Delta HMI communication cable
Posted 07 December 2007 - 02:02 AM
Posted 08 December 2007 - 11:54 PM
there is some electronics that converts RS485 (PLC) to and from RS232 (PC or HMI).
mentioned simpler cable is admirable attempt to bridge the gap but unfortunately it doesn't
meet the specs for such conversion. 485 is differential and more importantly, 485 signal
levels are different from RS232.
RS232 standard says that "1" is signal between -3V and -25V, "0" is between +3 and +25V.
note that nowdays one can come accross some ports that don't follow RS232 standard and even allow
0V as "1" but this is clearly not something to rely on and assume as common!
RS485 standard says that potential difference between pins of 0.2V or greater is
required and absolute value of either signal relative to ground (0VDC) is not important
(it will be within supply voltage which is tipically 0-5VDC).
For example if you look at one of the two signals (other signal is just inverted) then
"0" may be 2.4V and "1" may be 2.6V... BUT - this is not guaranteed and in fact can shift
(within limits of supply range) so for example "0" and "1" signals could be 1.7 and 1.9V.
Because 485 signal levels are not guaranteed to be particular value relative to 0VDC
almost anything is possile (0.1 and 0.3V to gnd for "0" and "1" is as good as
4.3 and 4.5V - or 0.2 and 4.8 - as long as the other signal line is at least 0.2V different...)
Compare that with RS232 signal spec and we clearly have someting that doesn't overlap.
This is why Beijer cable packs some electronics to do conversion.
Mentined simpler cable can work in some cases because:
- certain RS232 ports do accept 0VDC as "1"and levels over 3V as "0"
- 485 signal difference doesn't have to be as low as 0.2V - it may be 4V for example
so will one device with 485 interface work with other 232 interface when using simple cable?
it might but this is pure luck. it is not hardware of devices that don't work to be blamed,
it's the others who deviate from standards that create confusion and give people false hope...
same goes for any RS232 device for example that is supposedly powered from RS232 port.
this may work here and there but creators of such designs are just fooling themselvs (and others...),
those designs are flawed becaus they don't follow standard, they trade compatibility with standard
to achive limited gain somewhere else.
if you have this working with SC09, i would just leave it. if you want cheaper or DIY alternative,
(looks like you tried it already), then make proper 232/485 converter or lookup ready-made unit
and just terminate it properly. for example check this out:
it is about 1/3 to 1/2 of the price for SC09 but you still need to do some wiring on the PLC end.
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