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Texas Instruments Signal Integrity versus Data Rate and Cable Length for RS-485 Transceivers Application notes
Application Report
SLLA431 – December 2018
Signal Integrity Versus Data Rate and Cable Length for
RS-485 Transceivers
Eric Schott
ABSTRACT
This document contains lab data for THVD1450 and THVD1429 RS-485 transceivers operating over a
range of different cable lengths and data rates. Jitter measurements are provided for a variety of test
combinations between 1 m to 1 km and 100 kbps to 50 Mbps (depending on the device being tested).
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2
3
4
5
Contents
Introduction ................................................................................................................... 2
Jitter ........................................................................................................................... 2
Measurement Setup ......................................................................................................... 2
Lab Results ................................................................................................................... 3
Summary .................................................................................................................... 14
List of Figures
1
Jitter Measurement Set-Up ................................................................................................. 3
2
THVD1450 Differential Jitter
3
THVD1450 Output Jitter .................................................................................................... 6
4
1 Meter at 1 Mbps ........................................................................................................... 6
5
50 Meters at 1 Mbps
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
...............................................................................................
6
........................................................................................................ 6
1 Meter at 20 Mbps .......................................................................................................... 6
50 Meters at 20 Mbps ....................................................................................................... 6
1 Meter at 50 Mbps .......................................................................................................... 6
50 Meters at 30 Mbps ....................................................................................................... 6
100 Meters at 1 Mbps ....................................................................................................... 7
150 Meters at 1 Mbps ....................................................................................................... 7
100 Meters at 10 Mbps ..................................................................................................... 7
150 Meters at 10 Mbps ..................................................................................................... 7
100 Meters at 20 Mbps ..................................................................................................... 7
150 Meters at 20 Mbps ..................................................................................................... 7
200 Meters at 1 Mbps ....................................................................................................... 8
1000 Meters at 1 Mbps ..................................................................................................... 8
200 Meters at 10 Mbps ..................................................................................................... 8
200 Meters at 20 Mbps ..................................................................................................... 8
THVD1429 Differential Jitter .............................................................................................. 11
THVD1429 Output Jitter ................................................................................................... 11
1 Meters at 1 Mbps ........................................................................................................ 11
50 Meters at 1 Mbps ...................................................................................................... 11
1 Meters at 10 Mbps ....................................................................................................... 11
50 Meters at 10 Mbps ..................................................................................................... 11
1 Meters at 20 Mbps ....................................................................................................... 11
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Introduction
27
28
29
30
31
32
33
34
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.....................................................................................................
100 Meters at 1 Mbps .....................................................................................................
150 Meters at 1 Mbps .....................................................................................................
100 Meters at 10 Mbps ....................................................................................................
150 Meters at 10 Mbps ....................................................................................................
100 Meters at 20 Mbps ....................................................................................................
200 Meters at 1 Mbps .....................................................................................................
200 Meters at 10 Mbps ....................................................................................................
50 Meters at 20 Mbps
11
12
12
12
12
12
13
13
List of Tables
1
THVD1450 Jitter Test Results
2
THVD1429 Jitter Test Results
.............................................................................................
.............................................................................................
3
9
Trademarks
All trademarks are the property of their respective owners.
1
Introduction
The THVD14xx family of transceivers has been designed to work in a range of applications, facilitating the
transfer of data over relatively long distances while maintaining immunity against the electromagnetic
noise present in many industrial applications. Different variants within this family can be selected based on
particular applications needs ranging from different pin outs, half/full-duplex, data rates, loading
characteristics, ESD protections and more. All devices in this family can work with supply voltages
between 3.0 and 5.5 volts.
All of these devices operate within the standard set by TIA and EIA for RS-485 transceivers which
specifies features such as signal amplitude, input sensitivity, and input impedance. However, several
characteristics such as cable length, cable type, connectors, data rates, and bus topologies are not
defined by the standard. This document’s purpose is to provide a reference to tests in a controlled
environment to help system designers understand how the factors of cable length and data rate can affect
data integrity and provide insight so educated design decisions can be made.
