10. RS485 SYSTEM TESTING & RESULTS
The plan is to periodically report System testing and results in this Part 10 as replies. All tests have been carried with the coaxial cable simulator set at 3300 ft.
10.1 SLAVE BAREFOOT WITH EXT. PSU AND NO USB CONNECTED PC
One thing is to test this RS485 COMM System in the lab with Master and Slave units a foot from each other, another thing is to have it working reliably in the field. Last week I was worried at the idea of testing with an external supply connected to the Slave and in fact I popped one Modem in the blink of an eye, because I had not joined the modems with the ground conductor. System use with external supplies poses several problems which we will analyse in this section.
RECOMMENDATIONS FOR NOT BLOWING THE MODEMS
The modems are based on the Half Duplex MAX485 chip and I have uploaded the System schematic, from the Modem point of view. The MAX485 chip is a differential & balanced type communicator, but unfortunately it is not galvanically decoupled from the plus and minus supply terminals. Furthermore the chip data sheet does not provide any clue on stray voltage handling capability of the device, thus great care must be used in this respect, considering we are using a high sensitivity semiconductor chip. With reference to the schematic, as far as stray danger potentials are concerned, signal terminals A & B are at the same potential, being terminated by an internal 120 Ohm resistor. I advise adding a second 120 Ohm resistor across the terminals of each modem. Therefore there is no danger of damaging stray voltages between A and B.
The danger is represented by voltage differentials between the Master and the Slave grounds, which will make a relatively large current flow beween the ground pin of one Modem and the ground pin of the other while they are joined together by the twisted pair signal line. A transient capacitive current can also flow at switch-on when using an external supply for the Slave. These problems can be solved by using a third conductor joining the ground pin of each Modem and the common terminal of the PSU, but only if this common terminal does not reach external ground. This conductor will keep Master and Slave at the same ground potential. Chip failure modes are of two types: just blow and become totally inoperative or, much more insidious, the receiver portion will de-sensitise and all of a sudden OOS warnings will start to appear erratically.
Modems must be handled with care. They come in an anti-static envelope and must be kept there before use. They should be mounted on sockets, so there is no risk of damage with soldering and/or statics from the soldering iron and test/replacemen becomes quick and easy. When placing them on the Master or Slave assembly, the assemblies must be disconnected from USB lines, signal lines and external supplies, if used: this minimizes the danger of statics.
EXTERNAL SLAVE SUPPLY
Once the sketch has been uploaded thru the USB port, ideally the Slave should work barefoot. This mode of operation requires an independent + 9V supply. Several different switch mode supplies were tested for this job and with all of them the OOS count climbed up from zero to significant levels. Switch modes are noisy and the enclosed Picoscope Screen shows what to expect: noise peaks exceeding 5 V at the output. That the System could still work in these conditions proves the quality of the Modem noise rejection, but obviously a small linear PSU is the solution, considering that the Slave unit draws less than 90 mA @ 9V, or less than 1 W. I have enclosed PC screens and Photos. Will produce guidance for a miniature linear supply in next post.
With the already downloaded sketches, sketch_MASTER_485_7_5_b and sketch_SLAVE_485_7_2, no provision is made for temporary shutdown of mains at the Slave. This problem & its solution will be discussed in one of the next posts.