485 device recognizes commands and data addressed to it
- The device only talks when the PC commands it to do so
- When the RS-485 device receives the command to talk, it gates its TXD drivers on, sends the data, and disables its TXD drivers
- Other RS-485 devices remain in receive only mode when not being addressed
RS-485 Full Duplex Jumpers
Both jumpers left unconnected (not shorted).
RS-485 Full Duplex Wiring
The receiver end of the MAIN line is terminated in characteristic impedance. Stubs off the main line are not terminated.
Optional Grounding Arrangements
Proper operation of the cable circuit, according to TIA EIA: 485 A (1995), requires that the cable ground shield is not connected directly to the equipment ground shield. A current limiting resistor should be used in series with the shield to avoid possible large current flow due to differences in ground potential.
Configuration A
The circuit common of the equipment is connected to protective ground, at one point only, by a 100 ohm (plus or minus 20%) resistor with a power dissipation rating of 1/2W.
An additional provision may be made for the resistor to be bypassed with a strap to connect signal common and protective ground directly together when specific installation conditions necessitate.