Design Solution 18
Convert A Two Channel Full Duplex Data System
from RS-232 to RS-485
Date:
Designed by:
March 31, 2006
Mike DeLurio (mdelurio@sipex.com)
Part Numbers:
SP3220EB/EU and SP3071E/7E (VCC=3.3V systems)
SP3220EB/EU and SP3081E/7E (VCC=5.0V systems)
Application Description:
Converts a full duplex serial data communications
bus from RS-232 protocol to RS-485 protocol at data rates of 250kbps and
1Mbps.
Electrical Requirements:
Input Voltage
Input Current
ESD Protection
Circuit Description:
This RS-232 to RS-485 protocol converter uses two products for a complete
operational system. The circuit will comply with all EIA and ITU specifications for
RS-232 to RS-485/RS422 conversion. This is a rugged design that will handle
+/-15kV ESD levels on the bus pins. The design is compatible with 3.3V or 5V
systems requiring only one pin compatible device change. Only six external
capacitors are required, giving the design a small footprint. The SP3220 has
receiver enable control (/EN) and driver shutdown control (/SHUTDOWN) if
power saving is required. (Schematic figure 1)
The SP3070/6 (14 pin package) can be substituted for the SP3071/7 to also
shutdown the RS-485 transceiver. This provides power savings and extended
battery life for low power systems. (Schematic figure 2) /SHUTDOWN=low and
/EN=high
Part Numbers:
SP3220EB/EU and SP3070E/6E (VCC=3.3V systems)
SP3220EB/EU and SP3080E/6E (VCC=5.0V systems)
Table 1- Truth Table
3.0V – 3.6V or
4.5V - 5.5V
20mA fully loaded and switching
+/-15kV Airgap Discharge Method
/SHDN
0
0
1
1
SP3220
/EN
TXout
0
Tri-state
1
Tri-state
0
Active
1
Active
RXout
Active
Tri-state
Active
Tri-state
DE
0
0
1
1
SP3070
/RE
TXout
0
Tri-state
1
Tri-state
0
Active
1
Active
RXout
Active
Tri-state
Active
Tri-state
System
Mode
Sleep
Shutdown
Active - Full Duplex
Forbidden
May24-06 Rev C
SP3220 & SP3070 Full Duplex Serial Communication Protocol Conversion
Page 1 of 3
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