RS-485 and RS-422

[Richard Chambers]

Could you please recommend a book that would teach me the basics of RS-485
and RS-422 ? I have been in the library, using the computer to attempt to
find a book with RS-485 (or RS485, RS-422, RS422) in its title, but without
success. When I searched for "data communications" instead, I found a number
of books, but none of these had a section on RS-485 or RS-422, and they all
had no relevance to what I wanted to know.

Is there any difference between RS-485 and RS-422, or are they the same? Is
RS-485 the same as the four-wire (i.e. two twisted pairs) version of
ethernet, standard 802.3 ?

I have four power station electrical meters. The data from these meters are
tee-ed into a four-wire (two twisted pairs) RS-485 bus cabling system, which
is interrogated by a RS-485 to RS-232 converter. I want to find out how the
main interrogator can direct its interrogation first at meter 1, then at
meter 2, etc. What sort of signals is the RS-232 to RS-485 converter
putting onto the two sets of twisted pairs (that's all the wires we have) to
specify and call up first meter 1, then meter 2, etc.? How does each meter
"know" that it, and not one of the other meters, is being called upon to
download its data?

Richard Chambers Leeds UK.

 

[Joe Soap]

In response to what Richard Chambers

Could you please recommend a book that would teach me the basics of
RS-485 and RS-422 ? I have been in the library, using the computer to
attempt to find a book with RS-485 (or RS485, RS-422, RS422) in its
title, but without success. When I searched for "data communications"
instead, I found a number of books, but none of these had a section on
RS-485 or RS-422, and they all had no relevance to what I wanted to
know.

Is there any difference between RS-485 and RS-422, or are they the
same? Is RS-485 the same as the four-wire (i.e. two twisted pairs)
version of ethernet, standard 802.3 ?

I have four power station electrical meters. The data from these
meters are tee-ed into a four-wire (two twisted pairs) RS-485 bus
cabling system, which is interrogated by a RS-485 to RS-232 converter.
I want to find out how the main interrogator can direct its
interrogation first at meter 1, then at meter 2, etc. What sort of
signals is the RS-232 to RS-485 converter putting onto the two sets of
twisted pairs (that's all the wires we have) to specify and call up
first meter 1, then meter 2, etc.? How does each meter "know" that it,
and not one of the other meters, is being called upon to download its
data?

Useful info here: http://www.bb-elec.com/

 

[Andrew Tweddle]

Could you please recommend a book that would teach me the basics of RS-485
and RS-422 ? I have been in the library, using the computer to attempt to
find a book with RS-485 (or RS485, RS-422, RS422) in its title, but without
success. When I searched for "data communications" instead, I found a number
of books, but none of these had a section on RS-485 or RS-422, and they all
had no relevance to what I wanted to know.

Is there any difference between RS-485 and RS-422, or are they the same? Is
RS-485 the same as the four-wire (i.e. two twisted pairs) version of
ethernet, standard 802.3 ?

I have four power station electrical meters. The data from these meters are
tee-ed into a four-wire (two twisted pairs) RS-485 bus cabling system, which
is interrogated by a RS-485 to RS-232 converter. I want to find out how the
main interrogator can direct its interrogation first at meter 1, then at
meter 2, etc. What sort of signals is the RS-232 to RS-485 converter
putting onto the two sets of twisted pairs (that's all the wires we have) to
specify and call up first meter 1, then meter 2, etc.? How does each meter
"know" that it, and not one of the other meters, is being called upon to
download its data?

Richard Chambers Leeds UK.

Since I haven't found a good book on the subject, might I suggest
deriving the information from datasheets. I would suggest the datasheets
for RS485 and 75176 as common interface devices, and SCC8530 ? for a
controller IC. As you have probably deduced RS485 is a serial datacomms
format similar to RS232 , the significant difference being the levels
and the fact that it is multidrop(and so must have some sort of
addressing scheme) and not point to point.

Hope this helps

Andrew