Macurco DVP-120C Manual
REV – 1.0
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3.2.6.3
Baud Rates
At the baud rate of 19200 (default baud rate for Macurco MRS-485 and DVP-485) and with cables less than 1,000 ft.
in length, termination resistors are not recommended.
At the baud rate of 19200 and with cables longer than 1,000 ft., termination resistors are recommended.
It is recommended to use an RS-485 type of cable and 120 ohms for termination resistor.
An RS-485 network requires a 3-wire cable: a twisted pair and a third wire. It is difficult to tell whether shielding is
required or not in a particular system until problems arise, so it is recommended to always use shielded cable.
When using termination resistors use only 2 resistors, one at each end of the RS-485 transmission line (i.e. 1 at the
DVP-485B and 1 at the last/farthest MRS-485).
For any other baud rates calculate when (at what length) termination resistors are required. This is calculated by
dividing the length L by the ratio between the new baud rate and 19200 or (x/19200) where x is the new baud rate.
𝐿𝐿
(
𝑥𝑥
19200
𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏
⁄
)
For example, if new baud rate is 9600
1000
𝑓𝑓𝑓𝑓
(9600
𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏
19200
𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏
⁄
) =
1000
𝑓𝑓𝑓𝑓
0.5 = 2000
𝑓𝑓𝑓𝑓
Use termination resistors when cables are longer than 2000 ft.
For example, if new baud rate is 38400
1000
𝑓𝑓𝑓𝑓
(38400
𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏
19200
𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏
⁄
) =
1000
𝑓𝑓𝑓𝑓
2
= 500
𝑓𝑓𝑓𝑓
Use termination resistors when cables are longer than 500 ft.
Note
: Other manufacturers may have different recommendations on wire length and EOL resistor usage.
