High speed data transmission between computer system components and peripherals over long distances, under high noise conditions, usually proves to be very difficult if not impossible with single-ended drivers and receivers. Recommended EIA standards for balanced digital voltage interfacing provide the design engineer with a universal solution for long line system requirements. As was seen with general purpose balanced data transmission, the ability to cope with common-mode voltages allows a differential (balanced) receiver to receive and reproduce signals that otherwise would be unusable.
A comparison of the single-ended EIA RS-232-C and RS-423-A standards with the differential EIA RS-422-A and RS-485 standards is shown in Table 9-12. The balanced system of data transmission incorporates a differential driver transmitting on balanced interconnecting lines to a receiver with differential inputs.
The balanced voltage digital interface circuit is normally used for data, timing or control lines where the signaling rates are from 100 kbps to 10 Mbps. RS-422-A specifications do not place restrictions on minimum or maximum operating frequencies but rather on the relationship of transitional speeds to a unit interval.
Although single-ended transmission circuits are normally used at lower frequencies, differential transmission on balanced lines may be preferred under the following conditions:
- Interconnecting lines too long for effective unbalanced operation.
- Transmission lines exposed to large electrostatic or electromagnetic noise levels.
- When simple inversion of the signals may be desired (obtained by reversing the balanced lines).
A basic balanced digital interface circuit (seen in Figure 9-78) consists of three parts:
- The generator (G) or data line driver.
- A balanced transmission line.
- The loads, where a load may consist of one or more receivers (R) and the line termination resistor (RT).
The RS-422-A type of driver has a balanced (differential) output voltage source with an impedance of 100Ω or less. Its output differential voltage is in the range of 2.0 V minimum to 6.0 V maximum. Additionally, the output voltage of either output, with respect to ground, shall not exceed 6.0 V.
Output voltage balance is defined as follows:
The output differential voltage (VOD) shall not be less than 2.0 V with two 50Ω ( ± 1%) termination resistors connected in series between the outputs. The difference between opposite polarity differential output voltages must be less than 0.4 V (see Figure 9-79). The driver output offset voltage (VOS), measured from the junction of the two 50Ω terminators and driver ground, shall not exceed 3.0 V (either polarity). The magnitude of the change in VOS must be less than 0.4 V for opposite polarity differential output voltages. Drive output current, with either output shorted to ground, shall not exceed 150 mA. Output off-state leakage current, with any voltage between – 0.25 V and 6.0 V applied to either output, shall not exceed 100µA. As illustrated in Figure 9-80, output voltage transition times (tr) are the transitional times between 0.1 and 0.9 of Vss and must occur within 10% of a unit interval (tb) or 20 ns, whichever is greater.
Ringing and resulting overshoot and undershoot shall not, as illustrated in Figure 9-80, exceed 10% of VSS where VSS is defined as the difference between the two steady-state values of the output.
RS-422-A receiver characteristics for balanced-line applications are basically the same as those tor the RS-423-A single-ended applications. Basic receiver input requirements are as follows:
Differential data input threshold sensitivity of ±200 mV over a common-mode (VCM) range of 7 to 7 V. Input impedance of greater than or equal to 4 kΩ.
The receiver’s input voltage-current characteristics shall be balanced such that its output remains in the intended binary state with a differential input of 400 mV applied, (through 500Ω ± 1% to each input terminal as illustrated in Figure 9-81), and VCM is varied between – 7 V and 7 V.
An RS-422-A receiver shall not be damaged, when powered-on or powered-off under the following conditions:
- Driver output off (open circuit).
- A short across the data line.
- A short from either, or both, lines to ground.
RS-422-A drivers and receivers are compatible with CCITT recommendation V.11 and X.27. RS-422-A drivers and receivers arc not intended for operation with RS-232-C. MIL-STD-188C. MIL-STD-188-100, or CCITT recommendations V.28 and V.25.
A typical application of an RS-422-A is its use in communicating data from a central computer to multiple remote monitors or stations, such as airport arrival and departure monitors (sec Figure 9-82).
