终端电阻为什么选用120R?
CAN总线终端电阻,一般来说都是120欧姆,实际上在设计的时候,也是两个60欧姆的电阻串起来,而总线上一般有两个120Ω的节点,基本上稍微知道点CAN总线的人都知道这个道理。data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAS4AAABsCAIAAABbxcjHAAAgAElEQVR4Ae1dB1wTSRcfQKWJNAGVEJp0FMQCggVB/Tw9bCgWsNOxYO+K5ezo6aEi4qnYsB8WxK6nCFKkSROkCoqgSIdAMp+bTTabTYKUBLy73d8SZmfevHnzZv77ZqcCGo326NGj8PDwe513RURE3LlzJyQkJCMjA5IXqYEO0UBeXl5ISMjt27cjIiI6r+7fCw8Pf/jwYX19Pdi+fTv4aS4dHZ2ampoOKQgykf+0Burq6szNzX+aig+WL18O/Pz8fh6BFBUVKyoq/tN1hMx8h2igqqpKVVX156n5Pj4+oKam5vLly0ePHj3eSVdgYGBwcLCBgQEAQFVVtbKyskPKgkzkP62B2tpaPT09AIC2tvbx48cDAwM7qfofP3r06MWLFysqKsBPUiBjxowhofiTlMV/QQwMitbW1j9Jfn8WKI4aNYqE4k9SJ/4LYmBQtLS0/EnyS0LxJykIUowO1QAJRYHqJq2iQNWQASLQAAlFgUplQ1GN7LYRqCMyQHgaIKEoUJdsKPYioShQR2SA8DRAQlGgLtlQJAczBKqIDBCiBkgoClQmG4qkVRSoIjJAiBogoShQmWwoklZRoIrIACFq4L8FRQaD0XLdjbIjxxVbri2Ssr0aqKmt1dfXBwBYWg5pL692x0eRQo4rtluRJIN/oAb+W1YRK6CWmEc7O1sAgJqaSk1NFRaRdJAaEJEGGurr9PX7AgCsrDrfKqJ55LKKJSUlp0+fdnd337t3b15eHkELX758OXHiRHZ2Nt7/0aNH9+7dw/t8+vTpypUrDQ0NEMLr168/f/4cH5qenn7r1i28D+q2s/sVAKCsrFZRQUKRVz2kj5A10NBQr6tjBAAYNNgWzxr7pqqsrLx8+fLixYu3bNmSkpKCp4EQ1tXVnT17NikpCe+flJR0+fJlvOEpLy+/ePHix48fIYRPnz6NiIjA0+fm5oaGhlZVsSo8B4qhoaGamprDhw93dXUdNWqUqqrqiRMn8DH//PNPAICnpyfec9asWdLS0lFRUZhndHR0r169iouLIYSGhobOzs5YEIQwKCjIxMSksakJ8ywtbVq3tqhbt20AGALgNHJkUsQ9dMkiA0JMMxj5v93xU+b4JxCqmcrALR33E9/qkvCmwdExHQBXAAwAWDV3fl5mZiOeMjIy0sTExMLCYs6cOb/88ouSktL27dvxBFFRUQCA4cOH44G3d+9eAMDFixcxyvT0dEVFxcePH0MIp06dOnbsWCwINVQqKiq5ubmoJwuKsbGxkpKSe/bsodFoEEI6nb5jxw4AQExMDBZ54sSJc+fO7d+/PxYZQrho0SIAwODBg+vr61HKmJgYDQ0NFIqmpqbz5s3DOEAIg4NPmpmZYVAsKIQDB6UC8GrUqPi1a28tWhgjIxMPQPTxY8iLpFVQpNFoQUFBc+fOdXNzu3HjBj7Rf5K7BTWpRdkRFh9mYhxmzSCiRWK1mYiT8M2bN93d3V1cXI4dO4bVOg5fjqwsP05Mpkd4eLm8fCwA8TNnxq9bd2vSpAQA4tXV38TG1aERiouLVVVVPT09a2trUZ9z584BAK5evcriyKz2v/zyi7a2Nr7Rd/DgQeZHllp+fj5KmZGRoaqq+uTJEwihk5PThAkTMA4Qwps3b/bp0wdrfrKg6OTkNHLkSDwdhHDbtm0YFCMjI7W0tN69e2dtbb1z506M0sXFZezYsaampmvXrkU9Y2JiNDU1USj279/f3d0dI4YQXrp0wdzcrKkRsYqNjU2zZucA8ObAgS+NyBsAuZKT601N0uTkYpOSkCZuCy8arc7JyQm/EnTFihWtQnILExIlGU8lEmVibeTNqdedI+3WrVvxpTxlypS6uuqW5+XjxwZd3XgFhfQHD1nAgxBeu1YlKZU2xCq1uhqplhs2rKdSqY2NXHby999/v3nzJppQcXExhUKJiopydnZ2c3PDUt+9e7e5ufmwYcOmTp2KemZkZPTq1evp06coFKdMmYIRQwgfPHhAoVC4oEij0QwMDAgmGB8HQujt7T1u3DgI4a5du/r3748JOn36dC8vr6dPn3bv3v3Vq1cQQjwULSwsbG1tsb1DIiIiFi9ePHDgQHoTHUKY+rYGgJeO0z4Q0nr6pBaAVG/vsro6WF0FqyphlaBfZlBDAzxy5AxaQmJiYlhRhYW9bKBxRxfEB+/fTHIYGdNRWYkwr6iArRm1IeQV/4joBLuqYHUx/WMZLG3HXVYGyz7Dz6X0UlHgprwcIhrAdIJ38/XEEzBVxz8ungzHp6EBht+LAUAcAMApYwAOHAiqb2CKgSPmy7m+Hv5+uBqAxMDAr5ieUcfGzZUAvL51G3n7jxgxfObMmQQC/GNQUJC+vj6E8Pbt27169fr06RMaumPHDltb27S0NBkZmdOnT0MI8VB0dnYeOHAgHgsbN26kUCiYCUWsYnl5ec+ePQMDA/Hp4d1oz+/169chhGVlZWpqanfu3EEJnJycXFxcUKwaGRnR6fSkpCTMKlpZWfXo0eN///vfGOY1btw4TU1NS0tLtIV9/nwdALF/nuJYP7TGNNRDA4NCScliY5O3xsZvjIwT2Dfq5vwaGycYGyeamb2Sk5uOIBBfRKCLsvJic/NXRsZJ7OgC+RhxpcImM+EkxJ/AKMHIKLFv3zcLFmTUcV6yeM210V1SW+J0aobhbiPzAwPM97fxHrDfwnz/gH77+lvsGfgg60EbRREQ7djRfAODN4aGmG7xumIrkFVw+CDU/UMCbjKTBCPjJHPzKBXVFQB0IZSzjOzk/mYvjY0T2aWMj8txGxu/MTFN6tHjg5JyUUYG11sPQvjyZS0AMdt3IBC1sbHx9vYWkG/E28bGZs+ePSiBgYEB5t65c+fAgQMhhEePHlVSUioqKiosLMSs4sKFC2VlZfFYMDAwUFdXLywsRFkhUKTRaIaGhv7+/oTkaTQaiplr164BAObPn79s2bLly5dLSUlNnz4dJXZycpo9ezaKZx0dnZ07d2ZnZ2tpaWEN1IULF+LZnjlzZsCAAU3MbpsLFxEonjzJ+sjEPg0b6qG+foG0dOnQoZ8sh+ZaWuU1dw8tsBmWpqTEgSIbj+IUyrLhIzIsrQqHDG2WQ/P8BYbmW1kV2NgUalALAUgMvVyCz2Yb3HjDdSblLHAH2ru1bQNsh/0xrI33keHD/xhhdcQKLAFTTzvSIOeV1wbx8FG+fYM6OjEAvLeyKrYSiW55y6tw+PB3GhprUCgiaGQ3fxSUptoMS7EaWsBdSQiPCMOh1h9UVEoUFQvT0ji9huhXzMuX9QDE7thRDiG0t7cndHBACBsaGlAsxMfHi4mJOTo6+vr6rlixQk1NDWskYlCk0+lDhw6dNWvWhw8fKBQK1kCdOHEiXo137twhNlAhhI6OjuPHj8fTfTevM2fORE2lg4ODtbX1kSNH/P39Dx065Ovr27t374KCArQFjEIRQnjv3r0ePXr4+fmZmJhgUFywYAGe7Z9//olBMTOzRkz85cSJaA8Nhyo8HGlFLF/+GQEnA9J/dEMIz569iLVLMcezZ0i/7g+jt4cAQpidzeghnzrCNoPZ4cXJRZtdNEizC7LvuUElsTKJARkNsKHNNw3SGmDD/OsLui7p+vcnrlGlNosHITx3vhqA2N92ITakJQXUHg1jcZm2K7ZLF8Qq4q+goDNYKTPokEHnX+LoR8SpU5UAxB3+vYyQ/VWrvgIQfecOYhU2btxoYWFBIFi+fPnq1ashhCtWrDA2Nj58+LC/v//Bgwd37dqlqKj4999/QwhRKKKIzcjIUFZW9vX11dPTe/bsGYRwxowZDg4OeLZhYWF8oPjs2TNxcfHz589jpMHBwQCABw8eFBcXKysrv3z5EguCEKqrq2/duvX799706dNnzZqFBSF7yAGgpaVVUoJYiX79+nFeMMzX/qlTp8zNzVGrSKcz3N3zAIjduLEY21wqMrJKWztNUTE2Pa0VbT46vdHd3R0roa5du+L7ljDxROTw9s4D4M2zp6wOt3amcj/ngYRPl8U3l7STDxb9RfGLrku6ul13xxteLLS1jsYmxhDLVOWeqR+KhGZmWy7D4cOHpaWlsYJeuGhhYyNBjOZyWVraZGSU2F32zZUriAFk9h3Ckye/iIm9sR2VUlWNdB6+f/9eXl4e33Xy8OFDAMDBgwfpdLqOjs6xY8fwAltaWk6ePBlC+Ntvv1lYWKBQhBAeO3YMACAnJ4dih7cH9a+//lJXV+fqtkH57tixQ1paevr06X5+flOnTpWVld2xY8f3983mzZt1dHTQIXtMAk9PT1VV1dra2mnTpmH9RRDCb9++6ejoSEhIoFDU09PDAxVCGBgYaGhoiELx+xfm55LG0aNTAIg3NYmaOy9i/IRoAGIkJZMvhRI/rLGkm3FcunSpe/fuWpqa6FuqGUrhBr2JrxcXT5rhlCUUtjPOz5D0lYou4QzVtpMtDTZMDJ6ktEop9VtaO1khbZ/7NQBEL13a3gZ5myWJiorS0tKSlZW9dOkCnolgCDIQ242YRYTkZWRlH/UEAGJGjYqaPz/C0jIKgFgjo+S0VE6X6cmTJxUUFL6PPWzdunXOnDldunRZvHgxhDAkJKR79+5FRUX4dIOCgsTExEpKSvbs2WNsbIxBEUJoa4vMIUNr46RJk0aPHo2PeO3aNSUlJWxokDWYgVI8f/7c09PTwcHB1dX1xYsXzOYH48KFC+Hh4XgWEMKsrKxdu3aVlZXduXPn9u3b+NDXr18fOXIEHfAJDg7GOnhQmoSEhJCQELy4NbVw955cGdllAPQGwH7ylMioaPbIBp5vC9wMBkNTU9Pe3r4FtEImmTgpt2vX+JS3PJKze//RisKA9OaHWBJK38gsl5162rERcmpG+2W9/u468ASbH29pNyv61Ck5XbvGJScJpwnQNnns7e01NTXxtahVfHJzGxe5JgAwCYA+Xbsu8l2eXFxM7MhJTk5euXLlpEmT5syZg40ohoeH41uOqAClpaX+/v75+flxcXGnT5/GS5Wbm7t//360YyYsLOzy5ct4OTMzM48ePfrt2zfUkwuKeLoOdo8di0x8U1XtRWtsexlXVlZqaGjwDpB2QF4i7n8DIN53OTZbkA1BYtqC391MypXhq4AnuJN7lxivfc+VsHLoIWvtbTpFdcgsKNbFkoUOIZ1pN9j+gv8nJtaJiUU7zSCOPwmOIZKQkSNHamhotG/DB5q+PjLxbai1nUhEbD1TIUORwbxaLwYcZTeSCUWVqqq2b0ncSVBEajSNBocMyVHqmVDE837lrw0UqtzAzK3NU9vUe0TAiGpGK4at+fPn8T0eHwg8wB+xAZwQBmoKBL01OISYa/GSjwBEP3qCfsZzS48Rid7RfijW1dXq6em2fJEU3taJKH9ChmKbpRTKesVOhCKE8PSZMgBi/A8Se+d4dMKs+vyguO/v/cAD/Bn/J08UIXh8qP+g69d38CHLykZswj2xVdZ8Mvn59QoK8SNG5tAaWxexebZtCG0/FHkXSbXihdQGiVsQ5aeBInFLYn5V9Uf56SQossQqL2foaGf27ZtUVdW8uUCyxlvwZY2lRruNDXYZlTSIqkfE7/E24AmuZlzjVSSvPLw0O3aWARB14QKGZF6SDvIRBhRr0I36sS2JW6IBkWaPhKIw1fvb7s8ARJ8//+M2Nm/Bn0s6BzzAb093C1Mgbl7plek916qMCx7P2yfEKw93VFj+jaalna5vkFpZiR8fJ1B10KMooNhBogtO5ueBoh1xo/7mTQu/LHWuVYQQFhQ2KCmnWFunNTbxSs/rw8lDDaPG9pidygbV7BrhjIhwWHO7vG74dFvS7XlRq4f7z5xFBsEPHmR193Fz7egnYUCRdXyNFblRP6H0hLLNVKdDEZmL6/MZgNcPH3Fm87Fz2hwU7+fdB95iS28vZRML7z8yosZJOrIkUmqZ9PyrXFOgfpgYrREOHpKrrPymuJjFisPxh5FFQCBEKGIN1E5fG/vzWEXmNlNq7drxrVOgSKiUSQm1XbokTHcqbH78EF8/m2DTjHMzpXxlYj7Fov4/bC7io7fKTYONk05Nll8tn/rlbcsjhkd8A+D1qtW4gZCWRxYBpUigKAI5W8XyJ4Ni+85XrKmpoVKptrZcWySg6hBd5SaqmwGnTvnQpUtsYiL/iXsE6EIIY0vjZJbLTDs3nY5MABD5FZZ1C3iBDfc3tDwlh4m5UlKv8fNRWh5XFJS2trYaGhrtOaCatwdVFHK2iue/CopVVdWdNcSPV/qDB9UAvF6yBJnO3hLbuCzMV8xb/O57IQ/r40XCu6voVUMPD9XwoxbXc03gwtPg3fFx1RIS8TNn5uA9O9eNWsWqKoGjr7zvO4LAJBQJCuE8tu1bcfee3dY2w6wsh1tZjRxqNXLwYBsAukpJ9hjKfLSyHGFlaTN+/Pjk5GROSqJ3NdDo1tZZysqJhYWEmcp80n5X/U5tQ68RR4fXQf5WlE+cdnudSjgFPMCR138QOKGTNQmeXl4FAMQ+f8779UsgFNVjWFjY9wXoVlbDrSxHoCUrKdkDgK6DB9sMtbK1shxpZTXczs4uLCys5RKQUBSoq7ZAkQFNTEwAAIYakqYa4iYa4qZUcUtDuUH6suhjf82uFAUJAMCJoCCBCQs/AHkjnz1bBUD07t2ESe3MKWbcKe59sRe4g9PJIhnW506K8/SJVqK3U898v/nXph/0iOYXNHaXSxkxMp3eEW1njoR4l6sbsuZGR028H7WLiYaYqYb4IH3ZIQbdTZiF3l9TQksVKeXm1/viGUIISSgSFMJ5bAMUGRCamlr0p4qXRA7/FkX5GqmB3K+Yv4ibUptofPngIASKJ4M5KYnchUCx4luTjk5y374Z377hR+HYX6zs9lNJ0yejXcZGe4zLaFxzdNjhIpR1x5OdwBOEvr3SfBo7dhYBEHv9egVfsg6QE0Lo6ubdXQJEXzaqjtf7Gsku6EiNL8hNqYnX+vuCmZQY8Fm8jCPkjyQjocjRFcHVBihCBjQ1HThEW4yeqA+TAUwAMJF9JzEd2d3vHdcDAASdPEVIrgMe9+37AkDU2bMEw8iV8p9Jp4E72PWSOKz/o4rExaRtD+8qs5TX9fxf0DjuhjH7ZcFkWl7OoFJTjYyTq6o7QCKB+XB181HoBt7dUoHp3TiljBV3hljKDXVZceCz2FcgC54AEoo8KmF7tA2K/UwHDtQWq4vXR0oojnnHAxgPGPFMd4bc7YBOg2J+Ib2HYrylVQ5zc2Ykn4TqXE2vGfHHyF4bemVXce3yTCBja0j4/73CvCUWd3n2AVljzr64oHgyuAyA+MOHWdsocYWxI3TAf1c3H/luIO2mKnwrCePYhYsWdxyAb8UTrpJQFF45tAGKDMjoZ2rBF4oILGMB7FQoft/vZ5lvMQBxEQ9YrTsCxm7l3ALeYNkd4rA+gUx4OiZyiv70WtJXat6l+QziWwKhrG+Agwdn9OyZXFLC+kzsTCh2bQMUm5OXtIrE2oA9txeKb9hWEXtZdh4UMSwlJTd16Ro/aQprSy8ssxDCJkifcX6m9DKZ2LI4vH9HuumwaXLIlB4r5JO/8OlhDg+vASBu3XrW7rr4KTsdKSTzW9FHHgdFiDZ5sIIWaBVJKLapoNoERchpoPJCMa7zrSKEcNr0bAmJxIQE3HpoJlKjS2Okl0nPPD+rY4b1BZXJraw7wAusv7uem4BBZ0AHh3fduiVkZHbaGAYmEtJAxUMRAyHqEAhFjAEfB2kV+SgF9fq3QvHx41oAYn18iMvevcMWA29wL4/r5B+B2hFZQC2jduiRoZTNGnl1LOuHJhUT2yAuHj9nzk8xrE9CUWTlz4/xvxWKNBq0HpYq1yMtP58zMJdR8U5trZrdH3a1DOSgHqxBy08xIvcLTggGbuBQ9O/4lNzc8gCIf/mSOZ2lc+VDBjNIq4gvHBG7/61QhBBeuFgOQOyu3ZztXrc/2w48wNmUs2ylclDK9um4/6X1nw1+MzDbZ/YVfkFTzc5myMm9GWWH7dPTccLwTYmEIl+1iMrzXwhFdq9BZSU0MEjW1Mr4Vo7Yl5L6T4a7DI13m5QySkWlzVby3fl8J3AHF9NYp5Ft2/4ZgJjrN5obEW1lCu0iJ6HYLvW1NvK/EIo4Fezfj8xZ+fMUMqUmOPEkcAP7I/fjwjvPyWx8ZlW+U1unNub42CZY+/ULpGgkGptk1dS2pWEqio5WEoodWj/+vVBEKnRRcZOiUvLQoTlfGxptjw3vvbFPdvX7DtUvJzEGstUiz+UT5gN8QFTFi/MhdABeBxxDJ+KhO/nyUIvGIyMjIz09HTuFF0uEhCKmio5wdBgUCwsL09PT8VnKycm5fPny7du3v37lapJFRUVdunTp/Xs+mHny5ImHh8eECRPQY0X4bcjJbp6yO2WW++YD8Nbj2C3JFTJr7iCnLzAvjIzt0RH/meaO2+ZFl0TLLJdyOus30LqAQkn/XILbEJmbklfA169f1+FO0qqtrY2IiLhw4QLhfOzCwsJLly49f/6czj21nE6nHzt2zMzMTFtbW19fv2/fvmvWrKmo4Mx6JaHIq3MR+nQMFMvLy21sbFatWoXl5Ny5c7q6uqNHjx41apSpqSl6vAGdTvf09DQyMhozZoyOjg7+SGcajebq6iovLz927NgNGzbMnTtXXl7e1NT07VsBi+LZ9Tj1baOs8gvg4NBra683ZfGYAMwOVDYRzlfkTu40m2Cjy2UX8XELAIjevInVf9MSGQICAqhU6ufP6OJMWFRUNHr0aHNz83Hjxunq6m7btg1tst69e1dfX3/06NEmJiYzZszAoMtgMHx9feXl5f38/P7++++YmJjff/9dVVXVwsKivJx1rAUJxZYUhNBoRArFk8HIEqTIyMjBgwcDALCTSbKzs+Xk5EJCQtAzz6dPn25lZQUhDA0NVVdXR8+gDAgI6NOnD3oECITwwIED0tLS6OnqaObfv39vxbyw86K5lMKq8ci/udvvgplKC6640dmmkk3JDQu2b0f8x6X8OC9ZTOeUgmJKetaPl1miJ22uXr1aTEyMSqViDYpZs2ZZWlpWVyOjII8fP5aTk4uKimIwGHp6eugRLEVFRdra2r//zho7uX//vpSU1KNHj/CZzcrK0tDQ2LKFdawACUW8ckTuFikUz5w9V1dXN2jQoG3btllaWm7YwNpLIj09ffv27djHyaVLl/r06VNXV+fo6IidRdfU1KSrq4saxtLSUk1NTd6DKMvKyqhUKnayAj9lIVV+2b0NwFlu0QGuc4j4EXegHw6Kr+MZYt2iKENu5VazTFzzcixbtszOzm7NmjUGBgZlZaxFXkePHsUfT9+3b9+wsLC4uLg+ffpgNOvWrbO2tkabqfPmzSMcdYYmGhQUZGpqitKQUGy+IIQcKlIoBp863dTUhJ4iMnLkyDVr1vCVfuLEiQ4ODnQ6fciQIQcPHkRpGAyGjY3N2rVrv89ie/v2rZqaWmZmJm/02bNnEw51JdBkVWRq7tQH48eo9L2a+x6HAAJdhz6iYrCE8fYoA2IPxX+Zfej1oZZIkZeXx2Awbt68SaVSS0v5DMzcvn1bVlY2Jyfn+PHj/fr1w1oN586d09fXR88ms7GxWb+eMO0OSTwzM9PIyAhtjJBQbElxCI1GpFDEr1ccNmwYXyhu2LBBQUEB7WkwMzM7d+4clrdff/0VNZJv375VV1fPyuKzVamrq6ujoyMWBWmBcsNt++OtkqvlJ22/BkDMbzu5FgpzYnW0C+1KRX7fZ9UryCVYOiSb/DbW3N/8SyPnc5E7H0QRL1y4oKmpyQvF1NRUKpW6cuVKCOHatWttbGywcY579+717dsX/Vy0tbX18vIiMoXwyZMnysrKHz4gEwZJKPLqR4Q+nQhFBoPh4+MjLy8fGRmJ5nDAgAHoccvIrDQGw97eHj1eLycnR0FB4datW7yKGDBggK8vtnSVWHs/NnzS2aE74NDAvG+1psa5VGrilzIcDc7Jy1lEPoQ0N20qBeBV+KOmoykHgQcIeYt8P6MXBiG2B9d/vlB8+PChgoLC0qWs9V9+fn4DBw7Ejui8fv26np4eCsW1a9cOGjSIiyPzYfPmzcbGxmQDlVczIvfpLCg2NTXNmDFDX18f3wVqa2uL72U1MTFBD5ql0WhWVlboEbN4jbx8+bJHjx6xsaxdTHk7RU/EBQF3cPClP4Tw4MFSAF4Hn8I66wmgwDMWuRtNu/RLA0Ujxdw8nd4Ic+uz1Db0HnXcrh62aE0GLxSvX78uJyfn5+eHSX/lyhVNTU1syGfv3r3m5uaNjch4SUJCgqSk5JkzyCHe2PXu3TslJaWAANahV6RVxDTTEY4Og6KNjQ3WQKXT6YsWLbK1tcWPYqGHqg8fPhzNdlpamrq6OrZn3KNHj7p164bvuYmLi1NXV1+0aJEgNVUzNzvstbl3QQ0yq7O4uEFFJcXCIrWB51hUQRxE7X/sxBcAoo8Hss6lWR62XMxbIiLnfkvSvXDhAv5b8d69e+rq6teucZ2Qk5+fT6FQsKOgHRwcli3j7ENz+PDh3r17BwcH19fXf/9QDw8P79ev3+zZs7HU0Q01sFX8rO0asKVS5CIpTFNCcXQYFAcMGIA1nG7cuAEAGD9+vK+vr6ur66JFizZt2tTQ0FBUVGRhYTF//vzDhw/379/f3d0dn8eTJ08qKSn9+uuv1dXVJ06ckJKSmjJlCtYNi6dE3bcyw4AnWB2OdPyg18qVJQC8vn0bt4iRHdRR/zl7z9XWQPMB7/tQEkvZbeaYkhjp5dJO52fwXd1PkPDMmTNKSkrouGJlZaWxsbGcnJyvr6+Hhweq0pSUlO97Aqxbt05HR+fgwYNLliwxMjLCvrfLy8tDQkKGDBkCAAgODo6NjQUAKCoq7tmzB6MhrSJB56J9bCsUBw3UFqtF97YhLO7GLx0O4mwzFRgYePcua/PfZ8+erVy50tfX19PT08vL6zvktm7din7D5ObmLl68eENKwgUAABdXSURBVO7cuf7+/mhTCs1/SEjI2LFjtbS0unfvXlxc7O3tLSkpaWtrO3v2bKyzHq+pRtg4KXhi91U9ksqSMP+3qbWSUnETJrxnsA4bxUI6wRH211cAXm/dyukCRQ4OCJkhs0Im7jO2w4DAc8vj4+P37t2LDiR+/vx5y5Ytq1ev9vb29vLy8vT0dHd3R1v+DAbj0KFDLi4u3t7e+NlOHz9+XL169YoVK1xdXZ8+ffru3TsvL6/Vq1cvXLgQs6ICoRjL3LqBaRW7i4HFPti3OqHLjI9WyaXDfJSCerUViuxtpgSu4u8r3B3fPn36lJCQkJGRkZOT09jY+Pnz58zMzJSUlNTUVBqN0+LEOlAjP73qurSbS+gcQo/qrFlFAETHxHSiYWStlBwzJk+uR+z791wTUyPyIoA38L3LqdzEkuvAL1wcFKW4W6diyKOgBmqzEpJQJBYo9iwyKHbajm9o1jyue0gs7vK44DGWU9Tx7FktAK/d3Fq0VT4hrhAfo6IqAIh3dWNvYMNmXcOoHnXMTm1jr5xa7oX82DumA5c846CI7PhGZzV/mDhsBorsvPD9T0KRr1oQT+FC8WfYfBFCmFaRqrxG2T5wdAMkTiWj0+n2o/NlZePfv29RR6VAxbUvYMHCQnHx2JgYjj3H+J1POg88wd5nezEfbgcelNwhwn4iQJGBddgQ97bBuoKaNYhM8UgoCiylNkARMpBtpgZri9XH60PuBupPAkW/x37AHVx4y1qSS8h86OUaAF5v3tKiWWaEuEJ5zH7fKCWdPH58BvdKCRbvL41lJruNDXcZlNI4n5G4dDsUigpdQfpNVZiCWEXizWmgcqD4QyySUMQVJbezTVBkmJpaWOqIwWR9mAJgEs+dI3cvUB/3rfjDAuKWqX1PH2gfdPx0zfcPKKdhQ4hcHKurm4yMUimU1LIyos3kohPZw8aNBQDEhIcLPI9p3997gSc48wbb+IMpSsdhkJVzdDAj+7YqzJDkU8qZ4m9vUgi7gzNLujlBSSgKrFZtgCIDMkxNzU0p4jmPhhU9oRY8ohY80ix8pFnAvAsfaZRGmZ7ZbcE8vgY9M6NDoXgs7jhwAwe55nMSBfj98DcAogNPcGaZCVSQsANKyxiqqskDB72trWNLxVN1c2tz1DapjTw6qoa5HRZLBB4yYYtG5LfI1UtWAjw9a1TywqDwMbOUH2sWPtYseEQtfEwtealz/5SZpBjw8SFs7tycoCQUiVrGntsERdi/X//vSOstDyjMm6oIjCgyur26UuSBugKgKgL5bgAAcPLkSSyhjnF8Y1QM8bfS3KJZUIPfjxjtpWRXfQg/fmSoqiWam7+vr+d4il5CJK2jx4oBiDkZjGt88qu6y8NXAB9wLy9C9FIJTMHDwxMAoCILqApAnVmyBhRpgz5SGsxC11AESjJIKS9ZskQgC54AEoo8KmF7tAGKEMLTp09PnjLVwWGKg4PjpInT7Eb973tztE9vzUkTp010cHRwmPrrr5MWLlzIdyU+O2WR/L+SeQUZ1r/PWq3PjTOuMbr165E9ncJusZbJikQaLqYI4GprGf36pWlS3375iojGD4OsOG9K38islHU8M60R4o/E4uIo6ocXL17MmDHj118nOzhMnTjR0WGio7iYpIyU/GRWKTs6OEyZMWMGfnHWD0UioShQRW2DIoFdQWGhlJQUYXIMgaYDHmmwweGUg9zKHm++vPlhcmnpTdLS8WPH5mKUONw2gxGMXKADx4dIc/OvCgDidmzHW2wiDfrMgIyZF2ZJLpN6XRKD+BAkaiYN/vyE40uhUAbym0Tecu4kFAXqSihQzM7OlpKScnV1FZiMSAPY9fL5x+ddlnSde3lec5vw4+r0bJcccfG4qFesKaB4GXFUeO/WuNlSYXGa6HD06HQZ2eS8fHQDGy4rjZFhjvs59yW8xZfcJHyJYeGd4FBXV7ewsGhPwiQUBWpPKFDMysqSkpJqZma2wOTbG8CZqsKAjAXXFokvEX9cxLVJBDEFHMhevGwAIG7BAs5W+Uz4/AAhRIbEZ6ZIRBwiz69e1QMQ5+HJscPEqNzPDbBhVOAo1fVq76rfcYd02hMJRRGq/p8JRc5GhlidTypPUVytNP7kBM4iI1YYgg2MDK/KpiZoZ/9OWiYlK5vPUDueUijuOXPei4snxL8ROIaBpoJfpnguNQQ5kvU550hWvhkRingtYUJCsSVaaiPNPxOKWGY5VnHD4w3AE1xKv4SF4RwCDd2NG9UAxG/eLLR5cByocFyIIJmZTTKycRMc+OwoiZOT88pAY39t/NJvXz+D7YafGku4yBjc3PFhonSTUBShdv/hUGTVyKLaIu1t2gP2DfxKR4cK8aYF2RFAgF2E1dUM036pvfuklZWKtnKvX/8JgJh799AOW/QNwucFwSvE/sj9wB0EJ6AjtMyawEskwgrCxZqEIpc6hPvwj4YiViePxBwB7uBw9JE2KCcgoBSA+ICjwpwHhwmGylPymdGrV/yggdk0dHoP69VAoGLJTvDNr87rvVnd5vCwasYPWrZtyHtro5BQbK3GWkH/z4UiZukqGysH+g/U9NPKbyhoRc7ZhrLkE72XWqqZWXqt8BdOsWD1R0A5ANFnziAWG/Fi//GVlgBFCOGKeyuBFwjLCeNL35GeJBRFqG0MihWVlW1OplN6ULEqezX9KvAAmx5saq38GIcN6wsAiPvrL9yoBhbWWqZc9AiXmhqGkXGyjk56+TdkDKMlK/S5eEAY/yW++/LujiHTGmFHdC8RUud8v0L4Ayg2pzTWm/MfPJjB/dFD1FL7nzEoYpsRsa1FK3h3BhRZxd4IG34JHqewWiH5G59z7VuYh6wsmqRU0pixWY3ozBbM4LYwPg8Zvk5euVoLwCvcvo/4QJ6Y/DwYkDHn0lzJpZKvSl7xCxexH07e9kOxrrZOXw9ZKmBpaSliuVvKHvAl/PDhQ0RERHh4eF4en/Muc3JyUlNT8RHLysoSExPxPk1NTcnJyeiWstnZ2WlpaWgoTp94cmg/2h4AoNpTtb6hjh3ApzuBHcR/XIANxYU4MpE6OYMZTwufdl3SZf6l+YIy2EI55i/IBeBNZCTzY64dvDhRmS46g243OkdBISE/D1NvCyXiIntQ8EDMR9zrJp+dS7noRPzADUVOXgUky+eVxqDT9fSQZeVWQ5GjGZCLhyouLu7OnT***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
