Hi
I have ordered an Array 3721A electronic load. While, theoretically, the manufacturer lists an USB comms option for 372x series, I don't think it's available. Everyone seems to be using these loads either with optional GPIB expansion or serial to USB converters. From the
https://www.eevblog.com/forum/testgear/any-experience-with-scpi-and-chinese-dcprogrammable-loads/msg352717/#msg352717 post and thanks to MarkL's investigation, we learn that 372x implements "real" isolated RS232 using MAX202 transceiver.
First of, that's how I understand RS232 vs. "TTL serial" (please correct me if I'm wrong):
RS232: +3V to +15V for logic "0"; -15V to -3V for logic "1"
TTL serial: 0V for logic "0"; VCC (be it 3.3V, 5V, etc.) for logic "1"
Now, searching for a USB to RS232 converter for controlling my future DC load with my laptop, I came across countless similar-looking adapters with converter circuitry hidden in the DB9 plug and a short USB-A lead sticking out; fair enough. But pretty much all manufacturers/resellers, if they elaborate on the converter chip at all, almost invariably mention some variant of FTDI232. I even managed to find some pictures of the insides of one of these gizmos and, sure enough, there was lone FTDI232, few passives and maybe an LED or two if they were generous. No MAX232 or equivalent voltage level shifter to be seen. How is that supposed to work? How an FTDI232 alone, being a "TTL serial"/UART to USB converter is coping with high, bipolar voltage levels allowed within RS232 standard, let alone driving them into serial line? Do they rely on some crude diode clamping on the input for receiving? Is there some non-written agreement/expectation that "driving" 0V on TXD still counts as "negative" (i.e. logic "1") on the receiver side (despite not fitting in the standard-defined voltage window)? I'm confused, that doesn't seem right at all. I use FTDI-based gizmo for interfacing UART port of my microcontroller-based projects to PC but I wouldn't at all feel comfortable hooking it straight to "true" RS232-talking device.
To further add to my confusion, some resellers list Array 3312 adapter (
http://www.array.sh/yq-fje.htm), clearly marked as "TTL level" by the manufacturer, as suitable for "true-RS232" 372x series DC-loads as well.
Am I missing something? Where can I get a proper, standards-complying RS232 to USB converter?