Distribution of RS485 Signals
Note: contains some CERN specific information for General Timing Distribution applications
The CONV-TTL-RS485 board uses the Texas Instrument SN65HVD3088E
low-power RS-485 transceiver.
This IC is the fastest in the range and is suitable for applications
that requires signaling rates up tp 20Mbps.
RS-485 signal distribution
Data rate and bus length
As shown below on figure extracted from SN65HVD3088E datasheet, page 18, by increasing cable length, data rate capabilities are impacted, also introducing jitter on the RS-485 signal.
rs485-transceiver-cablelength-vs-datarate.png
For the purposes of timing signal distribution at CERN, the GMT signal is manchester encoded and is transmitted as a 1MHz signal. For this data rate and from the figure above, maximum distances would be about 100m. For longer distances reaching 300m, 5% jitter must be expected, whether receiver boards such as the CTRV (CERN specific) can handle this sort of jitter must be further looked at.
For single pulse distribution with Frequencies below 100kHz, the RS-485 system should be able to handle distribution for up to 1200m without signal quality being compromised.
Bus loading
Optional termination*
The RS-485 differential lines are routed to and from the P2 connector of
the motherboard. The tracks can be 100Ohm terminated, this is optional
and a solid-state relay is used to activate this option.
Two dedicated dip switches on the motherboard can be used to activate,
input or output termination separately. Note however that if termination
is enabled on the input, then it will be enabled on all RS-485 input
channel.
Similarly with output channels, if RS-485 termination is active, then
this will apply to all output channels.
RS-485 Signal characteristics
The following setup was used in order to measure RS485 signal characteristics coming out of a CONV-TTL-RTM-RS495-DB9 RTM.
Note that the output from a single output line on channel 1 is measured. The the RS485_p and RS485_n signals are both fed into a single input channel of the oscilloscope. For this specific setup, this was done with a DB9 to LEMO 0S 2-pin patch cable, then a LEMO 0S to LEMO 00 adapter was used to connect a LEMO 00 cable directly to the oscilloscope via a LEMO 00 to BNC adapter. The 50Ohm load is then connected to a LEMO Y splitter, in order to show waveforms with the 50Ohm load:
diff_50Ohm_load.png
And without the 50Ohm load:
diff_no_load.png
By feeding the differential pair RS485_p and RS485_n to two separate channels of the oscilloscope and performing a subtraction on the two waveforms, the no load differential signals looks like this:
diff_no_load_2osci-chans.png
To summarise depending on the measurement method, a no load measurement
of the differential pair on any of the output of the
CONV-TTL-RTM-RS485-DB9 RTM should lie between 7 and 8.6 Volts.
By loading the pair with a 50Ohm load, the differential voltage is
~3Volts.