Wichit Sirichote, firstname.lastname@example.org
Build a simple line driver/reciever that meets RS422A for the MiniLogger V1.0. Place a distant Minilogger and communicate over a full duplex serial line.
Since the Minilogger was installed at Korat for insolation recording, I have to retrieve the record every 15 days by using my notebook through the use of RS232 cable. One day I found a couple of 75179, 8-pin line driver in the shelf. I got the idea to provide the RS422 for the Minilogger and use my desktop to retrieve data instead. Because the Minilogger was placed near the window, about 10 meters from my desktop. Figure 1 shows a simplified diagram. The Minilogger already has RS232 port. I thought this version may not be used as a portable logger. So I cut the RS232 port and built the TTL-to-RS422 converter. At the desktop having RS232 port, so I built the RS422-to-RS232 converter. The cable was 4-wire phone cable having both end with RJ-11.
Figure 1: Simplified Diagram of RS422 for MiniLogger
Here is a complete schematic Rs422.pdf. The circuit for TTL-to-RS422 converter is very simple. The SN75179 has two drivers independently. Pin 3 is tied to TxD and pin2 to RxD pin. The left hand is TTL level while the right hand is RS422 level. The output is differential mode driving, so it has two wires for each driver. I used phone jack for cable connecting. The R1 is terminating resistor tied to the receiver. Figure 2 shows a sample board I have made resided in the Minilogger box.
Figure 1: The 75179 board, I used 330 Ohms for terminating resistor.
The second board is the converter that converts RS422 to RS232 for PC's COM port. As shown in the schematic, the 75179 first converts RS422 to TTL level, so we need to change the level to RS232 again, we can use MAX232 or DS275. The converter board needs +5V, I modified the +5V connector to use available PC supply for floppy drive. Figure 2 shows a sample of the RS422 to RS232 converter. The unit can plug into COM1 directly.
Figure 2: The RS422 to RS232 converter.
NOTE: JP1 and JP3 is phone jack. Notice that the pin designation for both connector is the same wiring, i.e. pin 1, 2, 3, 4 are tied to pin 6, 5, 8, 7 of 75179 respectively. However, the actual signal that tied both end is shown in the upper right hand block. So we have to use a cable that has wiring as shown below table. Fortunately, I brought the ready made 10m phone cable, it has such wiring already.
pin pin 1 4 2 3 3 2 4 1
If you need longer cable and may need to put the header at both end, be carefully put the right position before clipping the header to cable. Figure 3 shows a color wiring for both end header.
Figure 3: Sample wiring for header ( teeth side), see color wire placement.
I tested connection between my desktop and Minilogger with 10m cable at 9600bps, it worked fine. The circuit can be adapted for another device, if both are RS232, then you can build the R232-to-RS422 shown in Figure 2 two boards. Notice that there is no isolation between PC and Minilogger. For home use it works fine. However, if your device has installed in the factory, better add the optoisolator to provide galvanic isolation between PC and the device. See exemplary design of isolated RS422 for PC.
14 October 2545