Python rolling HEXs through MESM-6 UART

HEXs.py

#!/usr/bin/python3
from serial import Serial
from time import sleep

ser = Serial('/dev/ttyUSB0')

t0 = -10

hello = '00767938383F'
spin  = '240912'
spin1 = '010204081020'

def get_anim(pos, t):
    global hello, spin, spin1
    if t<0:
        return '00'
    if t<20:
        return spin1[t*2 % 12] + spin1[t*2 % 12 + 1]
    if t<50:
        return spin[t*2 % 6] + spin[t*2 % 6 + 1]
    return hello[pos*2] + hello[pos*2+1]

reverse = 0

while True:
    s = ':'
    for i in range(6):
        s += get_anim(i, t0-i*13)
    ser.write(bytes(s, 'utf8'))
    ser.flush()
    sleep(0.08)
    if reverse == 0:
        t0 = t0 + 1
    else:
        t0 = t0 - 1
    if t0 > 120:
        reverse = 1
    if t0 < -10:
        reverse = 0

ser.close()

uart3.bemsh

*
* UART & GPIO test.
*
start   start   '1'
* UART registers
U0_DT   equ     '77740'   , data fifo
U0_CT   equ     '77747'   , control and status reg
U0_CTS  equ     '77746'   , set bits
U0_CTC  equ     '77745'   , clear bits
* GPIO HEX
G0_DT   equ     '77756'
        ntr 3
* reset uart & gpio
        xta     '0'
        atx     U0_CT
        atx     G0_DT
* set divider
        xta     =b'505'      , 9600
        atx     U0_CTS
* enable rx tx
        xta     =b'1000'   , rxtx_en bit
        atx     U0_CTS
* -------------------------
s0      vjm     getch(2)
        aex     =b'072'
        u1a     s0         , wait for ':'
* ------ receive 12 4-bit nibbles
        xta     '0'
        a-x     =b'12'
        ati     3
        vjm     getch(2)   , get 1 nibble
        vjm     atoi(2)
        aax     =b'17'
        atx     buf2
* ------ shift and load next ------
l1      asn     64-4
        atx     buf2
        vjm     getch(2)   , get n-th nibble
        vjm     atoi(2)
        aax     =b'17'
        aox     buf2
        vlm     l1(3)
* ------ end
        atx     G0_DT
        uj s0
* helpers
* get_char        
getch   xta     U0_CT
        aax     =b'4000'
        u1a     getch
        xta     U0_DT
        aax     =b'377'
        uj      (2)
* atoi '0'-'9''A'-'F'
atoi    atx     buf
        a-x     =b'060'  , 0
        u1a     bad
        xta     buf
        a-x     =b'072'  , 9 +1
        uza     n1       , letter
        xta     buf
        a-x     =b'060'  , -0
        uj      (2)
n1      xta     buf
        a-x     =b'0101'
        u1a     bad
        xta     buf
        a-x     =b'0107'
        uza     bad
        xta     buf
        a-x     =b'0101'
        a+x     =b'012'
        uj      (2)
bad     uj      s0 , wait for ':'
*-------------------------
align   адрес   start+'1777'            , данные с адреса 2000
buf     конд    b'0'
buf2    конд    b'0'
        fin