In this application, a single twisted pair line is used to connect the central control terminal with several remote monitors distributed throughout the airport. Line termination would be at the most remote end from the control terminal (remote monitor 6). To minimize line noise it may be desirable to use two terminating resistors, with values of RT/2, one from each line to ground.
Even in less complex applications the good noise rejection capability of RS-422-A type circuits may be advantageous. Figure 9-83 illustrates a simple, short-line, dedicated single driver and single receiver application with very good noise handling capability.
This combination of RS-422-A driver and receiver will provide – 0.25 to 6.0 V common-mode capability and operate at speeds up to 20 Mbps. For improved negative noise voltage rejection capability it would be necessary to use a driver with more negative common-mode capability. The SN75172B driver is such a device with 7 to 7 V common-mode range. The SN75175 receiver also has a – 7 to 7 V input capability, allowing very good overall noise performance for the pair.
EIA RS-485 STANDARD
EIA standard RS-485, introduced in 1983, is an upgraded version of EIA RS-422-A. Increasing use of balanced data transmission lines in distributing data to several system components and peripherals over relatively long lines brought about the need for multiple driver/receiver combinations on a single twisted pair line.
EIA RS-485 takes into account RS-422-A requirements for balanced-line transmission plus additional features allowing for multiple drivers and receivers. Figure 9-84 illustrates an application similar to that of Figure 9-78, but with multiple drivers and receivers.
Standard RS-485 differs from the RS-422-A standard primarily in the features that allow reliable multipoint communications. For the drivers these features are:
- One driver can drive as many as 32 unit loads and a total line termination resistance of 60Ω or more (one unit load is typically one passive driver and one receiver).
- The driver output, off-state, leakage current shall be 100 µA or less with any line voltage from -7 V to 7 V.
- The driver shall be capable of providing a differential output voltage of 1.5 V to 5 V with common-mode line voltages from – 7 V to 12 V.
- Drivers must have self protection against contention (multiple drivers contending for the transmission line at the same time). That is, no driver damage shall occur when its outputs are connected to a voltage source of – 7 V to 12 V whether its output state is a binary 1, binary 0 or passive.
For receivers these features are:
- High receiver input resistance, 12 kΩ minimum.
- A receiver input common-mode range of – 7 V to 12 V.
- Differential input sensitivity of ± 200 mV over a common-mode range of – 7 V to 12 V.
Unit Load Concept
The maximum number of drivers and receivers that may be placed on a single communication bus depends on their loading characteristics relative to the definition of a “UNIT LOAD” (U.L.). RS-485 recommends a maximum of 32 unit loads (U.L.) per line.
One U.L. is defined (at worst case) as a load that allows 1 mA of current under a maximum common-mode voltage stress of 12 V (for a more detailed definition refer to the EIA Standard RS-485, page 4). The loads may consist of drivers and/or receivers but do not include the line termination resistors which may present an additional load as low as 60Ω total.
Example: Early production SN75172 drivers and SN75173 receivers.
- Driver output leakage current in the off state with 12 V output voltage is 0.1 mA max.
- Receiver input current at a Vin of 12 V is 1 mA max.
- The driver represents 0.1 mA/ 1.0 mA or 0.1 U.L.
- The receiver represents 1.0 mA/ 1.0 mA or 1 U.L.
- As a pair they represent 1.1 mA/ 1.0 mA or 1.1 U.L. Therefore, 32/1.1 or 29 pairs would represent the maximum recommended 32 unit loads.
Example: 1984 production SN75172B drivers and SN75173A receivers.
- Driver IO off at 12 V is 0.1 mA max
- Receiver IIN at 12 V is 0.6 mA max
Although there were no changes in the driver loading, the receive, now represents only 0.6 U.L. A driver-receiver pair represents 0.7 U.L. and therefore 32 U.L./0.7 U.L. per pair or 45 of these driver-receiver stations could be handled on one twisted pair data transmission line.