5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
Datasheet
media (not on each individual node). The nPULSE1 signal, in this mode, is an open drain or push/pull
driver and is used to directly drive the media. It issues a 200nS negative pulse to transmit a logic "1". Note
that when used in the open-drain mode, the COM20020ID does not have a fail/safe input on the RXIN pin.
The nPULSE1 signal actually contains a weak pull-up resistor. This pull-up should not take the place of
the resistor required on the media for open drain mode.
RT
RT
+VCC
+VCC
+VCC
RBIAS
RBIAS
RBIAS
75176B or
Equiv.
COM20020ID
COM20020ID
COM20020ID
Figure 5.4 - COM20020ID Network Using RS-485 Differential Transceivers
1
1
0
20MHZ
CLOCK
(FOR REF.
ONLY)
100ns
nPULSE1
100ns
nPULSE2
200ns
DIPULSE
RXIN
400ns
Figure 5.5 – Dipulse Waveform for Data of 1-1-0
In typical applications, the serial backplane is terminated at both ends and a bias is provided by the
external pull-up resistor.
The RXIN signal is directly connected to the cable via an internal Schmitt trigger. A negative pulse on this
input indicates a logic "1". Lack of pulse indicates a logic "0". For typical single-ended backplane
applications, RXIN is connected to nPULSE1 to make the serial backplane data line. A ground line (from
the coax or twisted pair) should run in parallel with the signal. For applications requiring different treatment
Revision 12-05-06
Page 22
SMSC COM20020I Rev D
DATASHEET
SMSC [ SMSC CORPORATION ]
