Specifications

RS-422/485 Application Note 33
© Copyright B&B Electronics -- Revised 2506
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax [phone]
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph 353-91-792444 – Fax [phone]
Chapter 5: Software
Introduction
RS-422 and RS-485 are hardware specifications. Software protocol is not
discussed in either specification. It is up to the system designer to define a
protocol suitable for their system. This chapter we will not attempt to define a
protocol standard, but will explain some of the issues that should be considered
by the system designer, whether writing or purchasing software.
RS-422 Systems
RS-422 system software differs little from the familiar point-to-point RS-
232 communication systems. RS-422 is often used to simply extend the distance
between nodes over the capabilities of RS-232. RS-422 can also be used as the
master node in a four-wire master-slave network described later in this chapter.
When selecting or writing software for RS-422 systems the designer should be
aware of the signals being used by the hardware in the system. Many RS-422
systems do not implement the hardware handshake lines often found in RS-232
systems due to the cost of running additional conductors over long distances.
RS-485 Driver Control
The principle difference between RS-422 and RS-485 is that the RS-485
driver can be put into a high impedance, tristate mode, which allows other
drivers to transmit over the same pair of wires. There are two methods of
tristating an RS-485 driver. The first method is to use a control line, often the
RTS handshake line, to enable and disable the driver. This requires that the host
software raise the RTS line before beginning a transmission to enable the driver,
then lower the RTS line after the completion of the transmission. Since only a
single RS-485 driver can be enabled on a network at one time it is important that
the driver is disabled as quickly as possible after transmission to avoid two
drivers trying to control the lines simultaneously, a condition called line
contention. Under some operating systems it can be difficult to lower RTS in a
timely manner and this method of driver control should be avoided altogether.
The second method of RS-485 driver control we refer to as Automatic Send
Data Control. This type of control involves special circuitry that senses when
data is being transmitted and automatically enables the driver as well as disabling
the driver within one character length of the end of transmission. This is the
preferred method of driver control since it reduces software overhead and the
number of potential pitfalls for the programmer.
34 RS-422/485 Application Note
© Copyright B&B Electronics -- Revised 2506
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax [phone]
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph 353-91-792444 – Fax [phone]
RS-485 Receiver Control
The RS-485 receiver also has an enable signal. Since RS-485 systems using
a two-wire configuration connect the driver to receiver in a loopback fashion,
this feature is often used to disable the receiver during transmission to prevent
the echo of local data. Another approach is to leave the RS-485 receiver enabled
and monitor the loopback data for errors which would indicate that line
contention has occurred. Although a good loopback signal does not guaranty
data integrity it does offer a degree of error detection.
Master-Slave Systems
A master-slave type system has one node that issues commands to each of
the “slave” nodes and processes responses. Slave nodes will not typically
transmit data without a request from the master node, and do not communicate
with each other. Each slave must have a unique address so that it can be
addressed independent of other nodes. These type systems can be configured as
two-wire or four-wire. Four-wire systems often use an RS-422 master (the
driver is always enabled) and RS-485 slaves to reduce system complexity.
Four Wire Master-Slave Systems
This configuration reduces software complexity at the host since the driver
and receiver are always enabled, at the expense of installing two extra
conductors in the system. The Master node simply prefixes commands with the
appropriate address of the slave. There is no data echo or turn around delays to
consider. Since each of the slave transmitters share the same pair of wires, care
must be taken that the master never requests data from multiple nodes
simultaneously or data collisions will result.
Two Wire Master-Slave Systems
Two wire configurations add a small amount of complexity to the system.
The RS-485 driver must be tristated when not in use to allow other nodes to use
the shared pair of wires. The time delay between the end of a transmission and
the tristate condition becomes a very important parameter in this type system. If
a slave attempts to reply before the master has tristated the line, a collision will
occur and data will be lost. The system designer must know the response time or
turn around delay of each of the slave nodes and assure that the master will
tristate its driver within that amount of time. B&B Electronics’ Automatic Send
Data control circuits tristate the driver within one character length of the end of a
transmission.