DIGITAL-LOGIC Microspace MSM586SEN Technical User Manual page 263

DIGITAL-LOGIC AG
4.3.3.1
4.3.3.1.1
RS-485 is a multi-drop extension to the RS-422 standard. It uses differential signals on twisted pairs for re-
ceive and transmit.
RS-485 systems can be half duplex 2-wire systems (one twisted pair plus signal common/ground) or full
duplex 4-wire systems. A RS-485 transmitter driver is activated to send data and is set to a high imped-
ance tri-state at the end of transmission. Driver control can be automatic using a Send Data circuit, or
manual by setting the RTS line or UART RTS control high for transmit, then low at the end of transmis-
sion. In a half duplex 2-wire system, the receiver is set to receive except when transmitting.
In a 2-wire system, all slaves and masters are normally in the receive mode. When one master transmits, all
slaves and masters receive the signal and response, and all slaves must be able to ignore commands and
responses to/from other slaves. Each slave must wait until transmit is finished plus a delay (for bus turn-
around), before responding.
In a 4-wire system, all slaves are connected to the transmitter of the master(s). All slaves connect to the re-
ceiver of the master(s). Each slave must respond only to commands addressed to it, but no turn-around de-
lay is needed. The slave can start responding immediately, even while receiving. Other slaves never hear
each other's responses.
4.3.3.1.2
RTS Control is relevant only if you are converting to 2-wire RS485 (where the serial UART is a Master or a
Slave) or to 4-wire RS485 (where the serial UART is a Slave). It is not required for RS422 which is a point to
point system only and on which the driver is permanently enabled.
RTS Control is a method with which the RS232 device (typically a PC) tells an RS232-RS485 con-
verter chip when it should enable its RS485 driver, i.e. when it should be transmitting. There is no techni-
cal reason why the converter cannot determine this by itself but it increases the cost of the converter. It also
makes it sensitive to the baud rate and character length (the number of bits).
With the LTC485 device, which is an interface converter chip only and does not monitor the data, an external
signal is required. When providing RTS Control, the RS232 device raises its RTS output immediately before
it starts to communicate, and drops it after the last stop bit of the message has been transmitted. The serial
UART uses this signal to control its RS485 driver. The advantages of using RTS Control is that the control is
simpler and therefore cheaper, and it does not care about the baud rate (within its limits) or the number of
bits, parity, etc.
The following diagram illustrates a message comprising of two characters and the RTS Control signal which
would be required to successfully transmit this message. Both characters are shown as 8-bit data (or 7 bits
with parity).
A more sophisticated converter does not need RTS Control because it generates it internally by monitoring
the data with a microprocessor. But you have to configure the baud rate etc on the converter.
The RTS Control function has to be written into the application program and is not an operating system
function which you can configure in e.g. the Windows Control Panel. Many RS485-oriented application pro-
grams have it, particularly those written for industrial applications. Some like LapLink or Therm95 do not. The
only way to establish if a particular application program provides RTS Control is to ask the programmer who
wrote it, or the vendor. If this is not possible, and no reliable information is available, you should assume that
RTS Control is not available and choose an "ADE" converter.
Do not confuse RTS Control with the more common operating mode of the RTS signal which is hardware
flow control and which is unsuitable for controlling an RS232-RS485 converter.
RS485 remark
RS-485
RTS Control
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MSM586SEN/SEV Manual V1.5E