Please note, I own neither a Data-20 nor the WiModem, so any advice I can give most probably requires you to check against the manuals. Also, you'll need to acquire at least some basic knowledge about how bytes are physically transmitted with RS232.
The PRG doesn't give this information, because in those days, the necessary information mostly was fixed and always conveyed by either the modem manufactorer or the BBS.
RS232 allows for different number of bits per bytes. Can be any of 5 to 8. 7-bit ASCII would only need 7 bits, if you need to transmit full 8 bits, you'll need to setup the transmission with 8 bits (d'uh), etc.
The simple 3-wire protocol needs only one line for signal ground (GND), and 2 signal lines in each direction. The receivers end of these lines is labeled RxD, the transmitters end TxD. That means, RxD and TxD are crossed along the cable.
While a signal line is idle, the sender puts a 0 signal (also dubbed "space") on it. To signify the beginning of a byte transmission, the sender puts 1 or 2 start-bits (a 1 signal, also dubbed "mark") on the line. That is already relevant for a correct transmission! If the sender uses 2 start bits, but the receiver only expects 1, it will mistake the 2nd start bit for a mark in the data. That means, both sender and receiver must agree upon the number of start bits.
Then follows the data. The bit order is not specified, but usually is lowest bit first.
If parity is specified, the data is possibly extended with another bit, the parity. If present, it is either chosen to make the number of all set bits, in data and parity combined, odd or even. That's what's meant with parity odd or even. The parity bit can also be chosen as always set, always clear, or not present at all. Parity, if used, checks for transmission errors on the bit level.
The combination of bits per byte, parity, and number of start bits is usually abbreviated in a short 3 letter combination like "8N1", which means
8 bits/byte,
No parity,
1 start bit.
That info is evidently missing from the WiModem manual!
The protocol also allows to specity the number of stop-bits. That is the minimal number of blanks a receiver can expect before the next byte is going to be sent - however, as you probably could guess, this is of minor importance for a correct transmission.
With the 3-wire protocol, special characters are employed between Modem and Computer to control the data flow. That means, the computer can tell the Modem (and thus, ultimately the BBS) to temporarily cease sending because it has to process the data. At a later time, it can ask for more data. The X-wire protocol implements this in hardware, with extra signal lines.
So much for the basics of RS232.
ral-clan wrote:[...]it works in my terminal programs on a 22-column VIC. With the 80-column card I get a garbled series of lines[...]
That's good news: at least something works, and we can exclude a hardware failure in the VIC-20 (of the VIA). Also, the WiModem seems to have quite relaxed demands upon the receiving end, otherwise it wouldn't be able to recognise your commands.
You'll need to replicate the exact settings of that 22-column terminal program with the OPEN127,2,... command for the Data-20 to stand any chance for a correct transmission in both directions. And then it still needs to be confirmed, that the terminal program of the Data-20 does a correct translation for ASCII (Edit: seems to work with your 1270 modem, so that is also confirmed).
Edit: you tell different logical channels for the OPEN command. What is given in the Data-20 manual, OPEN1,2,... or OPEN127,2,...? You can't just change the first number as you like, because the terminal internally uses exactly that number to set up the KERNAL I/O functions for input and output!
