RS485-signal-distribution.md

@@ -66,11 +66,48 @@ 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.
+is manchester encoded and is transmitted as a 1MHz signal.  
+<s>For this data rate and from the figure above, maximum distances would
+be about 100m</s>. For longer distances reaching 300m, 5% jitter must be
+expected. <s>whether receiver boards such as the CTRV (CERN specific)
+can handle this sort of jitter must be further looked at.</s>  
+Early tests have confirmed the increase of signal jitter with length of
+the copper cable. The test setup consisted of RS-485 signal distribution
+through varying length of twisted pair cable, with termination on both
+ends of the bus (input and output). The table below gives a summary of
+the results (though the reason for the decrease at 1150m is unclear).
+
+<table>
+<tbody>
+<tr class="odd">
+<td><b> Cable length</b></td>
+<td><b> Measured Jitter</b></td>
+</tr>
+<tr class="even">
+<td>150m</td>
+<td>~26 ns</td>
+</tr>
+<tr class="odd">
+<td>200m</td>
+<td>~26 ns</td>
+</tr>
+<tr class="even">
+<td>350m</td>
+<td>~69 ns</td>
+</tr>
+<tr class="odd">
+<td>650m</td>
+<td>~101 ns</td>
+</tr>
+<tr class="even">
+<td>1150m</td>
+<td>~88 ns</td>
+</tr>
+</tbody>
+</table>
+
+The Timing Receiver board (CTRV) has been tested to receive long
+distance signals, and has been able to cope with 100ns jitter.
 
 For single pulse distribution with Frequencies below 100kHz, the RS-485
 system should be able to handle distribution for up to 1200m without
@@ -135,7 +172,7 @@ By loading the pair with a 50Ohm load, the differential voltage is
 
 -----
 
-Denia Bouhired. May 31th, 2018
+Denia Bouhired. June 26th, 2018