Two devices were used in this series of tests. The THVD1450 is a high speed variant which supports data
rates up to 50 Mbps. The THVD1429 has similar operating characteristics to the THVD1450 with the
notable inclusion of an integrated TVS diode for increased ESD protection. This TVS diode adds some
amount of capacitance for each node and therefore is only specified to operate at data rates up to 20
Mbps. These two devices were tested and the results compared to observe the effect that added loading
on the bus can have on the capabilities of the system.
2
Jitter
The amount of jitter present in a system is a good indication of data integrity through a cable. Many factors
contribute to the amount of jitter in a system. The two variables that this document will focus on are cable
length and data rate. More on jitter and how it can be measured can be found in the application note for
Signal Integrity vs. Transmission Rate and Cable Length for RS-485 Transceivers (SLLA375).
3
Measurement Setup
The test setup included a PRBS generator configured to output random data at a configurable data rate.
The output of the generator was connected to a RS-485 half-duplex EVM. The device mounted on this
board had its driver enabled and receiver disabled. The outputs of the driver device were connected to
different spools of unshielded twisted-pair cables (Belden 3105A) of varying lengths. The other end of the
spool was connected to a second RS-485 half-duplex EVM which had its receiver enabled and driver
disabled. Both boards used 120-Ohm termination near the transceiver. The bus lines (A and B) on the
receiver board were connected to an oscilloscope using two 1-MΩ, < 10 pF probes configured as a
differential pair. The R pin of the receiver device was also connected directly to an oscilloscope channel
configured for 50-Ω termination. Lastly, the oscilloscope was connected to the trigger output of the PRBS
generator using a 50-cm cable and was set to trigger on this input.
2
Signal Integrity Versus Data Rate and Cable Length for RS-485 Transceivers
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To conduct the test, same-model devices (THVD1450s or THVD1429s) were populated on the transmitter
and receiver side of the setup. A cable length was selected and connected to both boards. The PRBS
generator was configured to output data at a given data rate and the results were viewed on the
oscilloscope. To measure jitter, the oscilloscope was set to infinite persistence. Using a histogram, the
peak-to-peak variation of the horizontal crossing point of the resulting differential eye diagram was
recorded; the results of this measurement are referred to as “Differential Jitter” in this document. Using the
cursors lined up at the first and last transition in the persistent figure, the jitter of the single output line was
measured and recorded; this is referred to as “Output Jitter” in this document.
The data rate of the PRBS signal was increased until it reached the device’s specified limit or the methods
of measurement could not differentiate different bit periods (past 80% jitter). At this point, a new cable
length was connected. This series of tests was done for the THVD1450 and THVD1429. Both devices
were supplied 5.0 V to VCC and a common ground was used by both transmitter and receiver.
Trigger Output
Oscilloscope
Differential High
Impedance Probe
Scope Probe
PRBS Generator
PRBS data
B
RS-485
Receiver
B
A
RS-485
Receiver
A
.305
Twisted
Pair Cable
Bundle
Figure 1. Jitter Measurement Set-Up
4
Lab Results
Table 1. THVD1450 Jitter Test Results
(1)
Cable
Length
(m)
Data Rate
(Mbps)
Bit Duration
(ns)
Differential
Jitter
(ns)
Eye Width
(ns)
Total
Differential
Jitter
Output
Jitter
(ns)
Eye Width
(ns)
Total
Output
Jitter (1)
1
1
1000.0
0.4
999.6
0%
3.2
996.8
0%
1
10
100.0
0.6
99.4
1%
3.1
96.9
3%
1
20
50.0
0.9
49.1
2%
3.1
46.9
6%
1
30
33.3
1.1
32.2
3%
3.2
30.1
10%
1
40
25.0
1.3
23.7
5%
3.8
21.2
15%
1
50
20.0
1.6
18.4
8%
4.2
15.8
21%
The level of acceptable jitter in a given application would be dependent on the overall serial communications system
implementation. Most systems can tolerate around 20% without a substantial increase in bit error rate, and some may tolerate
jitter levels as high as 50%.
