Table 10 - Setup Register
BIT | BIT NAME | SYMBOL |
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| DESCRIPTION |
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7 | Pulse1 Mode | P1MODE | This bit | determines the type of PULSE1 output driver used in |
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| Backplane Mode. When high, a push/pull output is used. When |
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| low, an open drain output is used. The default is open drain. |
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6 | Four NACKS | FOUR | This bit, when set, will cause the EXNACK bit in the Diagnostic |
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| NACKS | Status Register to set after four NACKs to Free Buffer Enquiry are |
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| detected by the COM20020. This bit, when reset, will set the |
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| EXNACK bit after 128 NACKs to Free Buffer Enquiry. The default |
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| is 128. |
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5 | ET3 | ET3 | This bit, | when set, scales down protocol timeout values of |
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| Response Time and Idle Time but not Reconfiguration Time to |
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| optimize network performance in short topologies. Provides a |
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| scaling factor of ÷ 3. Defaults to a zero. Must be reset to be |
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| ARCNET compliant. |
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4 | Receive All | RCVALL | This bit, when set, allows the COM20020 to receive all valid data |
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| packets on the network, regardless of their destination ID. This |
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| mode can be used to implement a network monitor with the |
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| transmitter on- or |
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| received with a destination ID equal to the COM20020's |
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| programmed node ID. This feature can be used to put the |
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| COM20020 in a |
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| and the COM20020 is not passing tokens. Defaults low. |
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3,2,1 | Clock Prescaler Bits | CKP3,2,1 | These bits are used to determine the data rate of the COM20020. |
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| 3,2,1 |
| The following table is for a 20MHz crystal: |
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| CKP3 | CKP2 | CKP1 | DIVISOR | SPEED |
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| 0 | 0 | 0 | 8 | 2.5Mbs |
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| 0 | 0 | 1 | 16 | 1.25Mbs |
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| 0 | 1 | 0 | 32 | 625Kbs |
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| 0 | 1 | 1 | 64 | 312.5Kbs |
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| 1 | 0 | 0 | 128 | 156.25Kbs |
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| 1 | 0 | 1 | 256 | Reserved |
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| 1 | 1 | 0 |
| Reserved |
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| 1 | 1 | 1 |
| Reserved |
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| NOTE: The lowest data rate achievable by the COM20020 is |
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| 156.25Kbs. A divide by 256 is provided for those systems that use |
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| faster clock speeds. Defaults to 000 or 2.5Mbs. |
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0 | Slow Arbitration Select |
| This bit, when set, will divide the arbitration clock by 2. Memory |
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| cycle times will increase when slow arbitration is selected. |
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| NOTE: For clock speeds greater than 20MHz, SLOWARB must be |
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| set. Defaults to low. |
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30
