Serial transceiver

This transceiver electrically isolates the TxD and RxD lines from the PC, provides an isolated power supply and also looks after the bi-directional conversion between TTL levels and symmetric RS232 levels.Frequently, a microcontroller system should or must have an electrically isolated connection to the RS232 interface of the PC. This can be necessary when the target system works at a completely different voltage level, or when earth loops must be avoided.The circuit described here can isolate potentials up to 2500 Veff or 3000 Vdc, depending on the type of optocoupler used. It uses only relatively easily obtained components, and it does not include any homemade inductors. Power for the circuit is provided by a mains adapter. A Maxim MAX232 interface driver is incorporated into the circuit to convert signals to the PC from TTL to RS232 levels (±12 V), and signals to the microcontroller system from RS232 to TTL levels.

Resistors:
R1 = 100kΩ
R2,R5 = 470Ω
R3,R4 = 1kΩ
R6 = 680Ω
Capacitors:
C1...C4 = 1µF 63V radial
C5,C7,C8,C10,C11,C12 = 100nF
ceramic
C6,C9,C13 = 10µF 63V radial
Semiconductors:
D1 = LED, high efficiency
D2 = 1N4002
IC1,IC2 = 74HC00
IC3 = MAX232
IC4,IC5 = 6N137
IC6 = NMV0505SA (Newport, Farnell)
IC7 = 7805
Miscellaneous:
K1 = 9-way sub-D socket, PCB mount, angled pins