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Lab Results
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Table 1. THVD1450 Jitter Test Results (continued)
4
Cable
Length
(m)
Data Rate
(Mbps)
Bit Duration
(ns)
Differential
Jitter
(ns)
Eye Width
(ns)
Total
Differential
Jitter
Output
Jitter
(ns)
Eye Width
(ns)
Total
Output
Jitter (1)
50
1
1000.0
3.2
996.8
0%
2.6
997.4
0%
50
2
500.0
3.5
496.5
1%
3.4
496.6
1%
50
4
250.0
4.0
246.0
2%
3.6
246.4
1%
50
10
100.0
4.2
95.8
4%
3.7
96.3
4%
50
12
83.3
9.1
74.2
11%
5.0
78.3
6%
50
14
71.4
10.2
61.2
14%
6.7
64.7
9%
50
16
62.5
9.8
52.7
16%
5.9
56.6
9%
50
18
55.6
8.0
47.6
14%
4.8
50.8
9%
50
20
50.0
9.1
40.9
18%
5.5
44.5
11%
50
22
45.5
7.5
38.0
17%
4.1
41.4
9%
50
24
41.7
10.4
31.3
25%
4.4
37.3
11%
50
26
38.5
11.2
27.3
29%
6.4
32.1
17%
50
28
35.7
13.3
22.4
37%
6.6
29.1
18%
50
30
33.3
14.7
18.6
44%
6.3
27.0
19%
50
32
31.3
15.0
16.3
48%
5.9
25.4
19%
50
34
29.4
11.1
18.3
38%
5.1
24.3
17%
50
36
27.8
13.7
14.1
49%
4.8
23.0
17%
50
38
26.3
15.7
10.6
60%
7.4
18.9
28%
50
40
25.0
15.8
9.2
63%
9.2
15.8
37%
50
42
23.8
13.9
9.9
58%
8.8
15.0
37%
50
44
22.7
15.1
7.6
66%
8.8
13.9
39%
50
46
21.7
16.9
4.8
78%
10.8
10.9
50%
50
48
20.8
N/A
N/A
N/A
14.1
6.7
68%
50
50
20.0
N/A
N/A
N/A
17.0
3.0
85%
100
1
1000.0
10.6
989.4
1%
4.8
995.2
0%
100
2
500.0
9.4
490.6
2%
4.4
495.6
1%
100
4
250.0
9.6
240.4
4%
4.8
245.2
2%
100
10
100.0
15.6
84.4
16%
5.8
94.2
6%
100
12
83.3
14.6
68.7
18%
6.6
76.7
8%
100
14
71.4
17.2
54.2
24%
7.6
63.8
11%
100
16
62.5
16.4
46.1
26%
6.4
56.1
10%
100
18
55.6
18.0
37.6
32%
6.2
49.4
11%
100
20
50.0
18.2
31.8
36%
7.6
42.4
15%
100
22
45.5
17.8
27.7
39%
8.2
37.3
18%
100
24
41.7
16.4
25.3
39%
7.6
34.1
18%
100
26
38.5
22.4
16.1
58%
8.2
30.3
21%
100
28
35.7
33.8
1.9
95%
8.2
27.5
23%
100
30
33.3
N/A
N/A
N/A
9.0
24.3
27%
100
32
31.3
N/A
N/A
N/A
9.2
22.1
29%
100
34
29.4
N/A
N/A
N/A
9.6
19.8
33%
100
36
27.8
N/A
N/A
N/A
9.6
18.2
35%
100
38
26.3
N/A
N/A
N/A
10.6
15.7
40%
100
40
25.0
N/A
N/A
N/A
11.4
13.6
46%
100
42
23.8
N/A
N/A
N/A
16.0
7.8
67%
100
44
22.7
N/A
N/A
N/A
15.4
7.3
68%
100
46
21.7
N/A
N/A
N/A
15.6
6.1
72%
Signal Integrity Versus Data Rate and Cable Length for RS-485 Transceivers
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Table 1. THVD1450 Jitter Test Results (continued)
Cable
Length
(m)
Data Rate
(Mbps)
Bit Duration
(ns)
Differential
Jitter
(ns)
Eye Width
(ns)
Total
Differential
Jitter
Output
Jitter
(ns)
Eye Width
(ns)
Total
Output
Jitter (1)
100
48
20.8
N/A
N/A
N/A
N/A
N/A
N/A
100
50
20.0
N/A
N/A
N/A
N/A
N/A
N/A
150
1
1000.0
23.6
976.4
2%
14.8
985.2
1%
150
2
500.0
