MAX487制作RS485总线接口模块

 

■ MAX487简介

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MAX487是用于低功耗RS-485总线收发器。

^{▲ MAX487管脚配置和典型应用电路}

存在的问题：

• 是否需要增加偏置电阻？在MAX487输出低电平的时候两线的差分是多少？
• 测量总线电压与输出电压之间的关系

 

01实验电路设计

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1.电路设计

^{▲ 实验电路原理图}

^{▲ 实验电路板PCB}

 

02电路测量

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1.MAX487静态电压

• 输出电压Vab=0.0035V
• RXD: 5V

2.Spipex485EEN静态电压

• 输出电压：Vab=0.0028V
• RXD: 0.0001V

3.电路输入输出之间的关系

□ MAX487输入输出电压关系

^{▲ MAX487输入和输出之间的关系}

inv=[5.11,5.11,5.11,5.11,5.11,5.11,5.11,5.11,3.30,3.29,3.28,3.27,3.26,3.25,3.24,3.23,3.22,3.21,3.20,3.19,3.18,3.16,3.15,3.15,3.14,3.13,3.12,3.11,3.10,3.09,3.08,3.07,3.06,3.05,3.04,3.03,3.02,3.01,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00]
outv=[-0.00,-0.00,-0.01,-0.01,-0.01,-0.01,-0.01,-0.02,-0.02,-0.02,-0.02,-0.02,-0.02,-0.03,-0.03,-0.03,-0.03,-0.03,-0.04,-0.04,-0.04,-0.04,-0.04,-0.05,-0.05,-0.05,-0.05,-0.05,-0.06,-0.06,-0.06,-0.06,-0.06,-0.07,-0.07,-0.07,-0.07,-0.08,-0.08,-0.08,-0.08,-0.09,-0.09,-0.09,-0.09,-0.09,-0.09,-0.10,-0.10,-0.10]

^{▲ MAX487输入输出之间的电压关系}

□ Sipex485输入输出之间的关系

^{▲ Sipexl485输入输出之间的关系}

inv=[0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.95,1.11,1.23,1.33,1.41,1.50,1.60,1.68,1.75,1.82,1.90,1.97,2.04,2.12,2.20,5.11,5.11,5.11,5.11,5.11,5.11,5.11,5.11,5.11,5.11]
outv=[0.00,0.00,0.01,0.01,0.02,0.02,0.02,0.03,0.03,0.04,0.04,0.05,0.05,0.05,0.06,0.06,0.07,0.07,0.07,0.08,0.08,0.09,0.09,0.09,0.10,0.10,0.11,0.11,0.11,0.12,0.12,0.13,0.13,0.14,0.14,0.14,0.15,0.15,0.15,0.16,0.16,0.17,0.17,0.18,0.18,0.18,0.19,0.19,0.20,0.20]

 

03输入输出之间动态关系

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1. TXD←总线

在485总线B端加入交流信号。A端接地。测量RO的输出波形。

^{▲ 测量电路}

测量信号波形：

^{▲ 输入波形和输出波形}

2.TXD→总线

• 补充这个实验

^{▲ 测量TXD到VAB之间的传递关系}

^{▲ TXD → Vab}

^{▲ TXD → Vab}

^{▲ TXD → Vab}

^{▲ TXD → Vab}

在上面测量过程中，MAX487在输出电压转换过程中出现了输入输出电压非单调变化的情况。对于这个原因初步猜测是输入电流的增加，使得输入电压微微下降。

下面在输入Vtxd:串联一个200Ω的电阻，测量其两端的电压，反映了输入电流的变化。

下面是给出了相应的曲线，可以看到输入电流的确是在输出反转前后出现了一定的波动，但这种波动（十几个微安）很小，应该不足以引起输入电压的变化。

^{▲ 在电压转换过程中的输入电流Vi变化情况}

★ 疑问：那么这其中究竟什么原因造成了前面的数据曲线？现在还不可而知。

 

※ 结论

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1. 对于MAX487组成的总线不需要另行进行偏置。Va
2. 对于Sipex485则需要进行总线偏置。它的输出在0V左右的时候呈现接收为低电平。

#!/usr/local/bin/python
# -*- coding: gbk -*-
#============================================================
# TEST2.PY                     -- by Dr. ZhuoQing 2020-07-12
#
# Note:
#============================================================

from headm import *

from tsmodule.tsvisa        import *
from tsmodule.tsstm32       import *

tspbeep(1200, 200)

dp1308open()

dp1308p6v(0)
tspbeep(1200, 200)
time.sleep(1)

#------------------------------------------------------------
inputv = []
outputv = []

#------------------------------------------------------------

setv = linspace(0, 0.1, 50)

for v in setv:
    dp1308p6v(v)
    time.sleep(1)
    meter = meterval()
    outputv.append(meter[2])
    inputv.append(meter[0])

    printf(meter)

tspsave('measure', inv=inputv, outv=outputv)
plt.plot(outputv, inputv)
plt.xlabel("Input(V)")
plt.ylabel("Output(V)")
plt.grid(True)
plt.tight_layout()
plt.show()

#------------------------------------------------------------
#        END OF FILE : TEST2.PY
#============================================================