any experience with SCPI and chinese dc/programmable loads? - Page 1
Forum section: electronics / testgear

author: linux-works
author_rank: Super Contributor
timestamp: December 23, 2013, 04:39:30 pm
content: I'm trying to decide which dc prog load to buy. one constraint is that I have to be able to write my own code to talk to its serial port, to control it.I am looking at the AT8511 from applent and also the M9711(etc) series from maynuo.the maynuo is not real rs232, its ttl and needs an active opto cable which is $60 or more and does not seem to be included in any of the ebay sales I have been browsing. it seems to use a binary (not ascii/scpi) protocol but the protocol is documented in the user guide (good).the applent looks like it has a nicer screen and logical button layout and it claims to be scpi-based and also real rs232 port on the back (no need for a cable). I was able to get a chinese language document on the applent (which has enough english for me to follow, I think) although its annoying I had to ask for it and its nowhere on the vendor's site and not included in the box, either.I'd like to hear of any real world experience in remote programming of these $500 and less chinese loads. I don't care at all about windows programs - I will be controlling via linux and/or arduino serial.scpi is familiar to me and I like it. but I will use the binary protocol if its better or more powerful or faster (I'd like to be able to send quick changes to the load over serial) or just less buggy.any experience or advice? any users of the applent unit?I'm looking to buy very soon (next few days or so).TIA/bryan

author: MarkL
author_rank: Unknown
timestamp: December 23, 2013, 09:15:54 pm
content: I've been using the Array 3723A (30A, 200V, 350W max):http://www.array.sh/yq-3721e.htmI've been using it through the optional GPIB interface, but I've also tested the built-in RS232 port and it works fine too (both port types speak SCPI). The RS232 port has real RS232 signal levels and is isolated, so there's no need for an external dongle/adapter. You can also get a USB port option, but I haven't tried it.I've been using it exclusively with Linux, so I know where you're coming from with non-standard, binary protocols to control instruments. There's really no excuse for that in modern equipment, IMO.I purchased mine here:http://www.circuitspecialists.com/array-3723a-electronic-load.htmlbut it is available from other distributors. Circuit Specialists provides good customer support and they will deal with Array on your behalf if there's any problems.The list price is US$769, which is a little more than your $500 target, but it's well made and works perfectly. I'd highly recommend it for anyone looking for a quality DC load.I had a B&K 8500 for a while, and even started writing a driver for their binary protocol, but it went back because it was too buggy. I was quite happy to find the Array unit after that experience. Array has some other models with different ratings depending on your testing requirements.

author: linux-works
author_rank: Super Contributor
timestamp: December 23, 2013, 10:23:22 pm
content: thanks for the reply, mark.how did you know that the rs232 port is isolated? I can't find anywhere that says that and can't find a schematic, either.what did you use for a programming reference? the user guide does have some scpi commands inline in the manual, but they refer to a different manual that is nowhere online or findable. I will call array to try to get it, but if you found the scpi command ref for this box, I'd love to get a copy.there are a few places that sell this model series and they are in the US, which is great for returns or calib/warranty work. its a huge buzz-kill to have a DOA unit and have to reship back to china; either now or later. I'll pay more just to deal with a US distributor and not hassle with expensive international shipping. (this is one thing the chinese need to learn to really be able to break into the world market; they need LOCAL distributors so that people like me will have less fear, re: future service).

author: MarkL
author_rank: Unknown
timestamp: December 24, 2013, 12:23:54 am
content: Quote from: linux-works on December 23, 2013, 10:23:22 pmNo, problem! Several reasons: I looked at it and I can see the isolators followed by a MAX202 driver, I measured the resistance from the front terminals to the pins on the RS232 port (readings are in the Mohms), I applied 70V to the front terminals and did not see it on the RS232 pins, and finally the RS232 ground pin (pin 5) is connected to the instrument case. But you're right, they *should* say this in their datasheet somewhere since a lot of their competition doesn't do it.I don't have time to do a tear down, but I did take some pictures of it below. You can see the isolators in one of the photos. You can't see the RS232 port on the inside shot, but it's under the power entry connector. That little board up top near the back is the optional GPIB.QuoteI had to ask Circuit Specialists for the "SCPI Programming Guide" and they in turn got it from Array. Most of the commands, though, can be gleaned from the user manual since they embed the commands with the description of front panel operations. If you want, shoot me a PM and I'll mail you what they sent me. It's in .doc format.QuoteI agree in general, and in this particular case I'm hoping Circuit Specialists will do a good job. I can't claim I've stressed them with any really awful technical issues with their products, but so far they've done fine on this unit and on other things I've bought from them in the past.

author: linux-works
author_rank: Super Contributor
timestamp: December 24, 2013, 12:29:43 am
content: well, I bought the 3720A. it was a reasonable price and I don't need high voltage or power, so this model should do me.I asked the seller for a copy of the scpi ref, but if you don't mind sending me a copy, that would give me an advanced start on writing some code. I just want to collect time/value pairs to send to some other process (another group, actually). but doing some setup and verification is also part of the task, so having a complete guide to the command set would be useful.

author: metalphreak
author_rank: Frequent Contributor
timestamp: May 03, 2014, 04:59:12 pm
content: Digging up an old thread here, but I'm starting a python project to communicate with my Maynuo M9712C.The manual section on communications makes it look rather complicated, but in actual fact it's just implementing the Modbus protocol. Something like minimalmodbus in python will handle all the CRC stuff and the frame formatting.Mine also came with a USB to TTL COM cable, but wasn't shown in the listing. I just made sure to ask the seller if it was included in the price before buying.