21.2
478.8
4%
12.0
488.0
2%
150
4
250.0
20.4
229.6
8%
10.4
239.6
4%
150
6
166.7
22.0
144.7
13%
14.0
152.7
8%
150
8
125.0
24.0
101.0
19%
12.0
113.0
10%
150
10
100.0
30.8
69.2
31%
14.4
85.6
14%
150
12
83.3
26.8
56.5
32%
15.2
68.1
18%
150
14
71.4
25.6
45.8
36%
12.0
59.4
17%
150
16
62.5
31.6
30.9
51%
14.4
48.1
23%
150
18
55.6
31.2
24.4
56%
17.2
38.4
31%
150
20
50.0
28.0
22.0
56%
18.4
31.6
37%
150
22
45.5
36.4
9.1
80%
16.7
28.8
37%
150
24
41.7
39.0
2.7
94%
18.4
23.3
44%
150
26
38.5
N/A
N/A
N/A
20.8
17.7
54%
150
28
35.7
N/A
N/A
N/A
22.8
12.9
64%
150
30
33.3
N/A
N/A
N/A
24.8
8.5
74%
200
1
1000.0
27.6
972.4
3%
11.6
988.4
1%
200
2
500.0
30.0
470.0
6%
12.0
488.0
2%
200
4
250.0
27.6
222.4
11%
14.4
235.6
6%
200
6
166.7
28.8
137.9
17%
14.4
152.3
9%
200
8
125.0
31.2
93.8
25%
19.2
105.8
15%
200
10
100.0
40.8
59.2
41%
20.0
80.0
20%
200
12
83.3
34.4
48.9
41%
19.2
64.1
23%
200
14
71.4
28.0
43.4
39%
20.0
51.4
28%
200
16
62.5
34.4
28.1
55%
23.2
39.3
37%
200
18
55.6
42.8
12.8
77%
21.2
34.4
38%
200
20
50.0
N/A
N/A
N/A
24.8
25.2
50%
1000
0.1
10000.0
1088.0
8912.0
11%
704.0
9296.0
7%
1000
0.2
5000.0
1136.0
3864.0
23%
696.0
4304.0
14%
1000
0.4
2500.0
912.0
1588.0
36%
688.0
1812.0
28%
1000
0.6
1666.7
944.0
722.7
57%
600.0
1066.7
36%
1000
0.8
1250.0
936.0
314.0
75%
648.0
602.0
52%
1000
1
1000.0
840.0
160.0
84%
712.0
288.0
71%
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100%
1m
50 m
100 m
150 m
200 m
90%
80%
70%
Jitter
60%
50%
40%
30%
20%
10%
0
0
10
20
30
Data-Rate (Mbps)
40
50
D001
Figure 2. THVD1450 Differential Jitter
100%
1m
50 m
100 m
150 m
200 m
90%
80%
70%
Jitter
60%
50%
40%
30%
20%
10%
0
0
10
20
30
Data-Rate (Mbps)
40
50
D002
Figure 3. THVD1450 Output Jitter
6
Signal Integrity Versus Data Rate and Cable Length for RS-485 Transceivers
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Figure 4. 1 Meter at 1 Mbps
Figure 5. 50 Meters at 1 Mbps
Figure 6. 1 Meter at 20 Mbps
Figure 7. 50 Meters at 20 Mbps
Figure 8. 1 Meter at 50 Mbps
Figure 9. 50 Meters at 30 Mbps
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Figure 10. 100 Meters at 1 Mbps
Figure 11. 150 Meters at 1 Mbps
Figure 12. 100 Meters at 10 Mbps
Figure 13. 150 Meters at 10 Mbps
Figure 14. 100 Meters at 20 Mbps
Figure 15. 150 Meters at 20 Mbps
Signal Integrity Versus Data Rate and Cable Length for RS-485 Transceivers
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Figure 16. 200 Meters at 1 Mbps
Figure 17. 1000 Meters at 1 Mbps
Figure 18. 200 Meters at 10 Mbps
Figure 19. 200 Meters at 20 Mbps
Table 2. THVD1429 Jitter Test Results
(1)
Cable
Length
(m)