终端电阻的作用
CAN总线终端电阻的作用有3个:
1、提高抗干扰能力,让高频低能量的信号迅速走掉
2、确保总线快速进入隐性状态,让寄生电容的能量更快走掉;
3、提高信号质量,放置在总线的两端,让反射能量降低。
一、提高抗干扰能力
CAN总线有“显性”和“隐性”两种状态,“显性”代表“0”,“隐性”代表“1”,由CAN收发器决定。下图是一个CAN收发器的典型内部结构图,CANH、CANL连接总线。
总线显性时,收发器内部Q1、Q2导通,CANH、CANL之间产生压差;隐性时,Q1、Q2截止,CANH、CANL处于无源状态,压差为0。
总线若无负载,隐性时差分电阻阻值很大,内部的MOS管属于高阻态,外部的干扰只需要极小的能量即可令总线进入显性(一般的收发器显性门限最小电压仅500mV)。这个时候如果有差模干扰过来,总线上就会有明显的波动,而这些波动没有地方能够吸收掉他们,就会在总线上创造一个显性位出来。所以为提升总线隐性时的抗干扰能力,可以增加一个差分负载电阻,且阻值尽可能小,以杜绝大部分噪声能量的影响。然而,为了避免需要过大的电流总线才能进入显性,阻值也不能过小。 二、确保快速进入隐性状态
在显性状态期间,总线的寄生电容会被充电,而在恢复到隐性状态时,这些电容需要放电。如果CANH、CANL之间没有放置任何阻性负载,电容只能通过收发器内部的差分电阻放电,这个阻抗是比较大的,按照RC滤波电路的特性,放电时间就会明显比较长。我们在收发器的CANH、CANL之间加入一个220PF的电容进行模拟试验,位速率为500kbit/s,波形如图,这个波形的下降沿就是比较长的状态。
为了让总线寄生电容快速放电,确保总线快速进入隐性状态,需要在CANH、CANL之间放置一个负载电阻。增加一个60Ω的电阻后,波形如图,从图中看出,显性恢复到隐性的时间缩减到128nS,与显性建立时间相当。
三、提高信号质量
信号在较高的转换速率情况下,信号边沿能量遇到阻抗不匹配时,会产生信号反射;传输线缆横截面的几何结构发生变化,线缆的特征阻抗会随之变化,也会造成反射。
能量发生反射时,导致反射的波形与原来的波形进行叠加,就会产生振铃。
在总线线缆的末端,阻抗急剧变化导致信号边沿能量反射,总线信号上会产生振铃,若振铃幅度过大,就会影响通信质量。在线缆末端增加一个与线缆特征阻抗一致的终端电阻,可以将这部分能量吸收,避免振铃的产生。
别人进行了一个模拟试验(图片都是我抄过来的),位速率为1Mbit/s,收发器CANH、CANL接一根10m左右的双绞线,收发器端接120Ω电阻保证隐性转换时间,末端不加负载。末端信号波形如图所示,信号上升沿出现了振铃。
若双绞线末端增加一个120Ω的电阻,末端信号波形明显改善,振铃消失。
一般在直线型拓扑中,线缆两端即是发送端,也是接收端,故线缆两端需各加一个终端电阻。
而在实际应用过程中,CAN总线一般都不是完美的总线式的设计,很多时候是总线型和星型的混合结构,这个时候一般都将CAN终端电阻布置在线束最远的两端,来尽量的模拟CAN总线的标准结构。 为什么选120Ω?
什么是阻抗?在电学中,常把对电路中电流所起的阻碍作用叫做阻抗。阻抗单位为欧姆,常用Z表示,是一个复数Z= R+i( ωL–1/(ωC))。具体说来阻抗可分为两个部分,电阻(实部)和电抗(虚部)。其中电抗又包括容抗和感抗,由电容引起的电流阻碍称为容抗,由电感引起的电流阻碍称为感抗。这里的阻抗是指Z的模。
任何一根线缆的特征阻抗都可以通过实验的方式得出。线缆的一端接方波发生器,另一端接一个可调电阻,并通过示波器观察电阻上的波形。调整电阻阻值的大小,直到电阻上的信号是一个良好的无振铃的方波,此时的电阻值可以认为与线缆的特征阻抗一致。
采用两根汽车使用的典型线缆,将它们扭制成双绞线,就可根据上述方法得到特征阻抗大约为120Ω,这也是CAN标准推荐的终端电阻阻值,所以这个120Ω是测出来的,不是算出来的,都是根据实际的线束特性进行计算得到的。当然在ISO 11898-2这个标准里面也是有定义的。
为什么功率还要选0.25W?
这个就要结合一些故障状态也计算,汽车ECU的所有接口都需要考虑短路到电源和短路到地的情况,所以我们也需要考虑CAN总线的节点短路到电源的情况根据标准需要考虑短路到18V的情况,假设CANH短路到18V,电流会通过终端电阻流到CANL上,而CANL内部由于限流的原因,最大注入电流为50mA(TJA1145的规格书上标注)这时候120Ω电阻的功率就是50mA*50mA*120Ω=0.3W。考虑到高温情况下的降额,终端电阻的功率就是0.5W。 不仅仅是CAN的总线是120R,其实485也是这样,但是这个终端电阻不是随便加的,得根据现场调试来决定要不要加
页:
[1]