I need to interface generic USB scanners (not a single, defined, model) with an embedded Linux device (mips32/MT7620 running Linux 3.18, if it matters).
All scanners operate in "keyboard emulation mode" and, sure enough, if plugged into a desktop Linux send their data directly as keyboard input to console.
This does not happen on device (i.e.: I do not see any char unto the serial debug console, which makes sense as it misses all X input subsystem).
All scanners present themselves as Input device on /dev/input/event0.
My current attempt is using python evdev; code is quite straightforward:
import asyncio
import evdev
edev = evdev.InputDevice('/dev/input/event0')
async for event in edev.async_read_loop():
if event.type == evdev.ecodes.EV_KEY and event.value == 1:
handle_event(event)
Note: this is just an extracted snippet; I can post a runnable example, if needed.
This is essentially this answer (I'm using asyncio, can that be a problem?) and it seems to work, but actually loses events.
If barcode scanner sends events back-to back I seem to start losing events quite early (after about 16 events <= 8 chars!).
If I can insert between-char delay (~1ms is enough) then everything works as expected, but that's not an option on many scanners.
What am I missing?
For whoever is in the same predicament: I was unable to find a Pure Python working solution.
I resorted to a very simple "C" program (most of it are tables):
#include <stdio.h>
#include <fcntl.h>
#include <linux/input.h>
#include <unistd.h>
#include <signal.h>
#include <stdlib.h>
void INThandler() {
exit(0);
}
char ttab[] = {
0, 27, '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', '-', '=', '\b', /* Backspace */
'\t', 'q', 'w', 'e', 'r', 't', 'y', 'u', 'i', 'o', 'p', '[', ']','\n', /* Enter key */
0, 'a', 's', 'd', 'f', 'g', 'h', 'j', 'k', 'l', ';','\'', '`', 0, /* Left shift */
'\\', 'z', 'x', 'c', 'v', 'b', 'n', 'm', ',', '.', '/', 0, /* Right shift */
'*',
0, /* Alt */
' ', /* Space bar */
0, /* Caps lock */
0, /* 59 - F1 key ... > */
0, 0, 0, 0, 0, 0, 0, 0,
0, /* < ... F10 */
0, /* 69 - Num lock*/
0, /* Scroll Lock */
0, /* Home key */
0, /* Up Arrow */
0, /* Page Up */
'-',
0, /* Left Arrow */
0,
0, /* Right Arrow */
'+',
0, /* 79 - End key*/
0, /* Down Arrow */
0, /* Page Down */
0, /* Insert Key */
0, /* Delete Key */
0, 0, 0,
0, /* F11 Key */
0, /* F12 Key */
0, /* All other keys are undefined */
};
char ntab[] = {
0, 27, '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', '-', '=', '\b', /* Backspace */
'\t', 'q', 'w', 'e', 'r', 't', 'y', 'u', 'i', 'o', 'p', '[', ']','\n', /* Enter key */
0, 'a', 's', 'd', 'f', 'g', 'h', 'j', 'k', 'l', ';','\'', '`', 0, /* Left shift */
'\\', 'z', 'x', 'c', 'v', 'b', 'n', 'm', ',', '.', '/', 0, /* Right shift */
'*',
0, /* Alt */
' ', /* Space bar */
0, /* Caps lock */
0, /* 59 - F1 key ... > */
0, 0, 0, 0, 0, 0, 0, 0,
0, /* < ... F10 */
0, /* 69 - Num lock*/
0, /* Scroll Lock */
0, /* Home key */
0, /* Up Arrow */
0, /* Page Up */
'-',
0, /* Left Arrow */
0,
0, /* Right Arrow */
'+',
0, /* 79 - End key*/
0, /* Down Arrow */
0, /* Page Down */
0, /* Insert Key */
0, /* Delete Key */
0, 0, 0,
0, /* F11 Key */
0, /* F12 Key */
0, /* All other keys are undefined */
};
char stab[] = {
0, 27, '!', '@', '#', '$', '%', '^', '&', '*', '(', ')', '_', '+', 0, /* Backspace */
0, 'Q', 'W', 'E', 'R', 'T', 'Y', 'U', 'I', 'O', 'P', '{', '}', 0, /* Enter key */
0, 'A', 'S', 'D', 'F', 'G', 'H', 'J', 'K', 'L', ':', '"', 0,'\n', /* Left shift */
0, 'Z', 'X', 'C', 'V', 'B', 'N', 'M', '<', '>', '?', 0, /* Right shift */
'*',
0, /* Alt */
' ', /* Space bar */
0, /* Caps lock */
0, /* 59 - F1 key ... > */
0, 0, 0, 0, 0, 0, 0, 0,
0, /* < ... F10 */
0, /* 69 - Num lock*/
0, /* Scroll Lock */
0, /* Home key */
0, /* Up Arrow */
0, /* Page Up */
'-',
0, /* Left Arrow */
0,
0, /* Right Arrow */
'+',
0, /* 79 - End key*/
0, /* Down Arrow */
0, /* Page Down */
0, /* Insert Key */
0, /* Delete Key */
0, 0, 0,
0, /* F11 Key */
0, /* F12 Key */
0, /* All other keys are undefined */
};
int main() {
char devname[] = "/dev/input/event0";
int device = open(devname, O_RDONLY);
struct input_event ev;
int shift = 0;
char line[4096], *p = line;
signal(SIGINT, INThandler);
fputs("starting\n", stdout);
//fputs("starting\n", stderr);
while (1) {
read(device, &ev, sizeof(ev));
if (ev.type == 1) {
if (ev.code == 42)
shift = ev.value;
else if (ev.value) {
//printf("Key: %i State: %i\n", ev.code, ev.value);
char *t = shift? stab: ntab;
char ch = t[ev.code];
//printf("Key: %02d State: %d [%c]\n", ev.code, ev.value, ch);
if (ch == '\n') {
*p = '\0';
fputs(line, stdout); fputc('\n', stdout); fflush(stdout);
//fputs(line, stderr); fputc('\n', stderr); fflush(stderr);
p = line;
} else
*p++ = ch;
}
}
}
}
... driven from Python:
async def handle_events(self):
log.debug("handle_events(%s): st start", self.dev)
while True:
if self.hid is None:
log.debug("handle_events(%s): checking...", self.dev)
self.hid = await asyncio.create_subprocess_exec(
'/usr/local/bin/readEvents',
stdout=asyncio.subprocess.PIPE
)
try:
out = await asyncio.wait_for(self.hid.stdout.readline(), 10)
except asyncio.TimeoutError:
log.debug("handle_events(%s): timeout on startup message", self.dev)
self.hid.terminate()
self.hid = None
else:
out = out.strip()
if out != b'starting':
log.debug("handle_events(%s): bad startup message: %s", self.dev, out)
self.hid.terminate()
self.hid = None
else:
log.debug("handle_events(%s): startup msg received", self.dev)
else:
if self.hid.stdout.at_eof():
log.info("handle_events(%s): killing subprocess", self.dev)
rc = await self.hid.wait()
log.info("handle_events(%s): subprocess exited with status %s", self.dev, rc)
self.hid = None
await asyncio.sleep(10)
if self.hid is not None:
line = await self.hid.stdout.readline()
if not line:
log.error('handle_events: readline() ERROR:')
self.hid.terminate()
self.hid = None
else:
line = line.strip()
log.info('handle_events: received line: "%s"', line)
await self.process_line(line)
I'm sure Python driver can be simplified. This is verbatim from my (working) application.
HiH!