Data Rate
(Mbps)
Bit Duration
(ns)
Differential
Jitter
(ns)
Eye Width
(ns)
Total
Differential
Jitter
Output
Jitter
(ns)
Eye Width
(ns)
Total
Output
Jitter (1)
1
1
1000.0
2.6
997.4
0%
3.2
996.8
0%
1
2
500.0
2.4
497.6
0%
2.8
497.2
1%
1
4
250.0
2.4
247.6
1%
3.0
247.0
1%
1
6
166.7
2.4
164.3
1%
3.2
163.5
2%
1
8
125.0
2.4
122.6
2%
3.4
121.6
3%
1
10
100.0
2.8
97.2
3%
3.4
96.6
3%
1
12
83.3
2.8
80.5
3%
3.8
79.5
5%
1
14
71.4
3.0
68.4
4%
4.8
66.6
7%
1
16
62.5
3.4
59.1
5%
4.8
57.7
8%
1
18
55.6
4.0
51.6
7%
5.4
50.2
10%
1
20
50.0
4.2
45.8
8%
5.4
44.6
11%
50
1
1000.0
10.2
989.8
1%
5.4
994.6
1%
50
2
500.0
11.0
489.0
2%
7.4
492.6
1%
50
4
250.0
17.2
232.8
7%
14.6
235.4
6%
50
6
166.7
17.6
149.1
11%
13.4
153.3
8%
The level of acceptable jitter in a given application would be dependent on the overall serial communications system
implementation. Most systems can tolerate around 20% without a substantial increase in bit error rate, and some may tolerate
jitter levels as high as 50%.
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Table 2. THVD1429 Jitter Test Results (continued)
Cable
Length
(m)
10
Data Rate
(Mbps)
Bit Duration
(ns)
Differential
Jitter
(ns)
Eye Width
(ns)
Total
Differential
Jitter
Output
Jitter
(ns)
Eye Width
(ns)
Total
Output
Jitter (1)
50
8
125.0
16.4
108.6
13%
13.4
111.6
11%
50
10
100.0
18.4
81.6
18%
15.6
84.4
16%
50
12
83.3
21.2
62.1
25%
21.0
62.3
25%
50
14
71.4
24.0
47.4
34%
22.0
49.4
31%
50
16
62.5
24.8
37.7
40%
23.8
38.7
38%
50
18
55.6
24.4
31.2
44%
22.0
33.6
40%
50
20
50.0
28.8
21.2
58%
20.8
29.2
42%
100
1
1000.0
10.8
989.2
1%
6.4
993.6
1%
100
2
500.0
14.6
485.4
3%
9.6
490.4
2%
100
4
250.0
14.0
236.0
6%
9.8
240.2
4%
100
6
166.7
18.4
148.3
11%
14.0
152.7
8%
100
8
125.0
15.0
110.0
12%
10.8
114.2
9%
100
10
100.0
21.0
79.0
21%
15.4
84.6
15%
100
12
83.3
22.2
61.1
27%
18.2
65.1
22%
100
14
71.4
22.8
48.6
32%
11.8
59.6
17%
100
16
62.5
23.6
38.9
38%
15.0
47.5
24%
100
18
55.6
22.6
33.0
41%
15.8
39.8
28%
100
20
50.0
28.2
21.8
56%
17.6
32.4
35%
150
1
1000.0
19.0
981.0
2%
8.6
991.4
1%
150
2
500.0
18.4
481.6
4%
10.2
489.8
2%
150
4
250.0
20.0
230.0
8%
11.2
238.8
4%
150
6
166.7
27.2
139.5
16%
18.2
148.5
11%
150
8
125.0
19.8
105.2
16%
14.0
111.0
11%
150
10
100.0
29.8
70.2
30%
19.0
81.0
19%
150
12
83.3
34.0
49.3
41%
21.8
61.5
26%
150
14
71.4
23.4
48.0
33%
16.0
55.4
22%
150
16
62.5
35.2
27.3
56%
24.8
37.7
40%
150
18
55.6
33.8
21.8
61%
25.4
30.2
46%
150
20
50.0
40.6
9.4
81%
27.8
22.2
56%
200
1
1000.0
29.2
970.8
3%
12.2
987.8
1%
200
2
500.0
28.4
471.6
6%
15.6
484.4
3%
200
4
250.0
31.0
219.0
12%
20.0
230.0
8%
200
6
166.7
33.4
133.3
20%
21.8
144.9
13%
200
8
125.0
29.4
95.6
24%
23.0
102.0
18%
200
10
100.0
40.8
59.2
41%
29.8
70.2
30%
200
12
83.3
38.4
44.9
46%
30.8
52.5
37%
200
14
71.4
33.6
37.8
47%
33.6
37.8
47%
200
16
62.5
40.6
21.9
65%
30.0
32.5
48%
200
18
55.6
N/A
N/A
N/A
37.2
18.4
67%
200
20
50.0
N/A
N/A
N/A
39.4
10.6
79%
Signal Integrity Versus Data Rate and Cable Length for RS-485 Transceivers
Copyright © 2018, Texas Instruments Incorporated
SLLA431 – December 2018
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Lab Results
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100%
1m
50 m
100 m
150 m
200 m
90%
80%
70%
Jitter
60%
50%
40%
30%
20%
10%
0
0
5
10
Data-Rate (Mbps)
15
20
D003
Figure 20. THVD1429 Differential Jitter
1
1m
50 m
100 m
150 m
200 m
0.9
0.8
0.7
Jitter
0.6
0.5
0.4
0.3
0.2
0.1
0
0
5
10
Data-Rate (Mbps)
15
20
D004
Figure 21. THVD1429 Output Jitter
SLLA431 – December 2018
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Signal Integrity Versus Data Rate and Cable Length for RS-485 Transceivers
Copyright © 2018, Texas Instruments Incorporated
11
Lab Results
12
www.ti.com
Figure 22. 1 Meters at 1 Mbps
Figure 23. 50 Meters at 1 Mbps
Figure 24. 1 Meters at 10 Mbps
Figure 25. 50 Meters at 10 Mbps
Figure 26. 1 Meters at 20 Mbps
Figure 27. 50 Meters at 20 Mbps
Signal Integrity Versus Data Rate and Cable Length for RS-485 Transceivers
Copyright © 2018, Texas Instruments Incorporated
SLLA431 – December 2018
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Lab Results
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Figure 28. 100 Meters at 1 Mbps
Figure 29. 150 Meters at 1 Mbps
Figure 30. 100 Meters at 10 Mbps
Figure 31. 150 Meters at 10 Mbps
Figure 32. 100 Meters at 20 Mbps
SLLA431 – December 2018
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Signal Integrity Versus Data Rate and Cable Length for RS-485 Transceivers
Copyright © 2018, Texas Instruments Incorporated
13
Summary
www.ti.com
Figure 33. 200 Meters at 1 Mbps
5
Figure 34. 200 Meters at 10 Mbps
Summary
The correlation between cable length and jitter can be clearly seen from these measurements. The
specific points at which certain jitter thresholds are reached depend on many elements of the system
being implemented and may vary from the results of this test, but the trend indicated by these readings
may serve as a guide for reference.
14
Signal Integrity Versus Data Rate and Cable Length for RS-485 Transceivers
Copyright © 2018, Texas Instruments Incorporated
SLLA431 – December 2018
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